UTICAJ RECIRKULACIJE NA KARAKTERISTIKE SAGOREVANJA BIOMASE

Transcription

1 UTICAJ RECIRKULACIJE NA KARAKTERISTIKE SAGOREVANJA BIOMASE A. M. Eric a, D. M. Djurovic a, S. Dj. Nemoda a, D. V. Dakic b, B. S. Repic a a Univerzitet u Beogradu, Institut za nuklearne nauke Vinča, Laboratorija za termotehniku I energetiku, P.O.Box 22, 11 Beograd, Srbija b Univerzitet u Beogradu, Mašinski fakultet, Inovacioni centar, Kraljice Marije 16, 1112 Beograd 3, Srbija Apstrakt: Biomasa je u Srbiji najznačajniji obnovljivi izvor energije, pa je stoga neophodno neprestano promovisanje njenog korišćenja u energetske svrhe i gde god je moguće izvršiti zamenu fosilnog goriva biomasom. U ovom radu prikazani su rezultati eksperimentalnog istraživanja sagorevanja biomase u eksperimentalnim uslovima. Korišćena su dva tipa biomase kukuruzovina i pšenična slama. Eksperimenti su izvođeni sa, i 2% recirkulacije. Recirkulacija je simulirana uvođenjem azota, kao inertnog gasa, zajedno sa vazduhom. Eksperimentalno istraživanje je pokazalo da se pojedine faze sagorevanja usporavaju i do 33% sa uvođenjem 2% recirkulacije u odnosu na sagorevanje bez recirkulacije. Dobijeni rezultati mogu biti od koristi za analizu i poboljšanje karakteristika procesa sagorevanja i razmene toplote. Osim toga, recirkulacija značajno utiče na snižavanje maksimalne temperature u ložištu, pa samim tim i na proces topljenja pepela biomase, koji predstavlja veliki problem prilikom procesa sagorevanja. Ključne reči: Biomasa, recirkulacija, brzina procesa, topivost pepela RECIRCULATION EFFECTS ON BIOMASS COMBUSTION CHARACTERISTICS A. M. Eric a, D. M. Djurovic a, S. Dj. Nemoda a, D. V. Dakic b, B. S. Repic a a University of Belgrade, Institute of Nuclear Sciences Vinča,Laboratory for Thermal Engineering and Energy, P.O.Box 22, 11 Belgrade, Serbia b University of Belgrade, Faculty of Mechanical Engineering, Innovation Centre, Kraljice Marije 16, 1112 Belgrade 3, Serbia

2 Abstract: Since biomass is the greatest renewable energy resource in Serbia it is necessary to promote its wider use and wherever is possible to switch fossil fuels with biomass. In this paper has been shown experimental results on biomass combustion in laboratory conditions. Two types of biomass were tested corn stalk and wheat straw. The experiments were performed without recirculation and also with and 2% of recirculation. Recirculation was simulated by introducing nitrogen, as an inert gas, in the air for combustion. Experiments have shown that combustion phases, with a 2% of recirculation, were up to 33% slower in relation to the experiments without recirculation. Obtained results will help improving combustion characteristics, altering the heat generation, heat transfer and reaction rate. Besides that, recirculation of flue gases can contribute in reduction of biomass ash melting. Key words: Biomass, recirculation, process rate, ash melting 1. INTRODUCTION In terms of sustainable energy development in Serbia, as well as in the whole world, there is a growing need for using the renewable energy sources. A need for the utilization this kind of energy sources is dictated by the market, on one side, as well as by environmental protection, on the other. As a country in transition, there are so many conditions Serbia needs to fulfil before the accession to the EU. With regard to sustainable energy resources, it is necessary to meet conditions defined by EU in order to reduce environmental pollution, greenhouse gas emissions and encourage innovation in this area. Technically utilizable energy potential of the renewable energy sources in the Republic of Serbia is very significant and estimated at around 6 million tons of oil equivalent per annum - of which 3.3 Mtoe in the production of biomass, 1.7 Mtoe potential of hydro-energy,.2 Mtoe geothermal,.2 Mtoe wind power and.6 Mtoe in solar energy. The northern part of Serbia, Vojvodina, is a region with large agriculture production, mainly wheat, corn, sugar cane, soya, etc. Presently, only a small amount of waste biomass is used for energy production, for several reasons low price of electricity, small energy density of biomass, problems and cost in its gathering, transportation and preparing for combustion in large industrial or district heating facilities, the lack of suitable technologies for burning bulky waste biomass, etc. The best opportunity to use waste biomass for energy production in industrial and district heating facilities is when waste biomass is used for energy production near the place of its collection - in large agricultural enterprises. These cases are optimal, and most efficient, both from energetic and

