The Effect of Different Combination of Agricultural Residues on the Quality of HTC Hydro-Char Energy Yield

Transcription

1 University of Natural Resources and Life Sciences, Vienna Department of Material Sciences and Process Engineering Institute of Chemical and Energy Engineering The Effect of Different Combination of Agricultural Residues on the Quality of HTC Hydro-Char Energy Yield Master thesis A Thesis by SAMAR SEYEDSADR Supervisor: Univ.Prof. Dr. techn. Christoph Pfeifer Co-Supervisor: Dipl.-Ing. Dr.techn. Rafat Al Afif Student number:

2 ABSTRACT Department Institute Material Sciences and Process Engineering Chemical and Energy Engineering Name of the thesis The Effect of Different Combination of Agricultural Residues on the Quality of HTC Hydro-char Energy Yield Date Number of pages Program Level of studies Natural Resources Master s Thesis Management and Ecological Engineering Accumulation of solid waste and demand of energy are increasing along with population growth. Thus, improving Hydrothermal Carbonization (HTC), which is considered as a remarkable and sustainable method of treatment as well as non-harmful way of biomass conversion to energy and soil fertilizer, is seemed to be a big step towards waste treatment and energy utilization. The present study was performed in order to discover if HTC technique could be a promising method so as to treat residues in biogas plant to valuable and high quality hydro-chars. The HTC experiments were carried out with a 1.5L reactor under the same condition of 200ºC and pressure of 15 bars, using different combinations of two agricultural residues (maize and barley) and biogas sludge; collected from a farm-based biogas plant located in the town of Utzenaich, Austria. Collected raw materials were 2 agricultural residues (AR); maize and barley silage, as well as anaerobically digested sludge (SD). All substrates successfully carbonized through 8 HTC tests, which were different in type and the fraction of selected inputs. Subsequently, qualities of all samples, 8 hydro-chars plus 3 raw materials were analyzed through elemental and calorific value analysis. Also organic concentration in liquid phase of HTC samples was investigated. And as a final analysis, potential germination test were applied on all 11 samples (raw materials plus hydro-chars) as well. The results have shown that over 70% of hydro-chars and liquid were recovered at the end of each HTC tests. In each test, gaseous products from HTC were calculated and shown 5-6% of the total HTC products. Moreover, quality of hydro-chars was compared to raw materials. According to this comparison carbon content increased by the range of % in most cases, but as an exception, carbon content and higher heating value (HHV) from HTC of biogas sludge increased by 1.4% and 13% respectively. Whereas there is a clear increase in HHV of the other HTC samples ranged between 31-36%. The highest HHV with MJ/Kg found in hydro-char from barley silage. The reduction of O/C atomic ratio of HTC samples range from 36 to 45% comparing with raw samples, whereas O/C atomic ratio of biogas sludge was reduced only 5%. Correspondingly, atomic H/C ratio also decreased in hydro-chars in a range of 21-30% for most cases and 14% for sludge. Therefore, decreasing of atomic O/C and H/C ratios of samples after HTC illustrates good process of HTC. In addition, quality of hydro-chars was compared with a selective brown coal quality; the low difference between brown coal and all hydro-chars quality lead to suggest that hydro-chars could be replaceable as a coal. Moreover, the combination of feedstock materials before the HTC process leads to improve hydro-char properties significantly in comparison to mixtures of hydro-chars 1

3 produced separately. The concentration of organic matter in the liquid phase, which was for most samples about 49%, recommends using the HTC liquid phase as feedstock for biogas plant. However, further anaerobic digestion experiments are needed for determination of the biochemical methane potential from the HTC liquid phase. As a result of germination tests, the range of 90-95% germination rate represents no phytotoxic effect. However, by comparing total germination between hydro-chars and raw materials, about 1% reduction in germination rate has displayed in the results. As a conclusion, quality analysis of hydrochars produced from maize, barley, biogas sludge and different combinations of them, indicates energetic use of HTC as well as soil amendment role of its by-products. 2

4 Table of Contents 1. Introduction 1.1 Background and Problem Statements 1.2 Research Objectives 2. Fundamentals 2.1 Biomass; structure and properties Role of Biomass in Hydrothermal process 2.2 Overview of Hydrothermal Carbonization Process Background of Carbonization and char production Hydrothermal Carbonization Process Chemistry of HTC and the Energetic Aspects of the Reaction Heat of Reaction, Energy Balance and Other Factors Products of HTC Solid Phase Liquid Phase Gases Influence of Process Parameters Temperature Residence Time Pressure PH Solid Load, Particle Size and Combination of Feedstock 2.3 Use of HTC Hydro-char Soil Amendment Carbon Sequestration Activated Carbon adsorbents Other Applications 2.4 HTC Reactors Batch Reactor and continuous Reactor Reactors at BOKU university Optical Cell Self-build Reactor 3. Materials and Methodology 3.1 Materials 3.2 Methodology hydrothermal carbonization experiment procedure 3

5 HTC reactor set-up Analytical methods Elemental Analysis Calorific value Germination Potential test 4. Results and discussion 4.1 Process Parameters 4.2 products of HTC and Mass balances Characteristics of Hydro- char Discussion on hydro-char quality and fuel characteristic 4.3 Seed Germination Discussion 5. Conclusions and Recommendations 6. References 7. Appendix 4

6 List of Figures Figure 1. Chemical structure of lignocellulosic material; (a) Building blocks/units of Lignin; (b) Xylose unit of hemicellulose; and (c) One chain of Cellulose polymer Figure 2. Schematic form of Lignocellulose in cell wall of a plant Figure 3. Biomass Sources for energy production Figure 4. A comparison of different energy exploitation routes for carbohydrates Figure 5. HTC reaction pathways for lignocellulosic biomass; Note: Liquid biocrude is a complex liquid solution of condensation, aromatization and polymerization products Figure 6. Examples of the Use of HTC Hydro-char Figure 7. Aluminum cube with copper pipes Figure Figure 8. Stainless steel cylinder-like container Figure 9. Arduino hardware Figure 10. Biogas plant. Utzenaich, Austria Figure Liters self-built reactor used for HTC experiments at Chemical and Energy Engineering Institute, BOKU University Figure 12. Milling device used to prepare samples for Elemental Analysis, Calorimetric Test and Germination test, at Waste Management Institute, BOKU University Figure 13. IKA C200 Oxygen Bomb Calorimeters, Pelleting Press and Pressure Device Figure 14. a. Schematic view of inside a Bomb Calorimeter b. Bomb Cell, 50J cotton strings, and other accessories Figure 15. Shows the order of three phases in a box, an example of before (a) and after (b) germination test Figure 16. Each box covered and isolated with a plastic cover Figure 17. Early stage and short rooted germination Figure 18. Temperature and Pressure Condition of HTC Process over time Figure 19. Measured Elemental Analysis And Calculated Oxygen For Raw Materials And HydroChars Figure 20. A comparison between carbon content before and after HTC; raw materials calculated for T4f to T8f Figure 21. A comparison between atomic H/C and O/C ratios; before and after HTC tests Figure 22. Comparison between atomic H/C and O/C ratios of calculated fresh feeds (Tf), hydrochars (Th) and brown coal Figure 23. Van Kerevelen diagram (A. Kauriinoja, 2010); a composition of beech wood and torrefied beech wood Figure 24. Comparison of Van Kerevelen diagram and hydro-chars Figure 25. Correlation between C intensity and HHV and comparison between raw materials, hydro-chars and brown coal 5

7 Figure 26. Carbon content and higher heating value correlation diagram Figure 27. Comparing HHV between hydro-chars and calculated raw materials Figure 28. Comparing effect of combination of inputs (measured) and combination of hydro-chars (calculated) on changes of C% Figure 29. Comparison of total germination rate Figure 30. Effects of HTC process on seed Germination 6

8 List of Tables Table 1. Composition of lignocellulose components in various lignocellulosic biomasses Table 2. Feedstock properties relevant to thermal conversion processes Table 3. Comparison of reaction conditions and typical product yields for thermochemical conversion processes with char as a product Table 4. Effects of hydrothermal carbonization on changes in cellulose properties Table 5. Results from different biomass hydrothermal carbonization experiments Table 6. Effects of certain hydro-chars on different MOs Table 7. Comparison of Chemical Reactors Table 8. example of some HTC reactors in different studies Table 9. Elemental analysis data and calorific value of raw materials Table 10. Summary of Analytical Methods Table 11. Experimental Design of the conditions and Substrates Mixture Table 12. Methodology Abbreviations Table 13. Substrates moisture content calculation and drying condition Table 14. Mass Balance Table 15. Elementary compositions of raw materials and hydro-chars Table 16 Brown Coal elemental data from ECN Phyllis2 Classification Table 17. Calorific Value (Heat of Combustion) Resulted from Bomb Calorimeter device and Carbon content Table 18. The effect of different combination of maize, barley and sludge on carbon content and HHV of hydro-chars Table 19 Leachate-effect phase results Table 20. Volatile-effect phase results Table 21. Sand + Hydro-char effect phase results 7

9 AKNOWLEDGMENT I wish to appreciate the encouragement of my professors Prof. Christoph Pfeifer and Dr. Rafat Al Afif and would like to thank them for their guidance throughout preparing the present research. I would like to acknowledge my deep debt to Prof. Gerhard Soja for his supervision on germination test at AIT Institute in Tulln. I am also thankful to my friends, and all who helped me conduct my research at Chemical and Energy Engineering Institute. Last but not least, my heartful gratitude goes to my family for their unfailing patience and moral support. To them, 8 I dedicate this study.

