Problems of Solid Biofuels made of Plant Biomass

Problems of Solid Biofuels made of Plant Biomass Michael Ioelovich *1 Designer Energy Ltd 2, Bergman Str., Rehovot, Israel *1 bd895892@zahav.net.il Abstract-The calorific values and density of the heat energy of some plant biomasses (wood, bagasse, corn stover, etc.) have been studied in order to evaluate their suitability as solid biofuels. It was found that the biomasses have a low density of the heat energy that hinders effective energy generation during combustion. To improve fuel properties of the plant materials, a pelletization technology was used. As a result of compacting of the powdered biomass samples together with binders, a considerable increase of the bulk density and density of the heat energy was observed. Co-granulation of a low-quality coal together with the powdered biomass results in the better energetic characteristics and reduced emission of the harmful sulphur dioxide. Novel fuel type - a dispersion of fine biomass powder in liquid fossil fuel has been developed. Introduction of the powdered biomass into the combustible liquid contributes to rise of the energetic potential of the dispersed fuel. An additional advantage of the developed fuel is the reduced emission of SO 2 and CO 2. Moreover, addition of the powdered biofuel permits save the non-renewable fossil fuel and reduce cost of the final dispersed fuel. Keywords- Biomass; Biofuel; Calorific value; Density of heat energy; Pelletization; Dispersed fuel I. INTRODUCTION As known, the non-renewable fossil resources are nowadays the main energy sources, which cover more than 80% of the world's energy needs [1]. A considerable attention in recent years is given to alternative energy sources, and especially to plant biomass as energy source, which in contrast to the fossil fuels is continuously renewed in the nature. Total resources of plant biomass in the nature reach 1.5 trillion tons [2]. Moreover, as a result of the photo-bio-synthesis, a mass of the plants increases approximately on 100 billion tons annually. However in fact, only small part of the synthesized biomass can be utilized. The share of the biomass-based energy has about 10% of the world energy consumption [3]. To generate the bioenergy, a non-edible plant biomass should be used in order to prevent the competition with food and feed industries [4]. This biomass type involves residues of agricultural plants (e.g. stalks, husks, cobs, etc.), forest residues (e.g. sawdust, twigs, shrubs, etc.), waste of wood, textile, pulp, paper and cities, as well as some plant species (e.g. Miscanthus, Switchgrass, Bermuda grass, etc.). Agriculture, forestry, pulp and paper industry, as well as cities create vast amounts of lignocellulosic residues. Moreover, huge amounts of algae are not utilized yet and can be used as appropriate source for energy production. The not-edible plant materials are related to abundant, renewable and inexpensive biomass type. Total amount of such biomass that is accumulated annually in the world is estimated in 10 billion tons at least. Only in USA annual accumulation of the biomass is about 1 billion tons [5]. Currently, main population of the world living in underdeveloped and developing countries, especially in rural areas, uses solid residues and wastes of the plant biomass (straw, bushes, twigs, firewood, sawdust, etc.) to meet their energy needs [6]. In many countries, these resources account for over 90% of household energy consumption. To generate the bioenergy, the non-edible biomass can be burned directly as a solid biofuel or after its conversion into secondary fuel: carbonized solid fuels (charcoal), liquid biofuels (bioethanol, biodiesel, bio-oil) or gases. However, carbonization, liquefaction and gasification processes are accompanied by a loss of the biomass and reducing in yield of the final biofuels. An additional problem is the compositional and energetic heterogeneity of the secondary fuels obtained as a result of the biomass conversion. Recent investigations have shown that the most efficient way for the energy production is the direct burning of biomass as a solid biofuel [7, 8]. However, use the biomass for