3 economic points of view. Biomass from agricultural production is most commonly collected in the form of bales of various sizes and shapes, and it would be most appropriate to use it in that form. In principle, the following combustion technologies can be distinguished: - fixed bed combustion, - fluidized bed combustion, - dust (pulverized fuel) combustion. Technologies enabling biomass use for energy generation are mainly dependant on biomass characteristics. Over the past couple of years, in the Laboratory for Thermal Engineering and Energy, considerable efforts have been made to develop a technology which would enable biomass bales of various sizes and shapes to be used for energy production. As a result of these efforts pilot plant with 1. MW power and cigarette type of combustion has been built [1-]. In order to improve combustion process various experiments has been performed and it is going to be presented in this paper. The fuel properties and process conditions affect the combustion characteristics, altering the heat generation, heat transfer and reaction rates. The air flow rate is the key process parameter that determines the amount of oxygen available and convective heat transfer [6]. Also, biomass has a wide range of variety in physical properties, which significantly change the process rates and detailed phenomena. Besides that, ash composition is a major concern in biomass combustion process since high presence of alkali metals in biomass may cause slagging, fouling and ash agglomeration. In this paper has been presented combustion tests for two type of agricultural biomass corn and wheat straw for different air flow rates and also with different recirculation flow rate. The ignition propagation and char oxidation periods are described using the measurements of temperature, mass loss histories, and quantified into key process rates and parameters. Then, the effects of bulk density, particle size and channeling on the combustion characteristics are discussed. Elemental ash compositions are also analyzed to elucidate the behavior of different bottom ashes. 2. EXPERIMENT Mathematical models of pressed (baled) biomass combustion on a "cigarette" principle that have been developed within the project III 4211, represent stationary models which do not take into account the kinetics of the individual phases that occur in the combustion process. This primarily refers to the basic stages of drying, devolatilization, combustion of volatiles and final phase of char burnout. These processes have been approximated by a global combustion process involving all stages, and could be determined only by experiment on a real plant. This means that the input data

4 on the global kinetics could get only by complex experimental procedures at the plant which is in operation. To overcome difficulties of determining global kinetic parameters for pressed biomass combustion original experimental apparatus has been designed and constructed, which allows quick and easy way to do more experiments with large number of different types of biomass and thus generate a database on global kinetics of pressed biomass combustion process, Figure 1. Flue gas Insulation Cylindrical wall Ash Combustion zone Thermocouple Fresh biomass Grate Combustion gas Figure 1. Experimental principle This principle has been chosen as the most similar to the principle of the cigarette biomass combustion with the difference of distribution of the fluid for combustion. In furnaces with a cigarette principle of combustion fluid for combustion is supplied from the side and frontal into the combustion zone, while in counter current reactor direction of the flame front and combustion fluid is opposite. Since tested biomass has low melting point of the ash conditioned by high content of alkali metals, special attention must be paid to measures to avoid that. One measure which gives positive results is the recirculation of cold flue gases. Recirculation is reasonable to perform on large experimental or real plant, and in the laboratory is acceptable to approximate recirculation by adding an inert gas.