10 1. INTRODUCTION 1.1 Background and Problem Statements According to the European Technology Platform for Zero Emission Fossil Fuel Power Plants (European Commission, 2006), EU commission obligates policy makers, industries and any committee that are responsible for fuel and energy production to follow frameworks and apply environment friendly techniques in order to control emission, and secure energy supplies on a sustainable basis. Therefore, it led researchers to come up with the idea of using natural based materials instead of fossil fuels; although fossil fuels were an astonishing opportunities in 20th century (A. Lilliestrale, 2007). Since biomass has a high potential of energy production, it would be useful and would support sustainable consumption, if during a controlled combustion, released solid, liquid and gas were recovered, treated and refined (J.A J. A Libra et. al., 2011). Mega cities in developing countries encounter several serious dilemmas; as the population grows their demands also goes up; demand of food and therefore agriculture and also need of water resources, demand of power etc. (A. Lilliestrale, 2007). By increasing consumption, other problems such as air and soil pollution, water scarcity, pollution of groundwater and surface water resources, accumulation of municipal solid waste and aggregation of yard waste also may arise, as well as management and treatment problems due to the lack of wellorganized system and equipment. Significant problem of aggregation of yard waste and agricultural residues arises when farmers try to manage the waste and get rid of this huge amount of residues by combusting them on-site (CEC, 2014) or via slash and burn method (E. Wayne, 2012). Although experts consider it as the most basic way of thermal method of treatment, it may release a huge amount of carbon dioxide to atmosphere as well as other toxic and green house gases, and subsequently cause emission (CEC, 2014). Moreover, the history of terra preta that contained charcoal-like materials originate from fireplaces (E. Wayne, 2012) led scientists to come up with the most effective idea of a controlled hydrothermal method, named HTC (Hydrothermal Carbonization), besides other similar methods such as dry pyrolysis and gasification, in order to get the most advantages from by-products for soil amendment, and energy power HTC is a wet thermal conversion process that normally is run between C under autogenous pressures (J. A Libra et. al., 2011; M. Tripathi et. al., 2015; N. D.Berge, 2011). Hydrothermal carbonization of biomass could be one of the solutions of almost all of mentioned problems. Several studies, (J. A Libra et. al., 2011; N. D.Berge, 2011, H. Ramke et al., 2009, M. 9

11 Tripathi et. al, 2015, I. Oliveria et al., 2013) have been worked on the improvement of HTC technique, investigating different kind of biological materials as inputs, changing parameters and lab conditions to reach a better quality of hydro-char in order to be more effective and comparable to brown-coal using different reactors and autoclaves; also to be more reasonable so as to use hydro-char as a renewable energy source worldwide. But considering vast different available feedstock, several further studies have to be done. Relatively, researches on the HTC method, hydro-char and other products of this specific process are rather less and limited, in compared to studies, which have been done on dry pyrolysis (J. A Libra et. al., 2011). 1.2 Research Objectives The principal objectives of the present thesis are: 1- to explore the general feasibility of hydrothermal carbonization treatment of 3 biomass streams: 2 agricultural residues (AR), which are commonly treated in farmbased biogas plant in Utzenaich, Austria and anaerobically digested sludge (SD), as a supplementary system to biogas plants 2- to present a method in order to improve hydro-char quality by applying and investigating different combination of two mentioned waste streams by the use of HTC technique 2. FUNDAMENTALS 2.1 Biomass; Structure and Properties Scientists refer to organic materials as biomass. Different studies classified biomass in different groups such as different organic wastes from households, and industries for instance paper sludge, sugar cane residues, also waste from food processing, agricultural residues, crop and grass stover or herbaceous energy crops, silage, woody materials or short-rotation energy crops (A. Lilliestrale, 2007), wastewater treatment and biogas sludge, manure and other natural organic matters. However, the majority of the biomass, which is widely used for energy production, is woody biomass including different part of a tree, and agricultural biomass such as stalks, straw and shells of crops (M. Tripathi et. al, 2015). Plants consist mainly of cellulose (30% - 50 %) (C. J. Coronella et al., 2014), hemicellulose (10% - 35 %) (C. J. Coronella et al., 2014), or according to T. Shahzadi et al., about 10% - 25% lignin, 20% - 30% hemicellulose, and 40% 50% cellulose known as lignocellulos. These data are slightly different in each research 10

12 dependingon the region, genetic variability, species and sources of the plants (table 1); whether they are from wood or grass (T. Shahzadi et al., 2014). Table 1. Composition of lignocellulose components in various lignocellulosic biomasses (T. Shahzadi et al., 2014) These three component are known as lignocellulose, which is a three-dimensional polymeric composites formed by plants (figure 1). Lignocellulose is built up the cell walls (figure 2) of biomass. Cellulose is the material that is rich in the nature and consists of a homopolysaccharide chain of repeating (1,4)-D-glucopyranose units attached by β -1,4 linkages, with insolubility and crystalline Polysaccharide nature (A. Lilliestrale, 2007, T. Shahzadi et al., 2014). Hemicellulos is a large number of hetero-polysaccharide (A. Lilliestrale, 2007); repeated polymers of pentoses, and hexoses and monomers like xylans, mannans and glucans, with a lower chemical and thermal stability than cellulose. Lignin is the most complex and smallest component of lignocellulosic biomass, which is non-carbohydrate with a phenylpropane-based long chain polymer sealed around the two other components (A. Lilliestrale, 2007, T. Shahzadi et al., 2014). Phenyl-propane polymer has different structure depending on methoxyl groups defined by the plant source they were derived from; structure I exist in grasses, structure II present in coniferous wood and structure III found in deciduous wood. As it is illustrated in Figure 2, lignin contains three aromatic alcohols, coniferyl alcohol, 11

13 sinapyl alcohol and p -coumaryl alcohol (T. Shahzadi et al., 2014). In contrast to cellulose, lignin cannot be easily depolymerised in to smaller and its original monomers, therefore it remains mainly unchanged during a hydrothermal carbonization. This is the reason why biomass with low content of lignin is preferred (A. Lilliestrale, 2007). Figure 1. Chemical structure of lignocellulosic material; (a) Building blocks/units of Lignin; (b) Xylose unit of hemicellulose; and (c) One chain of Cellulose polymer (T. Shahzadi et al., 2014) 12

14 Figure 2. Schematic form of Lignocellulose in cell wall of a plant Role of Biomass in Hydrothermal process Hydrothermal process is invented to reuse biomass, as natural and renewable energy potentials, in the shape of hydro-char, bio-oil and gas (I. Oliveria et al., 2013), besides several other advantages mentioned in different article, which are discussed in the following sections. Accordingly, there is a potential inside each biomass to be considered as an available (13Gha total land area), biological renewable carbon and energy source, which with pyrolysis can be transformed in to three different forms of biofuel; solid, liquid and gaseous fuel (M. Tripathi et. al, 2015). Source of biomass (figure 3) that is going to be used in energy production processes is important. The reason is that sources represent generally the proportion of elements that each biomass comprises of; elements that can help to predict heating value of biomass and energy content of the end products (M. Tripathi et. al, 2015). Table 2, is an example of comparing different biomass by representing their important elements and also the energy contents (J. A Libra et. al., 2011). Hence knowing biomass sources influences selecting the right feedstock for the hydrothermal processes. Therefore, biomass is a complex composition of carbon, oxygen, hydrogen, sulfur, nitrogen, ash and small quantities of few other elements such as 13

15 heavy metals. The proportion of these elements in each biomass depends on plant species and growing condition. However in general view, the share of carbon is the most, among all other elements (M. Tripathi et. al, 2015). Figure 3. Biomass Sources for energy production Furthermore, carbon, hydrogen and oxygen are the main factors of the energy content of biomass, but still are found in less value as compare to the carbon content of fossil fuel. Very small amount of sulfur and nitrogen in biomass residues is needed to avoid raising green house gases emission and acid rains, although S and N can make energetic bonds with carbon and hydrogen as it happens by using fossil fuels. Therefore, it is environment friendly to avoid biomass that has high amount of S and N, which are found mostly in industrial waste as well as animal and human waste along with aquatic biomass (M. Tripathi et. al, 2015). By comparing these elements in different biomass, driven from different literatures (M. Tripathi et. al, 2015) it can be argued that woody and agricultural biomass produce more char and higher calorific value products, which are the results of their high carbon and oxygen content. Accordingly, composition of the materials that is used as the inputs of hydrothermal processes, influences the quality of the product (J. A Libra et. al., 2011); thus cellulose and hemicellulos, as well as lignin, which are fiber components (A. Lilliestrale, 2007), influence the product yield (M. Tripathi et. al, 2015). For instance lignin from coniferous species is found to generate larger char in comparison to deciduous lignin. In high temperature cellulose 14

16 forms the gaseous phase and transforms to volatile production, while in low temperature it reforms to char. In conflict to lignin, which is liable to char yield, cellulose and hemicellulose are responsible to produce volatile fractions; it is seen mostly in dry pyrolysis in higher temperature conditions (M. Tripathi et. al, 2015). In HTC, where the process stays in mild condition, lignin stays unchanged therefore biomass, the ones that contains more cellulose is preferred (A. Lilliestrale, 2007). Table 2. Feedstock properties relevant to thermal conversion processes (J. A Libra et. al., 2011) Further discussion of the related reactions during HTC of biomass is presented in the following sections. 2.2 Overview of Hydrothermal Carbonization Process Background of Carbonization and Char Production Examining the most fertilized soils such as terra preta soils indicates that 9 percent of the soil consists of carbon, which is a high fraction in comparison to normal surrounding soils, which contains 0.5% carbon. Char-coal-like soil in terra preta seems to have been created by charring or carbonization of organic wastes by heating them in low-oxygen source condition in order to use it as fuel (E. Wayne, 2012). High amount of carbon in the soil generally improves the chemical characteristics, such as improving the preservation of nutrient, consequently increasing the growing condition and soil germination capacity; but applying hydro-char improves this situation twice, compared to using chemical fertilizers. Researchers from Cornell University started to use hydro-char to 15

