"There are usually plenty of agriculture byproducts and wastes"
FLUIDIZED BED COMBUSTION OF AN AGRICULTURE WASTE CASE STUDY : COMBUSTION OF RICE STRAW Okasha, F.M. 1, El-Emam, S. H. and Zaatar, G. Department of Mechanical Engineering, Mansoura University, Egypt. Tel. +20-50-2232387 & Fax. +20-50-2244690 Email: email@example.com ملخص البحث .رٕجذ فٙ يصش ٔفشح يٍ انًخهفبد انضساػٛخ ٔانزٙ يبصانذ نٓب آثبس سهجٛخ ػهٗ انجٛئخ، ٔثصفخ خبصخ قش األسص ًٔٚكٍ رالشٗ أضشاس ْزِ انًخهفبد ثزٕظٛفٓب كأحذ يصبدس انطبقخ انًزجذدح ، ْٔزا انجحث ٚشكض ػهٗ دساسخ خصبئص احزشاق قش األسص فٙ انفشٌ ر٘ انًٓذ انًًٛغ ٔانز٘ ٚؼذ أحذ أفضم انجذائم انٕاػذح نحشق انًخهفبد انحٕٛٚخ ٔ اسزؼبدح .طبقبرٓب َّٔظشً ألٌ قش األسص يُخفض انكثبفخ ٔغٛش يُزظى انشكم فقذ رى رجٓٛض ػُٛبد السزخذايٓب كٕقٕد ٚسٓم رغزٚز ا إنٗ داخم انفشٌ، ٔرنك ثبسزخذاو ٔحذح ثسٛطخ أػذد خصٛصب نزنك. ٔػُٛبد انٕقٕد انزٙ رى رجٓٛضْب ػجبسح ػٍ قطغ .3صغٛشح يٍ قش األسص اسطٕاَخ انشكم قطشْب 21 يى ٔطٕنٓب حٕانٙ 01 يى ٔكثبفزٓب حٕانٙ 3..0 جى/سى ٙٔقذ أجشٚذ سهسهخ يٍ انزجبسة نذساسخ خصبئص احزشاق قش األسص ثبسزخذاو فشٌ ر٘ يٓذ يًٛغ قطشِ انذاخه 003 يى ٔاسرفبػّ حٕانٙ 0033 يى ٔكزنك دساسخ رأثٛش ػٕايم انزشغٛم انًخزهفخ ػهٛٓب. ٔفٗ أثُبء انزجبسة رى يالحظخ سالسخ األداء يٍ حٛث رغزٚخ انٕقٕد ٔيسزٕٖ رًٛغ حجٛجبد انٕسط أٔ ركزهٓب كُزٛجخ نٕصٕنٓب دسجخ حشاسح انزهذٌ أٔ َزٛجخ .الَصٓبس سيبد انٕقٕد، كزنك رى سصذ ٔيشبْذح انهٓت داخم انفشٌ ثبنؼٍٛ يجبششح يٍ خالل َظبسح صجبجٛخ ،ٔقذ رُبٔنذ انذساسخ ثبنششح ٔانزحهٛم رأثٛش ػٕايم انزشغٛم انًخزهفخ ػهٗ كفبءح االحزشاق، ٔاإلَجؼبثبد انًخزهفخ كًب رى أٚضب سسى يُحُٛبد رٕصٚغ دسجبد انحشاسح فٙ كال االرجبٍْٛ انقطش٘ ٔانًحٕس٘ داخم انفشٌ. شًهذ ػٕايم انزشغٛم كم يٍ سشػخ انزًٛغ، ٔاسرفبع انًٓذ انًًٛغ، ٔدسجخ حشاسح انًٓذ انًًٛغ ، َٔسجخ انٕٓاء إنٗ انٕقٕد .ٔرفٛذ انُزبئج انزٙ رى انزٕصم إنٛٓب إنٗ أٌ ػًهٛخ حشق قش األسص فٙ فشٌ ر٘ يٓذ يًٛغ رزى ثكفبءح فٙ حذٔد 98%ٔاٌ َست رشكٛض أٔل ٗاكسٛذ انكشثٌٕ ٔ اكبسٛذ انُزشٔجٍٛ فٙ حذٔد يٍ 002- 033 جضئ فٙ انًهٌٕٛ ٔيٍ 1.1-0.2 جضئ فٙ انًهٌٕٛ , ػه .انزشرٛت ABSTRACT In Egypt there is abundance of agriculture byproducts and residues that is still provoking environmental problems, in particular, rice straw. Utilizing of biomass for energy production alleviates the growing waste disposal problems and preserves the diminishing conventional fossil fuels. The present work is dedicated to investigate the combustion characteristics of rice straw in a fluidized bed. Rice straw has been prepared as pellets of diameter 12mm and length 10 mm by virtue of chopping and compression processes. The rice straw pellets have been burnt in an atmospheric bubbling fluidized furnace of diameter 300 mm and height 3300 mm. The experiments have been carried out under steady state conditions and by means of over-bed fuel feeding system. Experimental results demonstrate that combustion of rice straw in fluidized bed is successful with high efficiency. Post-combustion of volatile over bed is evidence that results in a peak temperature in freeboard. The peak temperature degree and location are sensitive to the operating parameters, especially fluidization velocity and excess air. CO and NOx level are relatively low ranging from 200 to 330 ppm and 175 to 270 ppm respectively. The obtained results show that at high level of the studied operating parameters (fluidization velocity, bed height, bed temperature and excess air), NOx emission increases with different degrees. KEYWORDS Combustion, Fluidized Bed, Biomass 1 Author to whom corresponding should be addressed INTRODUCTION environment. According to the Agriculture The worldwide greenhouse issue, the Engineering Researches Institute, the annual protocol of Kyoto, the more stringent potential of agriculture wastes in Egypt is environmental regulations and the limited about 22.5 million ton . About 35% of resources of fossil fuels guide toward them are utilized as animal feeding and growing utilization of renewable resources of fertilizer and 65% are available for energy energy. From this point of view, Biomass is production which is 7 million ton oil expected to be one of the most important in equivalent (TOE) the near future. Biomass is a renewable Because rice straw represents the worst energy that is a CO2 neutral fuel. Biomass, environment issue of agriculture wastes in in all its forms, provides about 7% of the Egypt, the current study is devoted to world annual energy consumption. In investigate combustion of rice straw in developing countries, it provides 35% of all fluidized bed. The main objectives of this the energy requirements [1- 3]. experimental work are: Fluidized bed combustion (FBC) is Preparation of rice straw in a form widely considered for burning different adequate