direct burning has some shortcomings. The initial biomass is a nondense heterogeneous material that consists of pieces of various sizes with different calorific values. These features of the biomass cause serious problems at burning of this solid biofuel in combusting chambers. For instance, the low bulk density of the biomass leads to reduced efficiency of the combustion. In this paper, some ways are discussed in order to improve specific combustion heat and energy density of the solid biofuel based on the non-edible plant biomass. Besides, use the non-edible biomass as a modifying additive to fossil fuels has been studied in order to save fossil source and reduce gas emission. A. Materials II. EXPERIMENTAL Some non-food biomass types were used and studied: cardboard, cotton linter, softwood chips and sawdust, bagasse of sugar cane, corn stover, wheat straw, switchgrass, olive pomace and fallen olive fruits. Softwood samples were delivered from Södra (Sweden), bagasse - from Petrobras (Brazil) and switchgrass from Nott Farms (Canada). Cardboard was obtained from Amnir Recycling Industries (Israel). Samples of cotton linter, corn stover, wheat straw and olives were supplied by agricultural companies of Israel. - 15 -

Besides, Poland s lignite was also studied, which was delivered by means of National Coal Supply Corporation (Israel). Heavy fuel oil No 5-6 was supplied by Oil Refineries Co (Israel). The solid samples were milled in a lab Waring"- mill, screened through a sieve to obtain the fraction with particle size of 1-5 mm and then dried at 105 C up to constant weight. B. Methods Lignin was isolated from softwood sawdust by means of TAPPI procedure T222: The determination of acidinsoluble lignin in wood. To obtain charcoal, the softwood chips were placed into porcelain crucibles covered with perforated lids and pyrolysed in a muffle furnace at 350 C for 1 h. In addition, the some substances were used as binders: waste of polyethylene and polystyrene, petroleum tar (softening temperature 40-50 o C), paraffin wax (softening temperature 60-65 o C) and bitumen (softening temperature 65-70 o C). The following liquid biofuels were also studied: - Olive oil squeezed out from olive fruits; - Commercial tall and castor oils; - Waste of used cooking oils; - Biodiesel fuel that was produced by transesterification of the olive oil with methanol according to the procedure [9]; - Bioethanol that was obtained by fermentation of the hydrolyzate obtained from the alkali pretreated corn stover by enzymatic hydrolysis method [10, 11]. The fermentation was carried out with the yeasts of Saccharomyces cerevisiae [12] in the laboratory fermentor "Biostat A Plus" (Sartorius AG). Distillation of the diluted ethanol was performed repeatedly in a vacuum evaporator at 60 C, until the 95% ethanol concentration. The chemical analysis of the materials was carried out by conventional methods [6]. Bulk density of the samples was measured according to ASTM E873. The high heating value (HHV, MJ/kg) of dry samples was determined in the bomb calorimeter type Parr 6400. The net (lower) specific combustion heat or calorific value (Q, MJ/kg) was calculated by the equation: Q = HHV 0.225 H (1) where H is percentage of hydrogen in the sample. The density of the heat energy (QD) was calculated as: QD = Q x BD (2) where BD is bulk density of the sample, kg/m 3. To calculate the maximal density of the heat energy (QDt), the specific gravity (SG, kg/m 3 ) of the matter should be taken into consideration: QD m = Q x SG (3) Emission of gases at burning of the samples was tested by means of the analyzer FLASH-2000. This instrument equipped with high-temperature combustion reactor and gas chromatograph. Various samples were burned in the reactor and the formed gases (CO 2, SO 2, etc.) were analyzed by means of the chromatograph. III. RESULTS AND DISCUSSION Results of the chemical analysis of the dried solid materials were shown in Tables 1, 2. TABLE I CHEMICAL COMPOSITION OF BIOMASS Solid fuel Cellulose % Hemicelluloses, % Lignin, % Cotton linter 93 4 0 Softwood 47 23 27 Bagasse 38 27 20 Cardboard 58 15 11 Corn stover 36 28 19 Switchgrass 37 28 18 Wheat straw 35 30 16 