17 improve soil fertility and carbon sequestration early in 21th century (E. Wayne, 2012). This also has to be emphasized that their fundamentals began with researchers activities in the early 20th century. Activities have been performed in order to understand natural coal formation (J. A Libra et. al., 2011, H. Ramke et al., 2009). Also Friedrich Bergius has done other studies on coal-fuel conversion and carbohydrates transformation to coal-like materials. In that time, the use of coal-like material was not an economical issue, in comparison to use of the real cheap coal. However researchers began the synthesis of carbonaceous materials again by the 21th century (M. Child, 2014). Among several method of carbonization, HTC is a useful thermochemical method to control and manage waste stream (high moisture content biomass) (M. Tripathi et. al, 2015) and also to dry and resize the huge amount of different kind of waste to some products that are more compact and valuable. Solid phase product of this process is called HTC hydro-char or hydro-char. Therefore, the products also have multiple functions in different scientific fields and commerce (J. A Libra et al., 2011) such as replicable coal-like materials and soil fertilizers. Five different thermochemical carbonization mechanisms (slow and fast pyrolysis, gasification, torrefaction and hydrothermal carbonization) are known worldwide; each method differs in properties of each product, and the process conditions such as how fast heat is transferred to fresh biomass particles (shown in Table 3). The organic material is heated during slow or intermediate pyrolysis over long period of time at about 400 C C, released 30 %- 35% gases, and yield 20%- 40% char. By-products during slow pyrolysis are gases, tar-like substances and solid char. If producing solid char is desired, lower temperature and longer residence times will be set (slow pyrolysis), but if more gases and liquid products needed, high temperature and lower residence time (intermediate pyrolysis) will be set. Fast or flash pyrolysis is used to produce up to 75% liquid for further function as fuels. As the name represents, the feed is rapidly heated at a high temperature in low or absence of oxygen, and the generated vapor is cooled fast to achieve liquid product. These processes are named generally dry pyrolysis (J. A Libra et al., 2011, M. Child 2014). During gasification process, partial combustion takes place with the help of addition of air, oxygen, carbon dioxide or their mixtures, which provides an oxidizing atmosphere. The main products are gases with yields up to 85%, including H2, CO, CO2 and CH4, which can be used directly as a fuel or as a syngas; furthermore to be processed to generate products like synthetic natural gas. However in large gasifiers large amount of char could be recovered as well, due to 16

18 their high amount of input of biomass, although char and liquid products are gained very little in gasification processes (J. A Libra et al., 2011, M. Child 2014). Table 3. Comparison of reaction conditions and typical product yields for thermochemical conversion processes with char as a product (J. A Libra et al., 2011) Torrefaction or mild pyrolysis occurs at low temperature ( C) and in an average residence times (1-3 hours). In order to evaporate water and reach to the desired temperature, torrefaction process is designed to begin with pre-heating, pre-drying, and further post-drying and intermediate heating, which need external energy. Main products are about 70% char and 30% torrefaction gas (M. Child 2014). Hydrothermal carbonization (HTC) is considered a better choice of method for the present study due to its advantages and its compatibility to high moisture content inputs and etc. HTC is an exothermic chemo-physical process, which is known as a fast way of coalification in order to convert biomass to hydro-char in aquatic phase under a controlled temperature and pressure (H. Ramke et al., 2009) showing more advantages in comparison to dry pyrolysis (N. D. Berge, 2011). Details are discussed in following sections. Figure 4 also shows carbon efficiency of HTC process decomposing carbohydrate, in comparison to other conversion pathways. This schematic comparison, which was made by A. Lilliestrale, represents a high recovery of carbon (almost 100%) during HTC process dehydration reaction (A. Lilliestrale, 2007). 17

19 Figure 4. A comparison of different energy exploitation routes for carbohydrates (A. Lilliestrale, 2007) Hydrothermal Carbonization Process Coalification of organic material is a process, which occurs under special conditions in nature, takes thousand of years to produce coal from carbohydrates. Hydrothermal carbonization (HTC) (J. A Libra et al., 2011) is an exothermic chemo-physical process, which is known as a fast way of coalification in order to convert moisturized and fresh biomass to hydro-char in aquatic phase under a controlled temperature and pressure (H. Ramke et al., 2009) showing more advantages in comparison to dry pyrolysis (N. D. Berge, 2011). HTC process is invented not only to avoid air pollution arise from agricultural residues combustion but also to reuse biomass as natural and renewable energy potentials in the shape of hydro-char, liquid and gas, which are the by-products of the process (I. Oliveria et al., 2013). Also hydro-char can be used as an isolation material in buildings, because of its high carbon content and as a sorption coal in drinking water treatment processing (A. Lilliestrale, 2007). On the other hand, in the subject of fuel and energy needs, scientists try to introduce methods such as HTC, which could be less a threat to ecosystem and human health; additionally, Hydrothermal carbonization has been investigated in several issues such as MSW treatment, since HTC process is made to be applicable with high moisture content biomass (N. D. Berge, 2011) as well as improvement of soil quality (J. A Libra et al., 2011, A. Lilliestrale, 2007) and energy production (A. Lilliestrale, 2007, M. M. Tripathi et. al, 2015). 18

20 Wet pyrolysis is known as Hydrothermal Carbonization (HTC), which gives the opportunity to use moisturized feedstock, agricultural residues and municipal wastes as the inputs, without any drying process as a pretreatment (A. Lilliestrale, 2007, J. A Libra et al., 2011). Also HTC produces higher solid yields in comparison to dry pyrolysis, as well as more water-soluble organic compounds and fewer gases, which is mainly CO2 (J. A Libra et al., 2011). Hydrothermal carbonization is a natural-based chemo-chemical process to coalification of organic biomass. This exothermic process is done under a mild temperature and pressure in aqueous phase (H. Ramke et al., 2009) in oxygen free atmosphere. This oxygen free condition allows biomass to be heated above its limit of thermal stability (M. M. Tripathi et. al, 2015). Fundamentals of this method have been started hundred years ago and recently it is done with more accurate and modern instruments (H. Ramke et al., 2009). HTC chemical process is based on a simple reaction, which is de-watering or dehydration of carbohydrates along with other reactions such as decarboxylation, aromatization and recondensation. This reaction is released about 950 KJ/mol heat (H. Ramke et al., 2009) per carbohydrate (C6H12O6) or according to A. Lilliestrale research 2135 KJ/mol heat (A. Lilliestrale, 2007), and also 5 molecules of water plus HTC hydro-char (C6H2O). However, the degree of dehydration of the carbohydrates is depending on the duration of the reaction, and additives can accelerate this process. Researchers believe that there may be a very few of loss of carbon in hydro-char but still the carbon efficiency is rather high in this process (figure 4). Additionally, since the carbon is transferred also to the liquid and gaseous phase, which can be collected and refined in further processes, all carbon is recovered and kept at the end products (H. Ramke et al., 2009). Theoretically, the carbon efficiency in HTC is near hundred percent in comparison to combustion, fermentation and anaerobic digestion (A. Lilliestrale, 2007). However, it is difficult to recover carbon from HTC liquid phase or so-called process water. The heat that released from dehydration is also recovered and returned to the process; therefore the loss of energy also would be low utilizing for heat and temperature. When there is no pre-treatment as de-watering and pre-drying processes before the main process, there would not be any other costly procedures involved, thus the net calorific value is high (H. Ramke et al., 2009) Chemistry of HTC and the Energetic Aspects of the Reaction HTC is used to make carbonized biomass, means processing the biomass hydrothermally and making products with higher carbon density. This happens by heating and decomposing 19

21 carbohydrates in the absence of oxygen. During heating the biomass, reactions that happen include hydrolysis, dehydration, decarboxylation, aromatization and re-condensation. In a schematic figure (figure 5) presented by M. Reza et. al., chemical reactions summarized step by step showing that chemical reactions starts with hydrolysis and ends to the production of water, gases and HTC Hydro-char. Dehydration is the predominant reaction in HTC, which is an exothermic reaction and it needs lower activation energy compare to other reactions. Under vapor pressure, carbohydrates need lower temperature to decompose so they exhibit less stability in hydrothermal conditions (J. A Libra et. al., 2011). In the first phase, when the dehydration and decarboxylation take place immediately after hydrolysis (M. Reza et. Al., 2014), different carboxyl, carbonyl and hydroxyl groups are formed by the cleaved and devolatilized biomass. The second phase is when gases and liquids are produced (M. M. Tripathi et. al., 2015). Part of cellulose starts to decompose as the temperature reaches to 100 C but part of it needs more than 180 C to start decomposing to glucose monomers (C. J. Coronella et al., 2014). However according to J. A Libra et. al., cellulose needs temperature approximately above 220 C to completely decompose and it is rather higher than the temperature needed for hemicellulose and lignin (J. A Libra et. al., 2011). In contrast, during hydrolysis, lignin shows more stability and reactive during HTC in Charles J. Coronella experiment, and remains unchanged at C. During hydrolysis, water reacts with cellulose or hemicellulose and breaks their bonds. Hemicellulose starts hydrolysis above 180 C and cellulose degrades to oligomers above 230 C (C. J. Coronella et al., 2014). Decrease in oxygen follows by reduction of carboxyl groups mainly from hemicellulose and cellulose, happen during dehydration and decarboxylation reactions. Hemicellulose and cellulose degrade in to monomers that further can reduce in to CO2 and H2O (C. J. Coronella et al., 2014). Some of the intermediate compounds that are produced during previous reactions are highly reactive. Three other reactions, condensation, polymerization, and aromatization, take place on the intermediates as the next step. Moreover, Charles J. Coronella believes that the formation of HTC-biochar is mainly characterized during these last three reactions (C. J. Coronella et al., 2014). 20

22 Figure 5. HTC reaction pathways for lignocellulosic biomass. Note: Liquid biocrude is a complex liquid solution of condensation, aromatization and polymerization products (M. Reza, 2014) During HTC about 950 KJ/mol heat (H. Ramke et al., 2009) per carbohydrate (C6H12O6) is released through a based simple chemical process (dehydration); or according to A. Lilliestrale research (2007) 2135 kj/mol heat per 3240 kj/mol of carbohydrates (A. Lilliestrale, 2007), and also 5 molecules of water plus HTC hydro-char (C6H2O). However, the degree of dehydration of the carbohydrates is depending on the duration of the reaction, and additives can accelerate this process. Additionally, since the carbon is transferred also to the liquid and gaseous phase, which can be collected and refined in further processes, all carbon is recovered and kept at the end products (H. Ramke et al., 2009). Accordingly, carbon efficiency in HTC is near hundred percent in comparison to combustion, fermentation and anaerobic digestion (A. Lilliestrale, 2007) Heat of reaction, Energy Balance and other Factors One of the factors, which should be considered for designing a reactor and in general to see if the project is worth to run, is heat of reaction. During chemical reactions of lignocellulosic biomass, heat is released and consumed. Feedstock used as the inputs, temperature and 21