for feeding and combustion. biomass fuels. The fluidized bed is a layer of Assessment of the combustor inert material (like silica sand) that is being behavior with regard to fuel feeding, fluidized using air delivered through the bed fluidization, sintering and burnout. via a distributor plate found at the furnace Measurement of the temperatures and floor. The operation temperature is so low gaseous concentrations at different that all NOx emissions are generated from points in the combustor. fuel nitrogen while the thermal contribution Evaluation of the influence of NOx is insignificant. The thermal inertia of operation parameters on different the fluidized bed is large, which stabilizes emissions and combustion efficiency. and maintains combustion and allows very Special attention is given to the role of the fuel flexible operation. In particular, the freeboard in the combustion process by tolerance for fuel moisture variation clearly determining axial profiles of temperature. exceeds other combustion technologies. The advantages of the bubbling fluidized bed EXPERIMENTAL boilers may be summarized as: high boiler Test Rig efficiency, high availability, excellent fuel The experimental test rig used for the flexibility, fast dynamic behavior, low present work is an atmospheric bubbling maintenance costs, low auxiliary power fluidized bed. Figure 1 shows a schematic of consumption, low emissions and minimum the test rig. A detailed description of the test operating personnel requirement [4-7]. rig can be found elsewhere . The In Egypt there is abundance of combustor is a cylindrical column of 300- agriculture byproducts and residues that is mm inner diameter and 3300 mm height. still provoking environmental problems A nozzle type plate is used to because most of them are burnt haphazardly distribute the primary air at bottom of the without energy recovery. This performance is combustor. The air serves in fluidizing bed multiplying greenhouse effect due to adding materials and burning fuel. emissions and heat to atmosphere. The column is implemented with 21 Alternatively, many of agriculture wastes can portals to insert probes for measuring be utilized as a biomass fuel realizing two purposes. The fluidized bed section contains fundamental goals. Utilizing of biomass fuel a heat exchanger system consists of three preserves the diminishing conventional fossil radial movable pipes. By virtue of this fuels and alleviates the growing waste system, bed temperature can be controlled by disposal problem. Another remarkable asset adjusting the pipes penetration lengths into is its comparatively low impact on the the bed and accordingly heat removal rate. The combustor is equipped with a about 0.05 g/cm3. Figure 2 is a photograph of continuous over-bed fuel pellets feeding some briquette rice straw prepared by the system using a paddle shaft. The shaft is unit. The proximate and ultimate analysis of driven by variable speed electric motor. rice straw are reported in Table 1 Downstream the feeder, the pellets move to the combustor by gravity. A hopper to feed Test Procedure and Operating Parameters bed sand particles is located on the top of the After charging a quantity of sand combustor column. corresponding to the required bed height, Flue gases coming out from the sufficient air premixed with LPG is delivered fluidization column pass through a cyclone to through the distributor to fluidize bed separate and to collect the entrained material. The bed is initially heated by particulates. Column parts are all insulated burning LPG. At 650 oC the rice straw pellets using blankets of thermal wool. are fed into the bed multiplying the rise rate Radial and axial temperatures profiles of bed temperature. Flow rate is then of the combustor are measured using type K adjusted at predetermined value. The bed thermocouples. The flue gas concentrations temperature is controlled by adjusting the are carried out using GA-40 plus gas penetration length of heat exchanger tube analyzer, which is able to measure O2, CO2, inside the bed. When the desired temperature CO, SO2 and NOx concentrations. is attained feeding of LPG is stopped and the Silica sand with a narrow size fuel feeding is totally switched to rice straw distribution (0.25-0.5 mm) has been used as pellets. After the unit stabilizes at the bed material. Its experimental minimum prefixed steady state conditions, different fluidization velocity is 5.6 cm/s at 850 °C, measurements are carried out. typical operating bed temperature. The influences of the most important operating