Olive pomace 25 24 33 Fallen olives 19 20 22 TABLE II COMPOSITION OF ADMIXTURES Solid fuel Ash, % Sulphur, % Lipids, % Cotton linter 1 0 2 Softwood 1 0 2 Bagasse 5 0 5 Cardboard 7 0 0 Corn stover 5 0 2 Switchgrass 3 0 3 Wheat straw 6 0 4 Olive pomace 2 0 9 Fallen olives 1 0 36 Lignite 8 1.5 0.5 The greatest amount of cellulose 93 %, was in the cotton linter, while the highest amount of lignin 33%, was observed for the olive pomace. Moreover, the olive samples were characterized by increased content of lipids. The plant samples were free of sulphur, while the solid fossil fuel lignite, contained about 1.5% of the sulphur. Besides, the lignite had relative high ash content. The thermochemical investigations showed that the high-lignified biomass types olive samples and the lignin itself, exhibited the greatest calorific value of 22-26 MJ/kg (Table 3). The specific combustion heat of the other investigated plant materials varied in the range from 15 to 18 MJ/kg. The specific heat of lignite was close to calorific value of olive pomace. To study of the effect of some admixtures on the calorific value of plant biomass, the artificial mixtures were prepared containing various contents of lignin, lipid (olive oil), mineral (silica) and moisture. The investigations showed that lipids and lignin - 16 -

contribute to the increase of the energy potential of the biomass, while mineral components and moisture reduce this potential (Fig. 1). TABLE III CALORIFIC VALUE OF SAMPLES Solid fuel Q, MJ/kg Softwood 18 Bagasse 17 Cardboard 15 Corn stover 15 Cotton linter 16 Switchgrass 16 Wheat straw 15 Olive pomace 22 Fallen olives 26 Lignin 25 Lignite 20. Thus, high-lignified and lipid-rich biomass will have an increased calorific value. This is caused with a higher specific combustion heat of lignin (25 MJ/kg) in comparison with polysaccharides (16 MJ/kg). Calorific value of extractive substances of the biomass lipids, waxes and resins, is about 37 MJ/kg, i.e. it is much higher than for the polymeric components of the biomass lignin and polysaccharides [6, 7]. For example, fallen olives containing 35% lipids and 22% lignin are distinguished by the highest calorific value of 26 MJ/kg among the investigated biomass samples. Fig.1. Contribution of lipids, lignin, minerals and moisture to calorific value of biomass The calorific value of liquid biofuels (bioethanol, biodiesel fuel, lipids) extracted from the biomasses significantly exceeds the specific combustion heat of the initial plant raw-materials (Table 4). TABLE IV CALORIFIC VALUES OF LIQUID BIOFUELS Liquid fuel Calorific value, MJ/kg Bioethanol 27.2 Tall oil 37.0 Castor oil 37.1 Olive oil 37.2 Used cooking oils 37.4 Biodiesel fuel 37.6 At the discussion it is important to understand, which way of the biomass utilization might provide the greatest heat energy: direct burning of the solid biomass or burning of liquid biofuel or solid charcoal that was obtained from this biomass? To do this, it is advisable to analyse the some typical examples for production of the heat energy. Example 1 Pathway 1: One ton of the cotton linter is burned directly. Pathway2: One ton of this biomass is enzymatically converted into glucose (yield 60%); the hydrolyzate is subjected to fermentation and distillation in order to produce the ethanol (yield of 90%); and finally the obtained bioethanol is burned. Example 2 Pathway 1: One ton of switchgrass is burned directly. Pathway 2: One ton of this biomass is pretreated with alkali (yield 55%) and then enzymatically converted into glucose (yield 70%); the hydrolyzate is subjected to fermentation and distillation in order to produce the ethanol (yield of 90%); and finally the obtained bioethanol is burned Example 3 Pathway 1: One ton of fallen olive fruits (dry weight basis) containing 36% oil is burned directly. Pathway 2: One ton of this biomass is squashed to obtain 250 kg of the vegetable oil, which is burned as a liquid biofuel. Example 4 Pathway 1: One ton of fallen olive fruits (dry weight basis) containing 36% oil is burned directly. Pathway 2: One ton of this biomass is squashed to obtain 250 kg of the vegetable oil, which is subjected to transesterification (yield of 90%), and the obtained liquid biodiesel fuel is burned. Example 5 Pathway 1: One ton of softwood is burned directly. Pathway 2: One ton of this biomass is pyrolyzed to obtain charcoal (yield 36%) that is burned. As it follows from the experiments, a most efficient way of the energy production is the direct burning of the solid plant biomass, while the burning of such amount of the liquid biofuel or charcoal, which can be obtained from the plant material, gives a much smaller energetic effect (Table 5, Fig. 2). In the fact, energetic deficit of the fuels is much higher, because the calculations didn t include energy consumption for the production of the final products and other expenses of the real technological process. Despite that usually the calorific value of the liquid biofuel and charcoal is higher than of the initial biomass, the total amount of the heat energy generated by burning of the fuels is lesser due to the limited yield from the plant material. - 17 -

TABLE V HEAT ENERGY OF INITIAL BIOMASS AND BIOFUEL* Example Q b, GJ Y f, kg/t Q f, GJ 1 16 275 7.4 2 16 177 4.8 3 25 250 9.3 4 25 225 8.3 5 18 360 10.1 *Note: Q b is combustion heat of 1 t of initial biomass; Y f is yield of biofuel (kg) from 1 t biomass; Q f is combustion heat of Y kg of biofuel Fig.2. Yield of heat energy of liquid biofuels or charcoal obtained from 1 t of initial plant biomass To provide the efficient energy production from bioethanol, the yield of this biofuel from 1 ton of biomass should be about 600 kg. However, it is impossible because such amount of bioethanol exceeds its theoretical yield, 567 kg per 1 t of cellulose. The production of energy from the biodiesel fuel can be efficient if the yield of the vegetable oil is about 680 kg per 1 ton of the crop. But this yield cannot be achieved in the practice because the existing oil-crops might generate much smaller oil volume, from 200 to 400 kg from 1 ton of the crop. An additional problem that production cost of liquid biofuels made of non-edible plant biomass should be enough low, about the same as the cost of fossil fuel, nearly $1/gal; but in reality it is not so. Currently production cost of biomass-based bioethanol and biodiesel fuel is about $3-4/gal that is much higher than cost of fossil liquid fuels. The problem of cost also hampers commercialization the liquid biofuels as energy sources. Moreover, additive of bioethanol to the fossil fuel causes reducing the specific combustion heat of the mixed fuel. Thus, the direct burning of the plant biomass as a solid biofuel can be the most efficient way for the energy production. However, any plant material generates 2-3 times less heat energy compared to fossil fuels coal, gasoline, diesel fuel, etc. Another serious problem of the plant biomass is low density of the heat energy. This energetic characteristic of initial plant materials was far from the maximal value (Table 6). The low density of the heat energy hinders application of these materials as solid fuels for the energy generation. There are several ways to increase the energetic potential of the plant biomass. The one way is impregnation of the biomass with appropriate high-energetic liquids or melts; e.g. after impregnation of cardboard with used cooking oils, or with melt of tar or paraffin wax, the combustion heat increases from 15 to 28-30 MJ/kg, while the energy density rises from 8 GJ/m 3 to 22-25 GJ/m 3. TABLE VI DENSITY OF HEAT ENERGY* Solid fuel QD, GJ/m 3 QDm, GJ/m 3 Softwood 9.6 27.0 Bagasse 2.2 25.5 Cardboard 8.0 23.0 Corn stover 2.0 22.5 Switchgrass 2.1 23.2 Wheat straw 2.3 22.5 Olive pomace 8.0 32.1 Lignin 7.3 35.3 Lignite 10.6 28.7 *Note: QD is density of heat energy of samples; QDm is maximal energy density More promising way to increase the energetic potential could be adding of waste polyolefin as a binder to plant material. As known, huge amounts of plastics are thrown out and pollute the environment. In 2011, in the USA about 32 million tons of waste plastics were landfilled including about 15 million ton of polyolefins polyethylene, polypropylene and polystyrene. After recycling and granulation the polyolefin-waste can be repeatedly utilized, in