23 residence time are the factors that affect the amount of heat released during hydrothermal processes. It can be calculated from the heats of all substances during reactions, and products (C. J. Coronella et al., 2014). One of the models used an equation to estimate the heat of reaction, showed cellulose degraded to C5.25H4O CO2 + 3H2O, and with calorimetric measurements, value of approximately 1 MJ/kgCellulose have been reported for HTC reaction. Consequently, this process has been reported as an exothermic process, while hydrolysis of cellulose happening in the initial phase of HTC is known to be endothermic (J. A Libra et. al., 2011). In other studies different approaches may be used to calculate the heat of chemical reaction. One of these approaches introduced as DSC or using Differential Scanning Calorimetry, applied on glucose, cellulose and wood (C. J. Coronella et al., 2014). Knowing the energy balance is important to be estimated, also when it comes to decision of using moisturized materials in order to understand if it is feasible to use such biomass or not, specially when the method of dry pyrolysis is applied. It is more crucial for dry pyrolysis, to decide how much moisture in materials is allowed according to energy needed for de-watering (J. A Libra et. al., 2011). Biomass with a low calorific value and high water content can be more useful to reach a valuable end product. As presented in the following table (table 4) derived from a study by D. Kim et al., an increase in carbon content of cellulose from 6.1 % to 35.0 % at 220 C by HTC process is observable; and also results exhibited that increasing the temperature from 180 C to 280 C resulted in a growth of calorific value from 16.5 to 27.7 MJ/kg (D. Kim et al., 2015). Table 4. Effects of hydrothermal carbonization on changes in cellulose properties (D. Kim et al., 2015) 22

24 Compared to the char result from dry pyrolysis, chemical structure of hydro-char is more similar to natural coal. Reactions during HTC cause HTC-char to exhibit low H/C and O/C ratios, means higher carbon than hydrogen and oxygen content; but still hydro-char has higher H/C and O/C ratios similar to natural coal. By increasing the temperature, the solid yields mainly convert to gaseous and heat, therefore H and O decrease and most of the carbon (6084%) remains in the hydro-char. As a result, a range of temperature between 180 C to 200 C are more preferable if the goal of the project is to reach a higher solid product; but it is still difficult to decide on an optimized temperature for reaching a higher amount of hydro-char, because it depends on the type and nature of the biomass (J. A Libra et al., 2011, M. Tripathi et al, 2015) Products of HTC Products in HTC are mainly solid and water-soluble organic compounds along with fewer amounts of gas, which is mainly CO2 (H. Ramke et. al., 2009) Solid phase The HTC solid product is a char that is elementally similar to lignite or sub-bituminous coal. In terms of its chemical characteristics, it is higher in carbon and lower in both hydrogen and oxygen in comparison to the original feedstock; that would be the evidence of both dehydration and decarboxylation (M. Child, 2014). As mentioned before in section, each biomass has different characteristics related to its elemental contents. These characteristics may represent in heat value result and solid yield production at the end product of HTC of the biomass. Therefore different substrates produce different solid yield per percent of dry matter, due to the condition of the process. It has shown and compared in table 5 that different results have been derived from different kind of sources of biomass under different set of temperature and time HTC condition. According to the same table hydrogen content in char from lignin tends to be lower, and higher carbon content in char produced from cellulose has been expected (A. Funke and F. Ziegler, 2010). 23

25 Table 5. Results from different biomass hydrothermal carbonization experiments (Robbiani, 2013) HTC produces liquor rich in dissolved organic components in solid phase, which primarily include volatile fatty acids (VOCs), such as formic acid and acetic acid, also phenols, furfurals, and their derivatives in different concentrations (Reza, et al., 2014). Researchers believe that a significant amount of solid particles cannot be recovered from liquid fraction; therefore the term total organic carbon (TOC) has been used to represent for those particle fractions. Parts of hydro-char, which are soluble in benzol-alcohol mixtures, alkaline solutions and ammonia, show a similar characteristic to natural coal, however one significant effect of HTC process is the elimination of hydroxyl and carboxyl groups, which makes a difference between char and natural coal (A. Funke and F. Ziegler, 2010) Liquid phase Roles of water in HTC process are known as a medium of heat transfer, solvent, reactant and also the product itself. During hydrolysis, when degradation of carbohydrates and proteins occur, large amounts of water are consumed, but also during subsequent dehydration reactions, large amounts of liquid water is formed, which referred to as dewatering (M. Child, 2014). Rather significant loads of various organic and inorganic compounds present in the liquid phase, is a result of the involvement of water in HTC. These compounds such as organic acids, sugars and the derivatives of both sugars and lignin compounds have been regarded as undesired side-products, however, researchers refer to liquid phase as valuable product that have to be recovered; otherwise it would be a potential loss (M. Child, 2014; A. Funke and F. Ziegler, 2010). As mentioned, the amount of different materials and particles found in liquid phase, denoted as Total Organic Carbon (TOC); however, it has been reported that the wastewater of HTC can be efficiently treated by typical aerobic and anaerobic methods (M. Child, 2014; A. Funke & F. Ziegler, 2010; H. Ramke et al., 2009). 24

26 The organic products found in the liquid phase can be used in anaerobic degradation so as to get methane out of this process. Since acidic and warm condition speed up HTC reactions, recirculation of process liquid products, which have these two qualities, is a benefit. Subsequently, recirculation of processed water would be a benefit to improve solid product yields, carbon levels within the solid product, dewatering properties of the solid product and HHVs. Water recirculation has been a major issue when it comes to the industrial scale regarding to reduction and the cost of waste water treatment, however, existence of heavy metal have to be considered as well (M. Child, 2014) Process condition, temperature and residence time influence also the yield of liquid production, since dehydration and decarboxylation are temperature sensitive. There is increased water formation by dehydration at higher temperatures and longer residence times, as well as higher acetic acid formation (M. Child, 2014) Gases The gas formed during HTC consists mainly of CO2 due to the process of decarboxylation. The CO2 concentration in the gas lays between 70 90% depending on substrate and severity of reaction (H. Ramke et al., 2009). Besides, CO, CH4 and H2 and other gaseous hydrocarbons present in minor fraction (M. Child, 2014). Rising temperature and also increasing decarboxylation reaction during HTC increase gaseous yield. This process is significantly important as it results in having lower ratio of O/C in solid products, thereby, reaching to a higher heating value, although by more CO2 production it seems more loss in carbon content; it should be noted that carbon loss through this process is low. Also it is stated that as gaseous yields increase, higher yields of both CH4 and H2 are reported along with a reduction of CO yield. Outcome of this process also would be a higher heating value as H/C ratios of solid products decrease accordingly. Accordingly, it is reported that CO2 measurement may use as a parameter to give information concerning reaction progress (M. Child, 2014; A. Funke & F. Ziegler, 2010) Influence of Process Parameters Parameters that can affect hydro-products compositions have to be considered so as to have a high quality of hydro-products. Accordingly, temperature, pressure, ph, solid load, along with reaction time and the nature of the feedstock, are the main factors that influence the hydro-char composition (M. Tripathi et al, 2015; Robbiani, 2013). 25

27 Temperature The effect of temperature in hydrothermal carbonization is well known and clearly is reported in researches on hydrothermal processes. According to Robbiani (2013), this process parameter has the biggest influence on characteristics of the products. The role of temperature on every reaction such as hydrolysis and polymerization of different substrates, say glucose, is to ease the degradation process; as it is reported by Funke (2010), dehydration of glucose may take several seconds at 270 C up to several hours at 150 C. Temperature also has an influence on the number of biomass compounds that can be hydrolyzed (Robbiani, 2013); as Funke (2010) reported hemicellulose is almost completely hydrolyzed at around 180 C, the most parts of lignin is degraded around 200 C and cellulose significantly hydrolyzed above 220 C (A. Funke and F. Ziegler, 2010). Accordingly, it seems that high temperatures cause higher rates of reactions. Therefore, both high temperature and longer residence time ( ) increase reaction severity, which lead to higher carbon content hydro-char products (Robbiani, 2013). Considering water loss and water formation during hydrolysis and dehydration processes, it is reported that by increasing temperature the overall amount of water formation tends to happen, however at temperatures of 200 C, a small loss of water was noticed (M. Child, 2014). Results of a research on pyrolysis showed a negative effect of rising temperature on the hydro-char yield, on the other hand caused a degree of increase in gaseous and liquid phase (M. Tripathi et al., 2015) Residence Time Generally, the process of hydrothermal carbonization of biomass is a slow reaction. In different researches a huge range of residence time have been reported, between some hours to several days. In several experiments, it has been observed that a longer residence time increases reaction severity, and consequently increases the yield of HTC-char, however it could be the opposite in other cases. This increase could be the result of polymerization of fragments in liquid phase, which during a longer reaction time leads to precipitate insoluble solids, so-called HTC-char. In experiments with shorter residence time a higher heating value of char is expected (Robbiani, 2013; A. Funke and F. Ziegler, 2010). Therefore, for higher hydro-char production, more sufficient time is needed for biomass components to react. More vapor residence time allows components to re-polymerize completely and it ends not only to more hydro-char yield production but also to a higher quality product (M. Tripathi et al, 2015). By re-polymerization, the organic loss is reduced in wastewater; therefore, recirculation of the 26