parameters have been studied. Fuel Preparation and Characteristics Static bed height has been varied between 20 As receiving conditions, rice straw cm to 40 cm. Bed temperature has been has a very low bulk density and relatively ranged between 700 oC to 900 oC. To have long and irregular shape. Because of these good mixing of bed material with moderate characteristics rice straw has very low energy bubble size, fluidization velocity range is content per unit volume and it is difficult to chosen to be 0.3-0.7 m/s (about 5-10 time handle. Moreover, it has high transportation Umf). Excess air range is 10%-30%. and storage cost. So, it is important to convert rice straw into a product of regular RESULTS AND DISCUSSION shape and of higher bulk density through the Performance of Operation Briquetting process. Steady state Experiments have been For the present work, a simple unit performed at different operating conditions. has been installed to densify rice straw. The The feeding of the fuel pellets was usually unit should be piston type briquetting that successful, however, in few cases the system mainly consists of a hydraulic press, pistons is blocked. This problem may be overcome and different dies. The unit produces biomass by using a vibrating feeding system or by fuel pellets in cylindrical form. passing secondary air through the fuel- At first, rice straw has been chopped feeding pipe. During the combustion the to about 3 cm length. The chopped rice straw fluidization behavior was normal and no is undergone pressure of 300 bar inside the problem has been encountered. Saving die. The final product is rice sraw pellets of analysis of bed materials doesn’t demonstrate cylindrical shape. The pellet is of 12 mm agglomerating or sintering. diameter and about of 10 mm length. The Direct observation showed the bulk density of pellets is about 0.73 g/cm3. occurrence of the flame in the freeboard. whereas the initial bulk of raw rice straw is This behavior is expected as rice straw has a high volatile content. A significant part of decreasing the bed height. Further the peak volatile escapes the bed to complete temperature moves down. The profiles combustion in the freeboard. This indicate that more heat is released in characteristic of biomass fuel combustion has freeboard due to post-combustion. With been recognized by different researchers [10- decreasing bed height, the gas residence time 13]. The measurements of axial temperature becomes shorter and more volatiles bypass confirm the existence of peak temperature in the bed without combustion. On the other the freeboard. The peak temperature degree side, decreasing the bed height reduces the and position are varied according to the heat transfer to the cooling tubes inside the operating conditions. The collected materials bed. in the cyclone were found relatively small. To study the effect of excess air maintaining fluidization velocity nearly Temperature Profiles constant, the fuel flow rate is varied while Temperature distribution in axial holding the airflow rate constant. In Figure 5, direction has been measured at different the axial temperature profile is presented as a operating conditions. The obtained profiles function of excess air factor. It is noticed that are plotted in figures 3-5. The temperature of with increasing the excess air factor the peak the bed zone is nearly uniform, however, temperature moves down while the there is temperature rise in the freeboard temperature of gases leaving the freeboard above the expanded bed. In fact the reduces. Increasing excess air implies that the temperature profiles reflect the occurrence of oxygen available for reaction becomes the post combustion in freeboard for all runs. greater and the rate of combustion reaction A considerable part of volatile surpasses the becomes higher. At excess air factor 1.3, the bed without combustion because of short combustion rate is the highest. Under these residence time and lack mixing with oxygen. conditions, the majority of combustibles burn The position of intensive combustion zone in inside the bed. The combustion of escaping freeboard moves according to the operating volatiles takes place in freeboard zone close conditions. to the bed. This is evident as the peak The influence of fluidization velocity temperature is being adjacent to the bed. At on axial temperature profile is plotted in excess air factor 1.2 the available oxygen figure 3. The temperatures of the bed and decreases and reaction rate decelerates. splashing zones are more uniform at higher Consequently, the