5 For this purposes nitrogen was used as the inert gas. Bearing in mind that the recirculation of cold gases affects the kinetics of the combustion process, this paper analyzes the influence of rate of recirculation on the global kinetics of the combustion process Experimental installation An experimental installation, shown in Figure 2, generally consists of three parts. Working part of the apparatus is the tube diameter of 72 mm and a height of 43 mm (Pos. 1), in whose bottom is grid (4). Working part of the apparatus is insulated with insulating material thickness of mm (2), in order to reduce heat loss to the environment. Previously prepared sample biomass (3) is placed in the working section of apparatus. Fluid for combustion is introduced into the distribution chamber () through the duct wall facing down, in order to ensure equal distribution (6) ~22V N Figure 2. Scheme of experimental installation The preparation and distribution of a fluid for combustion is performed by using special lines, consisting of a flow meter ( and 11) in order to determine exact ratio of air and nitrogen in the combustion fluid. The required amount of air is provided by a fan (13) with voltage regulator (14),

6 and the required amount of nitrogen is provided by the tank with the regulatory valve (1). The total fluid flow is regulated by finely adjustable valve (9). Data acquisition system is the third and independent part, which consists of high-precision scales (16) with automatic recording of data, thermocouples for measuring temperature in the middle of the biomass sample (8) and a computer for data acquisition (17). Data on changes in weight and temperature are recorded every 3s Fuel In experimental studies, presented in this paper, two kinds of fuel has been used, wheat straw and corn stalks. These fuels were chosen because they are the most common types of renewable fuels in Serbia and the most interesting from the point of exploitation. During experiments, special attention was paid on porosity of the sample fuel in the test section of the apparatus, in order to correspond to the porosity of the fuel in real terms, so that a certain amount of fuel were compressed to the desired density (porosity) by special tool. Proximate and ultimate analyses of fuel used in experiments are given in the following table. Table 1. Ultimate and proximate analysis of used fuel Analysis Proximate Ultimate C H N O W Volatile C fix Biomass [%] [%] [%] [%] [%] [%] [%] Ash [%] H d [MJ/kg] Corn stalk 42,,8,61 38,48 8,38 69,76 17, 4,36 13,981 Wheat straw 41,2 4,92,6 36,87,11 66,91 16,67 6,31 14,779 From the table can be seen that chemical composition of the used fuels is very similar, however it should be noted that the structure of the fuel varies, and therefore the kinetic parameters of combustion, and it is assumed that obtained results will confirm that Experimental procedure Performing experimental procedure begins by preparing samples of biomass. Preparation involves sampling, chopping large parts that would make a problem when pressing in the working section of the apparatus and weighing. The sample is then placed in the working section of the apparatus and is compressed to the desired size of porosity. After that, thermocouple is placed in the sample and the entire apparatus is placed on the scale. Then, the fluid for the combustion is provided (the mixture of air and nitrogen) in relation to desired degree of recirculation. Adjusting the ratio of air and nitrogen is carried out using valves and flow meter. Ignition of fuel is done by inserting pieces of burning biomass, when combustion process starts. Temperature and weight change of the sample has been recorded on the computer.

7 The end of combustion, and therefore the end of the experiment, was adopted as the moment in time when the temperature drops below a certain limit, which ensures complete combustion process. 3. EXPERIMENTAL RESULTS AND ANALYSIS In the shown experimental apparatus a larger number of experiments were performed. The experiments were performed without recirculation, with recirculation of % and 2% Corn stalk, =92 kg/m 3, =.62, V=2 l/h a) Corn stalk without recirculation b) Corn stalk with % recirculation Corn stalk, =92 kg/m 3, =.62, V V =16 l/h, V N =4 l/h c) Corn stalk with 2% recirculation d) Wheat straw without recirculation Wheat straw, =92 kg/m 3, =.62, V v =18 l/h, V N =2 l/h e) Wheat straw with % recirculation f) Wheat straw with 2% recirculation Figure 3. Sample mass and temperature change in time Corn stalk, =92 kg/m 3, =.62, V V =18 l/h, V N =2 l/h Wheat straw, =92 kg/m 3, =.62, V=2 l/h Wheat straw, =92 kg/m 3, =.62, V v =16 l/h, V N =4 l/h