28 process water is suggested as an economical way of increasing the residence time (Robbiani, 2013) Pressure Pressure is also considered as a parameter, which influences carbon concentration in hydrochar. Rise of pressure is a matter of rise of temperature and when the temperature reaches to 100 C and higher, the equilibrium happens, which is the saturated pressure of water. In this case, further evaporation rate is as much as the rate of condensation of water vapor. In a pressurized system containing biomass at the same temperature, the system would experience a higher pressure than saturated vapor pressure due to the formation of gases (Robbiani, 2013). In hydrothermal processes a pressure higher than the ambient pressure is needed. The higher the pressure the higher is the carbon concentration in the product; therefore, energy density or energy per unit volume of the hydro-char rises by the rise of carbon concentration as a result of high pressure. As pressure is rising, it is expected that dehydration and decarboxylation be depressed, but also it is expected, that it has a low impact on hydrothermal carbonization (M. Tripathi et al, 2015). Pressure is also a parameter to ease the removal of extractables from biomass and is a parameter to be considered as a compaction factor. Compaction affects physical characteristics, as can be done by chemical reactions, condensation and polymerization, trough hydrogen ion exchange, which is facilitated by higher pressure. It is believed that by compaction, the biomass/water ratio would be higher; therefore the rate of reactions in hydrothermal carbonization process may enhance indirectly (A. Funke and F. Ziegler, 2010) ph During hydrothermal carbonization process, different kind of organic acid may be formed, which cause a drop in ph; however a neutral to weakly acidic environment seems to be needed to reach a recreation of natural coalification. Formation of different acidic materials will act as reaction catalyzers, to improve reactions such as hydrolysis of cellulose, although at the end a higher ph value will be needed especially in the liquefaction use. As a conclusion, weakly acidic conditions enhance the overall rate of reactions of hydrothermal carbonization and increase the carbon content in the hydro-char (Z. Robbiani, 2013; A. Funke and F. Ziegler, 2010); though the effect of acidic condition to some reactions such as polymerization and condensation is still unknown (A. Funke and F. Ziegler, 2010). 27

29 Solid load, particle size and combination of feedstock As it is mentioned, a change in the ratio of biomass to water would change the reaction rate significantly. Since, progress in hydrothermal carbonization would raise solid load through evaporation, it would recover large parts of dissolved organic fraction as solid material; thereby, with a low solid load very low fraction of residues may remain. In order to have higher amount of hydro-char, the solid load should be as high as possible with a good mixture with water (Z. Robbiani, 2013). If the residence time is too short, it seems necessary to have a higher concentration of e.g. glucose monomers in the liquid phase, which easily increases the chance of polymerization reaction to reach larger fraction of solid precipitation. However, when the residence time is long enough to reach solution equilibrium, a higher biomass/water ratio is not that necessary (Z. Robbiani, 2013; M. Tripathi et al, 2015) Particle size of the input materials also influences the heat flow in to the core of the materials; most of the studies on this factor showed that bigger size of feedstock increase the distribution of the heat from the surface in to the core of the materials. A good transfer of heat makes a better decomposition and hydro-char formation (M. Tripathi et al, 2015). According to the study made by Oliveira et. al., 2013, combination of different agricultural residues improve carbonization and the quality of hydro-char comparable to brown coal and increase the mass yield of solid product, which could be used to complement biogas plants. According to the results the mass balance showed minimum to maximum of 94-98% carbon recovery. Regarding the carbon content and the yield of the hydro-chars, combination of separated digestrate (SD) with bedding material (BM), SD with dry straw (DS) resulted in a hydro-char yield of more than 65% on a dry matter basis. However using the same materials separately resulted in hydro-char yield lower than 60%. With increase amount of corn silage (CS) hydro-char yield decreases, however CS in combination with BM had opposite results. Also different combination of residues showed different results in carbon content at the end (I. Oliveira et al., 2013). 2.3 Use of HTC hydro-char Solid product of hydrothermal process has variety of potentials and advantages that make it practical in different fields of studies, industries and markets. In a study by J. A Libra et. al., it is introduced several applications for carbon materials in the fuel cell field, such as a solid phase for hydrogen storage, adsorbent, ion exchangers, as catalysts in low temperature fuel cells, or as the fuel itself, which increase the efficiency of the fuel cell. Other applications 28

30 (figure 6) have been named as soil fertilizer; soil amelioration, energy storage and also it could be applied for carbon sequestration (J. A Libra et al., 2011). Management of organic wastes is also known as one of the advantages by means of mitigation of climate change, reducing methane emissions from landfill, reducing industrial energy use and emissions released from recycling and waste reduction, regaining energy from waste, improving carbon sequestration in forests due to decreased demand for paper; and decreasing energy used in long-distance transport of waste (J. Lehmann and S. Joseph, 2009). Figure 6. Examples of the Use of HTC Hydro-char Soil Amendment Intensive use of agrochemicals caused loss in soil productivity with adverse environmental impact on soil and water resources in many regions. Hydro-char, which is produced from locally available and renewable materials in a sustainable way, provides a unique opportunity to improve soil fertility and nutrient-use efficiency. Also, farmers are able to convert organic residues and biomass fuels easily into hydro-char. In other words, farmers can have this opportunity that the residues of their farms recycled to a valuable fertilizer. Therefore, hydrochar is able to play a major role for sustainable soil management not only to improve soil productivity but also to reduce environmental impact on soil and water resources (J. Lehmann and S. Joseph, 2009). Carbon and nitrogen cycling is one of the most important factors in the soil that seems to be the role of microorganisms. To improve soil condition the soil ecosystem has to be changed in a way that improves microbial communities. Hydro-char is one of the substances, that can change the soil ecosystem and also microorganisms function abilities. Hydro-char used in 29

31 different researches showed different effects on root colonization of microorganisms (MOs) and on soil amendment due to the feedstock they have been derived (table 6) and also regarding to the production conditions (M. Reza and Sun, 2013; S. Steinbeiss et al. 2009). Table 6. Effects of certain hydro-chars on different MOs (F. Haiboeck, 2015; S. Steinbeiss et al., 2009) Carbon Sequestration As mentioned, hydro-char can be a powerful tool to be used against climate change. As organic materials rotting, greenhouse gases, such as CO2 and methane are released into the atmosphere. By carbonizing the organic material, much of the carbon converts into a fixed and more stable form. Adding bio-char in to the soil increases the rate of carbon sequestration. It means that the rate of decomposition of nutrients from soil is slowed down and the carbon is effectively isolated and therefore sequestered, and thus, improves soil quality. Being more precise, bio-char changes carbon from the rapid biological cycle into a slower bio-char cycle with a more stable characteristic. Accordingly, it is believed that by applying this method to keep carbon remain in soil, current global carbon emission is reduced 10 percent (J. Lehmann and S. Joseph, 2009; J. Hunt et. Al., 2010, M. Tripathi et al, 2015). More studies needed to be done to prove the potential of bio-char sequestration such as a simple comparison between global carbon fluxes. It is expressed in recent studies that almost four times more organic carbon is stored in soils than in atmospheric CO2; and the entire atmospheric CO2 has cycled once every 14 years. Furthermore, due to the annual uptake of CO2 by plants, large amounts of CO2 are cycling between atmosphere and plants (figure7), therefore, diverting only a small quantity of this large amount of cycling carbon into bio-char, would make a large difference to atmospheric CO2 concentrations (J. Lehmann and S. Joseph, 2009) Activated Carbon Adsorbents One of the applications of hydro-char is being used as an activated carbon adsorbent. During HTC processes sorption capacity of hydro-char increases, and it can be used to remove 30

32 a variety of organic and inorganic contaminants from water in wastewater treatment process aimed at laboratory uses or drinking water purposes (J. A Libra et. al., 2011). Hydro-char need to be activated with activation steps during hydrothermal carbonization; this method increases the surface area and pore size. Two different methods are introduced in J. A Libra article as physical and chemical activation. Physical activation is normally applied with the presents of activation agents such as CO2 or steam. Chemical activation is also being done with chemical activating agents such as potassium salts Other Applications Nanostructured carbon materials can also be generated from HTC process through different changing temperature, time and feedstock, to reach different size, structure and functionality of the hydro-char (J. A Libra et. al., 2011). Since carbon materials are stable at high or elevated temperatures and also against harsh reaction conditions, they can be used as catalyst supports or as catalysts on their own (J. A Libra et. al., 2011). Furthermore, coal-like particles produced with HTC have other important applications such as hydrogen storage, electrochemical energy storage with lithium-ion batteries or super capacitors or can be used as feed material for fuel cells (J. A Libra, et al., 2011; Z. Robbiani, 2013) 2.4 HTC Reactors In this section three types of main reactors that are used normally in chemical applications are summarized in Table 7 with some brief characterizations at section Further, two more small-scale reactors which used for the hydrothermal carbonization of biomass reviewed in pervious works at the Institute of Chemical and Eenergy Engineering, University of Natural Resources and Life Sciences, Vienna (BOKU), are explained. More details of this last table are available in index Batch Reactor and Continuous Reactor Design of Batch Reactor, Continuous Stirred Tank Reactor (CSTR) reactor, and Plug Flow Reactor (PFR) was depend on the mode of operation (Nanda, 2008). Some of their characters, principles and their area of applications are shown in table 7. Some more examples of reactors in different studies are mentioned as well in the following table 8. 31

33 Table 7. Comparison of Chemical Reactors (Nanda, 2008) Type of Reactor Batch Reactor Continuous Stirred Tank Reactor (CSTR) Plug Flow Reactor (PFR) Principle of work All reactants are added at the commencement and the product withdrawn at the completion of the reaction. They are conducted in tanks attached with impellers, gas bubbles or pumps. One or more fluid reagents are introduced into a tank reactor equipped with an impeller while the reactor effluent is recovered. A stepped up concentration gradient exists One or more fluid reagents are pumped through a pipe or tube. These are characterized by continuous gradients of concentration in the direction of flow Advantages Suitable for small scale production Suitable for processes where a range of different products or grades is to be produced in the same equipment Suitable for reactions requiring long reaction times Suitable for reactions with superior selectivity Highly flexible device By products may be removed in between the reaction It is economically beneficial to operate several CSTRs in series or in parallel. Reaction can be carried out in horizontal as well as vertical reactors Higher efficiency than a CSTR of the same volume PFRs may have several pipes or tubes in parallel Both horizontal and vertical operations are common They can be jacketed Reagents may be introduced at locations even other than inlet 32 Limitations Area of application Not suitable for large batch sizes It is a closed system in which once the reactants are added in the reactor, they will come out as products only after the completion of the reaction Batch processes are used in chemical (inks, dyes, polymers) and food industry More complex and expensive than tubular units All calculations performed with CSTRs assume perfect mixing At steady state, the flow rate in must equal the flow rate out, otherwise the tank will overflow or go empty Chemical industry especially involving liquid/gas reactions Not economical for small batches The tubular reactor is specially suited to cases needing considerable heat transfer, where high pressures and very high or very low temperatures occur