particular as a binder for plant biomass to improve fuel properties. In the papers [13, 14] was found that compacting of the powdered biomass permits to increase the density and strength of briquettes, which improves energetic characteristics of the biofuel. Compacting of coal fines is also the main way of utilization of the low-quality fuel for energy production [15]. In this paper, the pellets were prepared by the following technology: the milled material was mixed with small granules of a binder, introduced into press-form, heated, pressed at pressure 100 MPa for 30 sec; then the pellets were released from the press-form and cooled up to room temperature. Fig.3. Bulk density of initial sawdust (In) and after compacting of the powder without binder (N) and together with 10% binder paraffin wax (W), bitumen (B), Polystyrene (PS) and Polyethylene (PE) - 18 -

Pressing temperature of the powdered samples with tar, paraffin wax or bitumen binders was 100 o C. In the case of polymer binders, the temperature of pressing was about 180 o C. The results showed that compacting of the samples together with binders, and especially with polymer binders, considerably increases the bulk density (see for example Fig. 3). As a result of compacting of the powdered samples in presence of 10% PE-binder, the density of the heat energy increases many times (Fig. 4). characteristics of the mixed solid fuel, as well as reduces emission of the harmful sulphur dioxide during combustion (Table 7, Fig. 6). TABLE VII CHARACTERISTICS OF LIGNITE AND SAWDUST PELLETS Sawdust, % Bulk density, kg/m 3 QD, GJ/m 3 0 530 10.6 10 680 13.6 20 700 14.1 40 740 14.8 50 750 15.0 Fig.4. Density of heat energy for initial and pelletized biomass: Sawdust (SD), bagasse (BA), cardboard (CB), corn stover (CS), switchgrass (SG) and olive pomace (OP) Increased content of the binder (e.g. PE) in the pellets improves both calorific value and density of the heat energy (Fig. 5). In the case of 50% PE-additive, the combustion energy of the fuel reaches 31 MJ/kg and energy density 25 GJ/m 3, which are close to energetic characteristics of middle-quality coals. Fig.6. Emission of SO 2 at burning of lignite and sawdust pellets Novel fuel type is a dispersion of fine biomass powder into liquid fossil fuel. Example of this fuel type can be dispersions of the milled wood (sawdust) in heavy fuel oil. The dispersions were prepared by means of mixing of the biomass powder with the liquid in the Waring - mixer at 5000 rpm for 5 min. Due to small particle sizes of the biomass and increased viscosity of the liquid, the final dispersions were stable to phase separation. The experiments showed that introduction of the solid biomass particles into the combustible liquid raises the energetic potential of the dispersed fuel (Fig. 7). An advantage of the dispersed fuel is also the reduction of SO 2 and CO 2 emission with increase of content of the biomass powder in the liquid (Fig. 8). Moreover, addition of the powdered biofuel to the fuel oil permits save the nonrenewable fossil fuel and reduce cost of the dispersed fuel. Fig.5. Dependence of energetic characteristics of sawdust-based pellets on content of polymer binder Thus, the proposed pelletization technology improves the burning effectiveness of the solid fuels and also permits utilize the waste plastics. It is known that lignite is a cheap, but low-quality fossil fuel. To achieve the better quality of the solid fuel, a cogranulation method can be used, when lignite is compacted together with the powdered plant biomass in the presence of a binder, e g. 10% tar. Addition of the plant material, e.g. sawdust, to lignite improves density and energetic Fig.7. Combustion heat of 1 kg of the heavy fuel oil after introduction of the solid biofuel powder into the liquid fuel - 19 -