8 Experiments without recirculation were performed only with air as combustion fluid (oxidizer), with flow of 2 l/h. Experiments with recirculation were performed by adding nitrogen 2 l/h (% recirculation), and 4 l/h (2% recirculation), but also total flow rate were kept on 2 l/h. In the working part of the apparatus were placed 36 g of biomass to a height of mm above the grid, which corresponds to the desired porosity ε =.62. Mass of placed biomass corresponds to the power of about 9 W and excess air of about 2.3 assuming that the combustion takes place for 6-7 minutes. All three experiments were performed for both types of biomass, with three repetitions, for a total of 18 experiments. Experimental results carried out by the described procedure are shown in the Figure 3. As it can be seen from the Figure 3 every experiment was repeated three times for each experimental condition. Repetitions were performed to obtain more accurate results, and for that purpose has been made their averaging and rejection of individual values with the standard deviation beyond the allowable range of ± 2.%. For easier results monitoring, averaged values are then reduced to a dimensionless form, dividing by the initial mass of the sample, and the results of these types of samples are also shown in the Figure % % 2% Corn stalk, =, % % 2% a) corn stalk b) wheat straw Figure 4. Influence of recirculation on the global combustion kinetics % % 2% Wheat straw, =,62 % % 2% From Figure 4, for both types of biomass, can be clearly seen three stages of combustion: - Stage ignition and separation of moisture; - Phase of devolatilization and combustion of volatiles and; - Phase of char burnout process. The first phase is the shortest one and it lasts from inserting fired pieces of biomass till starting volatiles separation. This time period includes drying the sample biomass (separation of moisture), so that the moisture content is relatively low, it is expected that this time interval is the shortest one.

9 Process rate, [1/s] Process rate, [1/s] Since at this stage there is no intense burning, but usually there is a separation of moisture, the kinetics is the same for all three ratios of air and nitrogen. The second phase is longer, but the rate of separation of matter is maximal. This phase is the period of devolatilization and combustion of volatiles, until the beginning of char burnout process. Analysis of the impact of recirculation on the kinetics of this phase can come to the conclusion that the rate of allocations matter is the greatest when is used clean air and by increasing proportion of nitrogen in the oxidant deposition rate decreases, which means that for the highest degree of recirculation devolatilization kinetics is minimal. The third stage of combustion is the longest one and is char burnout phase. This is the period from the end of devolatilization till the end of char burnout, i.e. until the end of the entire combustion process. Also in this phase combustion process is the shortest when the clean air is the oxidant, while with the increase of the share of nitrogen increases duration of this phase. Rate of all three phases are shown in Table 2 and Figure. Table 2. Process rate Process rate, 1/s Ignition and drying Devolatilization Combustion of charcoal Recirculati on rate, % Corn stalk 2,88e-3 2,88e-3 2,88e-3,9e-3 4,e-3 3,39e-3,e-4 4,68e-4 3,97e-4 Wheat straw 2,46e-3 2,46e-3 2,46e-3 4,8e-3 3,61e-3 3,14e-3,8e-4 4,36e-4 4,6e Corn stalk,, Recirculation, [%] ignition and drying devolatilization combustion of charcoal a) corn stalk b) wheat straw Figure. Process rate Wheat straw,,62 Recirculation, [%] ignition and drying devolatilization combustion of charcoal From the evaluated results, it can be seen that the rate of the first phase is constant and does not depend on the degree of recirculation. Also, it is noticed that the rate of this phase is something greater with corn stalk.