34 Table 8. example of some HTC reactors in different studies Reactor Two-chamber reactor Study HTC of Lignocellulosic biomass Reference Reza, 2011 HTC of biomass for Batch reactor energy and crop Reza, 2014 production 160-ml stainless steel HTC of Municipal tubular reactor Waste Stream Grenolmatik ZHAW Autoclave ZHAW Diving Bottle TFC engineering, Buchs AVA-CO2 HTC of bio-waste/fecal sludge HTC of bio-waste/fecal sludge HTC of bio-waste/fecal sludge HTC of bio-waste/fecal sludge HTC of bio-waste/fecal sludge Umwelt Campus HTC of bio-waste/fecal Birkenfeld sludge Cube of Destiny Agrokraft HTC of bio-waste/fecal sludge HTC of bio-waste/fecal sludge 40-ml stainless steel Set up and process reactor (Optical cell) control N. D. Berge, 2011 Z. Robbiani, 2013 Z. Robbiani, 2013 Z. Robbiani, 2013 Z. Robbiani, 2013 Z. Robbiani, 2013 Z. Robbiani, 2013 Z. Robbiani, 2013 Z. Robbiani, 2013 F. Haiböck, 2015 Study of Parameter 1.7-l self-built reactor variation during C. Preinknoll, 2013 biomass HTC 1 L lab scale reactor (Hanwoul Eng. CO., HTC of Cellulose LTD., Gunpo, Korea) 33 D. Kim et al., 2015

35 Batch and continuous reactor Waste sludge materials M. Child, Reactors at BOKU University Optical Cell So far hydrothermal carbonization process has been operated with an optical cell, which is available in University of Natural Resources and Life Sciences (BOKU) in Vienna. This optical cell has been introduced in a bachelor thesis under the title of Set up and process control of an optical cell used for hydrothermal carbonization (F. Haiböck, 2015). The general idea of building up this optical cell was to allow the experimenter to have eyesight inside the cell through a sight glass so as to monitor the physical changes during chemical processes of hydrothermal carbonization. The optical cell is an aluminum cube in size of 151*151*118 mm with copper pipes (figure 7) and a stainless steel cylinder-like container with 44ml volume capacity (figure 8) located inside the cube and fixed with screws. Tubes, screws and the glasses used in the cell have to stand temperature up to 200 C and 200 bar pressure (F. Haiböck, 2015). Figure 7. Aluminum cube with copper pipes Figure Figure 8. Stainless steel cylinder-like container (F. Haiböck, 2015) In order to heat up the cell a Haake N6 temperature controller is used connected to a container with a pump that helps streaming the hot non-hazardous mineral oil through the flexible tubes to the optical cell cube (F. Haiböck, 2015). After doing a couple of pre-tests in 34

36 May 2016, tubes show leakage problems. Therefore, it was decided to heat up the cell electrically. A hardware combined with an open source software program called Arduino (figure 9) recorded time, temperature data every 10 seconds during the HTC process. A wire connector from the hardware attached to the body of the cell, transferred heat data to the software that has installed on a computer. Figure 9. Arduino hardware (F. Haiböck, 2015) Self-built Reactor The reactor was built by stainless steel material in a cylindrical shape. The reactor had about 1.5 L capacities. Reactor could have sealed with 12 screws and a washer. During each test the reactor is placed in a metal box isolated with wools all around it, plugged into the power and attached temperature and pressure sensors to the sensors coming from a power machine, which is manufactured by B&R company ( The power system programed to read temperature and pressure from reactor, and the desired temperature is also set on the system to control the heat of the reactor. Heating power safety can be set to a desirable percentage. And also to prevent over shooting, there is another temperature option to be set to max desired temp. Therefore, at this point the heating power is reduced 10% automatically for threshold safety. More safety options, operated information and data saving are presented at section. 35

37 3. Materials and Methodology 3.1 Materials Three biomass substrates were selected to apply for the present project run in the laboratory of the Institute of Chemical and Energy Engineering, BOKU University. Substrates were two agricultural Residues (AR) and sludge (SD) from a biogas plant. Materials used in this work were collected from a farm-based biogas plant located in the town of Utzenaich, Austria (figure 10). The two agricultural residues were: barley silage (BS) and maize silage (MS). Input raw materials used in Utzenaich biogas plant as the source for sludge were barley silage, maize silage, maize straw, and rape straw. Figure 10. Biogas plant. Utzenaich, Austria Therefore, every substrate was representatively sampled and transported in closed barrels of 1 liter; five buckets filled with sludge, plus four plastic bags with barley and maize silage. Chemical characters of raw materials are presented in table 9. Table 9. Elemental analysis data and calorific value of raw materials a Dry matter C H N S (% DM) (% DM) (% DM) (% DM) b HHV Substrates Ab Sludge SD Barley Silage BS Maize Silage MS (% FM) a Abbreviations for each substrate b Higher heating value Note: the value shown for each parameter is a mean value of a triple test 36 (MJ/Kg)

38 3.2 Methodology In general, the processes that have been done in the present study were first, drying each material in an oven, following with hydrothermal carbonization of each mixture, and then milling (figure 12) and preparing hydro-char products for elemental analysis, calorific value determination, and germination test. The following table (table 10) summarizes different measurements and devices used to analyse hydro-chars. Samples were taken from solids after each experiment. Table 10. Summary of Analytical Methods Hydro-char quality assessment test Unit Description Measurement of hydro- Calculating dry matter FM % chars solid fraction and Measuring instrument Memmert Universal drying Oven UNE400 raw substrates Solids Measuring the higher Calorific value MJ/Kg heating value of the IKA C200 Oxygen Bomb dried hydro-char and of calorimeter raw substrates Elementary analysis (C, H, N, S) DM % Measuring C, H, N and Eurovector EA 3000 S of the dried Hydro- CHNS-O Elemental char and raw Analyzer substrates Hydrothermal Carbonization Experimental Procedure An experimental design of the substrates mixtures presented in table 11 shows how many percentage of a substrate was applied in which test. So, totally eight tests were done started with applying 100% of each substrate. Then, fermented sludge considered as the main waste stream that needs to be managed; therefore four tests were done with different mixture of sludge and agricultural residues. Hence, in test 4, 20% of sludge and 80% of Barley silage, and in test 5, 50% of each was prepared to use for HTC. The same portions were applied for the mixture of sludge and maize silage in tests 6 and 7. In the last experiment, T8, a combination of one third of each substrate was used. 37

39 Table 11. Experimental Design of the conditions and Substrates Mixture HTC Temperature Time Substrates Tests ( C) (Hours) (%) SD BS MS T n n T n 100 n T n n 100 T n T n T n 80 T n 50 T n = not used Abbreviations that are used trough out the following sections summarized with a brief explanations in table 12. Table 12. Methodology Abbreviations Abbreviations Substrates Hydro-chars T1 = 100% Sludge T1f = Sludge (SD) T1h = Hydro-char of T1 T2 = 100% Barley T2f = Barley (BS) T2h = Hydro-char of T2 T3 = 100% Maize T3f = Maize (MS) T3h = Hydro-char of T3 T4 = 80% Barley+20% Sludge T4f = calculated fresh feed for T4 T4h = Hydro-char of T4 T5 = 50% Barley+50% Sludge T5f = calculated fresh feed for T5 T5h = Hydro-char of T5 T6 = 80% Maize+20% Sludge T6f = calculated fresh feed for T6 T6h = Hydro-char of T6 T7 = 50% Maize+50% Sludge T7f = calculated fresh feed for T7 T7h = Hydro-char of T7 T8 = 34% Barley+ 33%Maize+ T8f = calculated fresh feed for T8 T8h = Hydro-char of T8 HTC Tests 33%Sludge Note: Every item that is not mentioned as calculated, are really measured To manage the time, everything got prepared the day before, in which the test was going to be run. First step was to calculate the amount of extra water that was needed to add to each material for each test. Therefore, table 13, was prepared to present the moisture content of each material; the drying procedure has been done under the same methodology for each substrate, in which substrates were dried in a drying chamber at 102 C for 24 hours. Moisture content was computed by weighing samples before and after drying and then by subtracting these two 38

40 values. Values for moisture content and dry matter are presented in percentage in table 13. Table 13. Substrates moisture content calculation and drying condition Substrates Temperature Duration Calculation of moisture content (h) Weight (g) Before After drying drying Weight Dry Moisture difference matter content % % T1f T2f T3f The value of moisture content is one of the parameters that improve reactions in HTC process. Accordingly, before starting each HTC test, the value of moisture content was ready for the feedstock, which was necessary to calculate the extra water needed. Biomass ratio that was considered for each experiment was 80% moisture and 20% solid. Thus, it represents that there was no need to add water to fermented sludge in the first test (T1), since 89% of it was liquid and enough to run the reactor for a HTC test. Second HTC test (T2) has been done using 100% of barley. Since the moisture content was 51%, to have 80% moisture for the test, 435ml water added to 300 grams of barley. Every detail of calculation for material preparation was the same in each test, which are gathered in index 3. Barley and maize silage were used in the shape and size as their real size that is why grams of their solid phase were chosen differently according to the capacity that they occupied in the reactor HTC reactor set-up The hydrothermal carbonization experiments were carried out with an experimental HTC reactor used for research purposes, which located at BOKU, Department of Material Sciences and Process Engineering Institute of Chemical and Energy Engineering. 39