Fig.8. Emission of SO 2 (1) and CO 2 (2) at burning of 1kg of dispersed fuel depending on content of powdered biomass in liquid fuel IV. CONCLUSIONS The energetic characteristics of some biomass types have been studied in order to evaluate their suitability as solid biofuels. Is was shown that the most efficient way of the energy production is the direct burning of the solid plant biomass, while the burning of such amount of the liquid biofuel or charcoal, which can be obtained from the plant material, gives a much smaller energetic effect. However, the plant materials have a low density of the heat energy, which hinders effective energy generation. One way to improve the energetic characteristics of the biomass is impregnation with high-energetic liquids or melts. More promising way to improve fuel properties of the biomass samples a pelletization technology was used. As a result of compacting of the powdered plant materials together with binders, a considerable increasing of the bulk density, calorific value and density of the heat energy was observed. The proposed pelletization technology of the biomass together with polymer binders improves the burning effectiveness of the solid fuels and also permits utilize the waste plastics. Co-granulation of the low-quality coal and the plant biomass results in the mixed solid fuel having the better energetic characteristics and reduced emission of the harmful sulphur dioxide. Novel fuel type - a dispersion of fine biomass powder with liquid fossil fuel has been developed. Addition of the powdered biomass to the combustible liquid contributes to rise of the energetic potential of the dispersed fuel. An additional advantage of the developed fuel is the reduced emission of SO 2 and CO 2. Introduction of the powdered biofuel saves the non-renewable fossil fuel and reduces cost of the final dispersed fuel. 2 [3] K. A. Hossain, Global energy consumption pattern and GDP, Int J Renewable Energy Technol. Res., vol. 1, pp. 23 29, 2012. [4] M. Ioelovich, and E. Morag, Study of enzymatic hydrolysis of mild pretreated lignocellulosic biomasses, BioResources, vol. 7, pp. 1040-1052, 2012. [5] R. D. Perlack, L. L.Wright, A. F. Turhollow, R. L. Graham, B. J. Stokes, and D. C. Erbach, Biomass as feedstock for a bioenergy and bioproducts industry: The technical feasibility of a billion ton annual supply, Report of DOE & USDA, 2005. [6] D. Ahuja, and M. Tatsutani, Sustainable energy for developing countries, Sapiens, vol. 2, pp.1-16, 2009. [7] M. Ioelovich, Energetic potential of plant biomass and its use, Int. J. of Renewable and Sustainable Energy, vol. 2, pp. 26-29, 2013. [8] M. Ioelovich, Plant biomass as a renewable source of biofuels and biochemicals, Berlin, Lambert Acad. Publ., 2013. [9] H. Fukuda, A. Kondo, and H. Noda, Biodiesel fuel production by transesterification of oils, J. Biosci. Bioeng, vol. 92, pp. 405-416, 2001. [10] M. Ioelovich, and E. Morag, Study of enzymatic hydrolysis of mild pretreated lignocellulosic biomasses, BioResources, vol. 7, pp. 1040-1052, 2012. [11] M. Ioelovich, and E. Morag, Study of enzymatic hydrolysis of pretreated biomasses at increased solids loading, BioResources, vol. 7, pp. 4672-4682, 2012. [12] T. D'Amore, I. Russell, and G. Stewart, Sugar utilization by yeast during fermentation, J. Ind. Microbiology, vol. 4, pp. 315-324, 1989. [13] S. H. Larsson, M. Thyrel, P. Geladi, and T. A. Lestander, High quality biofuel pellet production from pre-compacted low density raw materials, Biores. Technol. vol.99, pp.7176-7182, 2008. [14] S. Warajanont, and N. Soponpongpipat, Effect of particle size and moisture content on cassava root pellet fuel s qualities follow the acceptance of pellet fuel standard, Int. J. of Renewable and Sustainable Energy, vol. 2, pp. 74-79, 2013. [15] F. Dehont, Coal briquetting technology, Report of Sahut- Conreur S.A., France, Raismes, 2006. Michael Ioelovich graduate of the Latvian State University. He worked at the Institute of Wood Chemistry, Latvian Academy of Sciences. He was graduated Ph.Dr. in 1974, Dr.Sc. in 1991 and Habilitated Dr. in 1992. From 2008 he works at Designer Energy Company (Israel) as Chief Chemist, develops and produces novel types of biofuels and biochemicals. He is author more than 200 scientific papers. REFERENCES [1] M. Ioelovich, Biofuels technology, problems and perspectives, a review, Journal SITA, vol. 12, pp. 106-126, 2010. [2] D. Klemm., B. Heublein, H.-P. Fink, and A. Bohn, Cellulose: fascinating biopolymer and sustainable raw material, Angew. Chem., vol. 44, pp. 2-37, 2005. - 20 -