10 Rate of the second phase is largely dependent on the degree of recirculation and with differences of 23% for wheat straw and 33% for corn stalk. Rate of this phase is also greater with corn. The third and the slowest stage depend on the rate of recirculation, similar like in devolatilization phase. The difference in rate is about 3% both for corn and wheat straw. Generally speaking, the rate of the slowest phase is an order of magnitude smaller than the devolatilization phase and similar both for corn and wheat straw. From the above analysis can be concluded that the recirculation of cold flue gases, which is advantageous from the standpoint of lowering the maximum temperature in the combustion chamber below the solubility of the ashes of biomass, decrease global burning rate. This means that the introduction of measures to prevent melting of the ash in the combustion process, must rely on increasing the volume of the combustion chamber, providing the same power. This observation opens the door to new research on the experimental apparatus for the combustion of biomass on the principle of cigarettes, whose structure is closer to the real plant. 4. CONCLUSION This paper presents an experimental study of burning agricultural biomass on an experimental apparatus for determining global kinetics. The study was conducted in order to determine the effect of cold flue gas recirculation to the global kinetics of biomass combustion in the furnace, which works on the principle of cigarette combustion. Recirculation is important from the standpoint of achieving lower maximum temperature in combustion chamber and to avoid a problem with the melting of the ash. For this purpose special experimental apparatus and experimental procedure has been developed. Experiments were conducted with two types of biomass, corn and wheat straw, with an analysis of the impact of recirculation. Recirculation was simulated by introducing nitrogen, as an inert gas, in the air for combustion so that the mixture corresponded to recirculation rate of %, % and 2%. The analysis of the experimental data has shown that recirculation process impacts on reduction of global reaction rate during agricultural biomass burning process. The most intensive influence of recirculation is reflected in the devolatilization and into char burnout phase, whiles the influence on drying and firing phase is negligible. Experiments have shown that combustion phases, with a 2% of recirculation, were up to 33% slower in relation to the experiments without recirculation. Conducted research could be of great use in designing new combustion chamber where is planned to solve the problem of solubility of ash biomass by introducing cold recirculated flue gas. In addition, the results could be of great importance in the mathematical modeling of combustion, both stationary and non-stationary transport processes.

11 ACKNOWLEDGEMENTS The authors thank the Ministry of Education, Science and Technological Development of Serbia for enabling funding of the projects III 4211 Development and improvement of technologies for energy efficient and environmentally sound use of several types of agricultural and forest biomass and possible utilization for cogeneration and TR3342 Fluidized bed combustion facility improvements as a step forward in developing energy efficient and environmentally sound waste combustion technology in fluidized bed combustors. REFERENCES [1] Bech, N., Wolff, L., Germann L., Mathematical Modeling of Straw Bale Combustion in Cigar Burners, Energy & Fuels, (1996), 2, pp [2] Mladenović, R., Erić, A., Mladenović, M., Repić, B., Dakić, D., Energy production facilities of original concept for combustion of soya straw bales, Proceedings, 16th European Biomass Conference & Exhibition From Research to Industry and Markets, Valencia, Spain, 2-6 June, 28, pp [3] Erić, A., Thermo mechanical processes during baled soya residue combustion in pushing furnace, PhD Thesis, University of Belgrade, Faculty of Mechanical Engineering, 2 [4] Erić, A., Dakić, D., Nemoda, S., Komatina, M., Repić, B., Experimental method for determining Forchheimer equation coefficients related to flow of air through the bales of soy straw, International Journal of Heat and Mass Transfer, 4 (211), 19-2, pp [] Eric, A., Dakic, D., Nemoda, S., Komatina, M., Repic, B., Experimental determination thermo physical characteristics of balled biomass, Energy, 4 (212), 1, pp [6] Ryu, C., Yang, Y. B., Khor, A., Yates, N.E., Sharifi, V. N., Swithenbank, J., Effect of fuel properties on biomass combustion. Part I. Experiments fuel type, equivalence ratio and particle size, Fuel, 8 (26), 7-8, pp