41 Figure Liters self-built reactor used for HTC experiments at Chemical and Energy Engineering Institute, BOKU University The reactor (figure 11) had about 1.5 L capacities. In order to use the reactor under a safe condition and to run the test as proper as it should be done, some safety points, had to be followed. Two third of the reactor has to be filled to have also a free space so as to avoid too much of pressure during HTC process. Therefore, totally one liter of materials, includes solid and liquid phase, were needed for each test, while this method changed in some tests considering the volume of silage. After fixing the reactor with its 12 screws manually with no more extra forces like using drill, a bit of nitrogen gas was blew over a tube to the reactor connected from temperature sensor in order to eliminate oxygen. In this case, every time the temperature sensor had to be removed and nitrogen tube was attached for a while. During each HTC test, the reactor has been placed in a metal box isolated with glass wool all around the reactor, plugged into the power and attached temperature and pressure sensors to the sensors coming from a power machine, which is manufactured by B&R company ( The power system programed to read temperature and pressure from reactor, and the desired temperature was also set on the system to control the heat of the reactor up to the 200 C. Heating power safety was set to 80%, and also to prevent over shooting, there was another temperature option to be set to 180 C; therefore, at this point the heating power is reduced 10% automatically for threshold safety. Inner pressure and temperature are recorded during each experiment. Temperature data is collected from the centre of the reactor by a temperature sensor. Data could be saved on a memory stick directly right from the power system or saved on a computer with the help of Automation Studio software, which was connected to the main power system and could read data from the power machine. Automation Studio software is linked to a program named OPC- 40

42 HTC.vi, which is encoded with Lab View software. OPC-HTC is encoded to show data (temperature, pressure and time) and at the same time can be used to save data on the computer as an excel file. Then the metal box was covered and fixed by four screws. The over pressure valve was also covered and fixed to the box for more safety in case of any trouble shooting and breaks. For power safety also there was a wire, which has to be connected to somewhere to the box or reactor so as to have a connection to the ground. By pressing a button on the power machine, system starts to heat the reactor. It takes about 10 minutes that the temperature reaches 100 C; at this point, the pressure valve was opened in every test, to get rid of gases and be sure of oxygen elimination. The starting point was when the temperature exceeded to the 180 C, and then after 6 hours, the system was shut down and was prepared for the next step. Condition and parameters of HTC process was constantly the same in all 8 HTC tests (table 11; Figure 18). The next step was cooling down the reactor. There are two ways of cooling down the reactor; one is easily leave the system to be cooled over night, or using cooling system. Cooling system is kind of a water-pumping machine, which can be simply connected by tubes to a pipe, attached to the body of the reactor. Pressurized cold water flows over the pipe, thus the reactor starts to be cooled down. After about 45 minutes, reactor reaches to a proper pressure and temperature, so as to be able to handle and carry on for the next steps. In this study the cooling system was used in most experiments for avoiding time consumption. Products of the HTC test had to be separated as solid and liquid phase, and be weighed right after emptying the reactor. Solid part was dried for 24 hours in an oven heated up to 102 C. Then again the dried matter was weighed to calculate water and solid fraction of the output product. Figure 12. Milling device used to prepare samples for Elemental Analysis, Calorimetric Test and Germination test, at Waste Management Institute, BOKU University 41

43 A simple test also has been done on the liquid phase in two steps of drying, in order to determine volatiles and remaining ash. Knowing volatile fraction of a liquid or HTC recycled water helps to know its heat potential and its overall usefulness. First step was simply drying 100 ml of samples at 102 C for 24 hours and then 550 C for one hour for the next step. Before and after each step, samples were weighed (index 2) Analytical methods Elemental Analysis One gram of each milled-sample of Hydro-chars, plus the raw materials themselves were sent to the Microanalytic Laboratory, Faculty of Chemistry at Vienna University for a double elemental analysis. Labeled samples have to be packed in 3 ml Eppendorf vials. According to the standard method description (J. Theiner, 2014), C/H/N/S analysis is performed by using a Eurovector EA 3000 CHNS-O Elemental Analyzer. For each individual analysis run, between 0.75 and 3.0 mg of a sample are taken. Mineralization of the sample material is done by flash combustion applying 25 kpa oxygen at 1,000 C. Results of the analysis (table 15) and discussion are presented in chapter Calorific value Calorific value and germination tests were also applied on the same samples. The value of heat of combustion for each sample was tested and calculated by a bomb calorimeter device (figure 13) at the Institute of Agricultural Engineering, Department of Sustainable Agriculture, University of Natural Resources and Applied Life Sciences located in Tulln, Austria. This test has been done twice on each sample, including raw materials and hydro-chars. Approximately one gram of each sample had to be prepared in shape of a pellet to be easily placed in a small glass sample cup, which then was located in to a cylindrical container so-called vessel bomb (figure 14). A 50J cotton string also was placed into the glass cup in contact to the sample pellet. After adding 1ml of water to the vessel, its door was sealed and the pressure of the vessel bomb was fixed to 30bar and was placed into the water container. The device was filled with water up to the marked zone, and then the rest of the process was automatically run after giving the actual weight of the pellet to the device. It takes about 15 minutes that the final result appears on the screen in the form of J/g (table 17, index 5). However, unit of MJ/kg was used in calculations. 42

44 Figure 13. IKA C200 Oxygen Bomb Calorimeters, Pelleting Press and Pressure Device a. b. Photo taken by Samar Seyedsadr, October 2016 Figure 14. a. Schematic view of inside a Bomb Calorimeter b. Bomb Cell, 50J cotton strings, and other accessories For percentage change calculation (details in chapter 4) between the measured C and HHV contents of the mixtures and their calculated contents from mono substrates, two steps were followed: First: compute the difference between measured and calculated values. Difference = measured value calculated value Second: divide the Difference by the calculated value and multiply the result by

45 Percentage Change = Difference calculated value Germination Potential test In order to examine the germination potential of hydro-chars and the effects that hydrochars may have on the soil structure, leachate and the volatile effect on the seed germination, a method from Buss and Masek (2014) paper gave an inspiration to be followed in the present study. By this means, all 8 hydro-chars and 3 raw materials were tested. Materials needed for germination test were some cress seeds, sand in the size of mm, small pots and boxes, and filter papers. This germination test was run in triple, at the green house of AIT institute in Tulln. In each boxes one sample was examined in three phases. First phase was a small pot with some holes filled with a mixture of 40 grams of sand and 1 mg of the sample; second phase included seeds on a pot to examine volatile effect placed above the first pot; and the last phase was for examining leachate effect, in which seeds where placed on a filter paper on the lowest level of the box, as it shown in the Figure seeds were applied for each phase in a box, in a total of 90 seeds. Each 33 boxes covered and isolated with a plastic cover (figure 16). After 72 hours the germinations were counted. In order to make comparison a bit more simple, the germinations categorized in four groups, well-germinated; early stage; short root and no germination (figure17). a. b. Figure 15. Shows the order of three phases in a box, an example of before (a) and after (b) germination test 44

46 Figure 16. Each box covered and isolated with a plastic cover Figure 17. Early stage and short rooted germination Results and analysis of germination test are presented at the section 4.3 and more details summarized in index 6. Statistical analysis was carried out for data obtained from germination tests using STATISTICA Software Version 7.1. Data were analysed by one-way ANOVA, followed by Duncan Test for post hoc comparison. The level of significance was set at p< RESULTS AND DISCUSSION Elemental analysis and calorific test were performed more than one time; therefore, after being sure that there is non-significant differences between each set of results by conducting standard deviation, an average was calculated between doubled and tripled results. The mean of the results are available in tables located in the related sections and a complete version of data are presented in appendix. It should be noted that in each comparison that O/C; H/C; carbon and HHV of fresh materials were needed, it was calculated only for combinations (T4fT8f) since these values was measured for SD, BS and MS (T1f-T3f). 4.1 Process Parameters The experiments carried out in accordance with several published standards for hydrothermal carbonization; namely a temperature of 200 C in reaction-time period of 6 hours. This situation is believed to be the optimal condition for HTC tests. As all tests have been done at the same condition, graph 18 is drawn to show the process parameters (internal 45

47 pressures-vessel and temperatures) over the reaction-time period of all tests. The rise in internal vessel pressure illustrated here indicates the magnitude of the gaseous reactions products (dominantly CO2) Temprature Pressure 4 Pressure [bar] Temprature [C ] Time [hour] Figure 18. Temperature and Pressure Condition of HTC Process over time Therefore, rising pressure along with rising temperature represents that there was no serious dilemma or any leakage in the system during HTC test, thus the condition worked properly accurate. 4.2 Mass Balance Products from HTC are in a solid, liquid and gaseous form. Products are collected in two different phases after HTC, in forms of liquid and hydro-char. Output recovery (FM%) is presented in table 14. The losses can be explained as gas phase, spill- or droplet losses when emptying the reactor or during separation of the output materials. Table 14. Mass Balance Output Recovery HTC Tests FM% T T

48 T T T T T T Characteristics of Hydro-char The main characteristic of hydro-chars are that they have higher C content and lower H/C ratios than the raw materials used as inputs. This is the result of the dehydration and decarboxylation processes during HTC. The following table 15 shows the characteristics of hydro-chars due to their composition of C; H; N and S. Oxygen percentage calculated due to this assumption that 100% of each substrates comprise of O; C; H; N and S. Figure 19 shows a comparison of these compositions between raw materials and hydro-chars. Table 15. Elementary compositions of raw materials and hydro-chars C H N S O Substrates (% DM) (% DM) (% DM) (% DM) (% DM) T1f T2f T3f T1h T2h T3h T4h T5h T6h T7h T8h According to the figure 20, which represents high carbon content in hydro-chars (T1h-T8h), it is clearly interpreted that HTC was significantly raised carbon content intensity in the products compare to the raw materials. A low ratio of O/C was also expected, which is 47

49 obviously represented by figure 21 as well, if comparing hydro-chars and raw materials. Atomic H/C and O/C ratios are discussed furthermore in section Comparing carbon content of hydro-chars (T1h -T8h) shows that lowest value of carbon belongs to the HTC of 100% of sludge (T1h), and higher results also belong to T3h and T6h shows a high carbon density for hydro-char gained from MS, and the combination of SD and MS. 70 T1f T2f T3f T1h T2h T3h T4h T5h T6h T7h T8h Elemental Content [%] O C H Elements N S Figure 19. Measured Elemental Analysis And Calculated Oxygen For Raw Materials And Hydro-Chars Calculating the carbon content of raw materials with the same fraction in each test and comparing them with the carbon analysis of the hydro-chars results have shown a range of % (in average about 13%) carbon increase in most cases (figure 20). The lowest change was about 1% carbon growth in biogas sludge hydro-char (T1h). More details comparison of carbon concentration is discussed in next section. 48

50 70 60 C [%] Raw Material 30 Hydro-char T1 T2 T3 T4 T5 HTC Tests T6 T7 T8 Figure 20. A comparison between carbon content before and after HTC; raw materials calculated for T4f to T8f 4.2.2Discussion on hydro-char quality and fuel characteristic In the figure 21, the atomic hydrogen/carbon ratio is plotted against the atomic oxygen/carbon ratio. Graphs of these ratios represents that during each HTC test, H and O fractions significantly were reduced, due to two main reactions, dehydration and decarboxylation. It is clearly observable when values of raw materials are compared to hydro-chars results. By this means, every ratio was calculated with the same fraction of raw materials that were used in each HTC test and named as before HTC. Therefore, the comparison is done based on before HTC and after HTC results. The higher amount of H/C and O/C belongs to BS (0.37; 2.98) and MS (0.37; 3.15). This reduction of volatile matters in hydro-chars is the result of a good HTC process, which represents also high quality of hydro-chars. Generally, characteristics of those hydro-chars are more coal-like, which have low volatile matter content over high concentration of carbon, as it is the same for the hydro-chars in the present study. Interpretation of the graph absolutely proved what has been expected from hydro-char quality. As it is understandable from the models and equations for HHV estimation, there is a reverse correlation between amount of oxygen and energy content of biomass materials. However, a positive correlation between hydrogen and HHV is predictable. Moreover, even before analyzing and comparing HHV results from calorimeter, H/C and O/C ratios show an improved situation for hydro-chars, which resulted in higher energy content of the final products. 49

51 Reduction of oxygen and more intensity of carbon (36-45%) after HTC process are illustrated by a difference between light green graph (quality before HTC) in Figure 21 and dark green graph. Also a similar change (21-30% reduction) can be seen in H/C ratio (compare pink and red graph). Atomic Ratio of O/C and H/C T1 T2 T3 T4 T5 T6 T7 T8 O/C after H/C after O/C before H/C before Figure 21. A comparison between atomic H/C and O/C ratios; before and after HTC tests Figure 22 illustrates the difference of O/C and H/C ratios between hydro-chars and calculated ratios for raw materials. According to HHV models reduction of oxygen is a good sign of significant improvement because of negative correlation between amount of O and HHV. Figure 22 clearly shows a reduction of these ratios for each case after HTC and linear trending between the most significant change, which is raw (T6f) and hydro-char (T6h) of 80%maize, and the less trending (5% O/C; 14% H/C) between raw sludge and its hydro-char (T1h-T1f). Brown coal driven from ECN Phyllis2 Classification (table 16), which is marked by a dark triangular on figure 22, shows a bit lower H/C and O/C ratio in comparison to hydrochars produced in the present study. Table 16 Brown Coal elemental data from ECN Phyllis2 Classification Coal C% H% O% Brown Coal HHV (MJ/Kg) 25.67

52 Atomic H/C Ratio Atomic O/C Ratio T1h T1f T2h T2f T3h T3f T4h T4f T5h T5f T6h T6f T7h T7f T8h T8f Brown Coal Figure 22. Comparison between atomic H/C and O/C ratios of calculated fresh feeds (Tf) hydro-chars (Th) and brown coal In the graph 24, status of hydro-chars is compared to a van Kerevelen diagram driven from a thesis by A. Kauriinoja 2010 (figure 23). Figure 23. Van Kerevelen diagram (A. Kauriinoja, 2010); a composition of beech wood and torrefied beech wood 51

53 In figure 24, it is clear that coal has a better O/C and H/C ratio among biomass and hydrochars; but still hydro-chars ratios are comparable to brown coal and much more improved than biomass, representing the effective role of HTC. By blending figures 22 and 23, it is indicated that hydro-chars are fit to the peat area. 24. Comparison of Van Kerevelen diagram and hydro-chars Figure 25 is drawn based on the table 17 comparing substrates by their carbon content and HHV value. What is shown in figure 25 is exactly the expected result of HTC; carbon content in hydro-chars is significantly higher in comparison to the raw materials. As it is obvious from the graph, SD, BS, and MS (T1f-T3f) have much lesser carbon percentage than hydro-chars T1h, T2h and T3h. Comparing the value of T6h and T7h represents a better result for T6h when there is a higher amount of MS (80%) in combination with SD (20%) as inputs. Also a similar case is observable when comparing T4h and T5h (better result for T4h). This outcome shows that using agricultural residues in combination with sludge improves the hydro-char quality due to carbon content and HHV; more AR as inputs, better would be the calorific values and carbon content of hydro-chars. 52

54 Table 17. Calorific Value (Heat of Combustion) Resulted from Bomb Calorimeter device and Carbon content C HHV (% DM) (MJ/Kg) T1f T2f T3f T1h T2h T3h T4h T5h T6h T7h T8h Substrates Moreover, figure 25 generally shows a nearly linear correlation between carbon content and HHV. Higher HHVs belong to the hydro-chars with more density of carbon, as it is the case for T3h (23.107MJ/Kg), T2h ( MJ/Kg), T4h and T6h. Carbon content and HHV of hydro-chars exhibited a low difference with brown coal, except for T1h, which is hydro-char of 100% SD. 53

55 30 T1f 25 T2f HHV (MJ/Kg) T3f 20 T1h T2h 15 T3h T4h 10 T5h T6h 5 T7h T8h Brown Coal C (%) Figure 25. Correlation between C intensity and HHV and comparison between raw materials, hydro-chars and brown coal By comparing HHV of three raw materials (T1f, T2f, T3f) with the HHV of their hydrochars (T1h, T2h, T3h), significant increases in the values are clearly observable (figure 26). Also the same result for carbon content is noticeable in Figure 26. The highest values for HHV and carbon are highlighted as well. In both cases it is related to T3h. However, the change in T1h was very low, which is believed to be a result of lignin bounds remaining in fermented sludge. This indicates that temperature during HTC was not high enough to destroy lignin to its monomers. In order to understand if applying maize and barley had any influence on improvement of hydro-chars HHV, comparison between T4h and T6h values with T5h and T7h is needed. Consequently, in both cases, more use of MS and BS (80%) in mixture by SD (20%) came up with a better HHV results in comparison to T5h and T7h. Carbon content and HHV of hydro-chars in the present study was also compared to the results driven from I. Oliveira et al. (2013) study. Although the condition of HTC tests was not the same, the comparison is still worthy to be made. Carbon content between ranges of 41% to 60% is totally comparable to the carbon content that found in I. Oliveira et al. (2013) results (48-70%) for agricultural residues and the mixtures. Similar results also can be observed between HHV of both studies. 54

56 T1f T2f T3f T1h T2h T3h T4h T5h T6h T7h T8h 0 HHV C Substrates Figure 26. Carbon content and higher heating value correlation diagram 55 C [%] HHV - MJ/Kg HHV C

57 For a HHV comparison between before and after HTC samples, figure 27 was drawn. Blue columns from T4 to T8, which represent HHV amount for calculated raw materials, show lower values in all cases in comparison to hydro-chars (red columns). However, in T1 just about 13% increase is observable in contrast with the rest of the samples that 31-36% growth is illustrated. Figure 27. Comparing HHV between hydro-chars and calculated raw materials Table 18 designed to show the effect of different combinations of the tests T4 to T8. The percentage changes between the measured C and HHV contents of all mixtures and their calculated contents from mono substrate were calculated. Positive results for C and HHV changes emphasises that higher values found in the measured carbon and HHV in comprison to the expected calculated values. Therefore, since mixture of feedstock before HTC leads to a better hydro-chars value, this thechnique of applying HTC on different combination of input substrates is significantly reasonable. Table 18. The effect of different combination of maize, barley and sludge on carbon content and HHV of hydro-chars Hydro-chars Change in C (%) C (%) Calculated Measured 56 Change in HHV (MJ/Kg)

58 T4h T5h T6h T7h T8h The Following diagram (figure 28) exhibited the changes of C% comparing what is expected to the measured values C (%) Calculated 50 Measured T4h T5h T6h T7h T8h Hydro-chars Figure 28. Comparing effect of combination of inputs (measured) and combination of hydro-chars (calculated) on changes of C% 4.3 Seed Germination Discussion A non-significant difference between triple germination tests expressed that the results are trustable. Total germination rate (figure 29), which is over 93% in most cases, exhibited a good result that represents no phytotoxic effect of hydro-chars. Non-significant differences between germination rate in 3 phases of each test, also represents no negative influence of leachate and volatile effects. Although total germination rate was high enough, comparing this result between raw materials (T1f, T2f, T3f) and T1h, T2h, and T3h, showed about 1-2 % reduction in germination (figure 30). 57

59 96 95 Germination rate [%] SD BS MS T1 T2 T3 T4 T5 T6 T7 T8 Substrate Figure 29. Comparison of total germination rate Seeds in raw sludge had better results of 95% of total germination. The maximum of total well grown was about 81%, which was related to hydro-char of MS50%+SD50% (T7h). The number of early-stage germination was higher in raw sludge. Hydro-char of BS50%+SD50% (T5h) had the highest number for not-germinated seeds. Among seeds placed in sands and hydro-chars, the ones, which were more exposed to MS and BS, had better results as well as T8h (SD+MS+BS). Raw SD also had more early-stage germinations in sand + hydro-char phase (index 6). 58

60 93% 94% T1f T2f T3f 93% 94% T1h T2h T3h 93% 95% Figure 30. Effects of hydro-chars on seed Germination Second phase was to test volatile effect. Seeds in hydro-char MS100% (T3h) had the most not-germinated rate of seeds. In the last phase, so called leachate effect, T7 h (MS50%) had the most total rate of germination and T4h (BS80%) had the most number of not-germinated seeds (index 6). Numbers of germinated seeds counted in different phases for each germination test were presented in tables 19, 20 and 21. The highest numbers marked with a star (*) and the lowest results exhibited with two stars (**). Table. 19 Leachate-effect phase results Leachate Effect Number of germinated seeds T1f 30 * T2f 30 * T3f T1h T2h T3h T4h T5h T6h T7h T8h * Highest results ** Lowest results 59 ** **