amount of combustibles fluidization velocity due to the higher that burns inside the bed lessens while the rigorous mixing of bed particles. However, peak temperature becomes higher. The same the freeboard temperature becomes higher trend and justification may be referred to with fluidization velocity increase and flue when decreasing excess air to 1.1. It is worth gases leave the combustor at higher noting that the temperature profile ends temperature as well. It is obvious that higher for lower excess air factor. Under the increasing fluidization velocity enlarges the condition of lower excess air, the reaction volatile combustion in freeboard zone and rate is slower. Accordingly, the flame shifts the peak temperature up along the stretches and the combustion process combustor height. These results should be continues releasing significant heat along the ascribed to the shorter gas residence time in combustor height. the bed. It is the average time period that a unit volume of gas remains in the bed and is Gaseous Emissions defined as the ratio of the expanded bed From environmental point of view, it height to the fluidization velocity. is important to determine different emissions Figure 4 shows the influence of bed issuing from the combustion process. height on the axial temperature profile. The Keeping the emissions under the freeboard temperature becomes higher while recommended national limits becomes mandatory. Measurements of CO and NOx bed temperature improves the combustion have been reported at different operation process and reduces the carbon monoxide. conditions. The reduction is significant at lower part of temperature range. For higher bed Carbon monoxide temperature more than 850 oC, the reduction The influences of operating of CO becomes insignificant. It is well parameters on the carbon monoxide known that the temperature rise increases the concentration in flue gases have been plotted rate of chemical reaction. Moreover, in figures (6-9). CO concentration is found to diffusion of gases somewhat enhances with be reduced with decreasing fluidization temperature rise. These may be the reasons velocity, increasing bed height, increasing of CO reduction with bed temperature rising. bed temperature and increasing excess air Figure 9 illustrates the influence of factor. Excess air factor has the highest excess air factor on carbon monoxide impact on CO whereas the bed height has the concentration. The figure shows that lowest effect. reducing excess air factor less than 1.2 has Figure 6 presents the influence of high impact on the combustion process fluidization velocity on CO concentration. where high level of CO escapes the The concentration of CO increases with combustor. At excess air factor 1.1 the fluidization velocity. At the lower range of carbon monoxide concentration records fluidization velocity, up to 0.5 m/s, The about 1550 ppm. On the other side measured CO concentration is in the range increasing excess air factor more than 1.25 (175-240 ppm) and the increment is low. does not lead to further considerable However, at higher range of fluidization reduction in CO concentration. velocity, the increment multiplies resulting in relatively high concentration of CO in flue Nitrogen Oxides, NOx gases. This trend of results should be Oxides of nitrogen from biomass attributed to the residence time effect on fluidized bed combustion originate almost combustion processes. The residence time exclusively from fuel bound nitrogen. Once either in bed or in freeboard becomes longer the volatiles are released, the nitrogen amine at lower values of fluidization velocity. fragments (-NH2) either oxidize to NO or are Hence CO and O2 have prolonged time for stripped of the hydrogen atoms and form N2. diffusion, mixing and reaction to produce The percentage oxidized is a function of the carbon dioxide. Further, lower fluidization combustion environment, with volatile velocity produces smaller bubbles which nitrogen conversion to NO ranging from 8% have slower rising velocity. These to 40% depending upon combustion consequences enhance mass transfer between technology and combustion conditions . bubble and emulsion phases improving Measurements of NOx emission in combustion processes. At lower fluidization flue gases have been carried out and velocity almost all CO converts to CO2 inside presented against the operating parameters in the combustor. figures 6-9. The NOx level is relatively low The influence of bed height on CO ranging from 175 to 270 ppm. The figures concentration is plotted in figure 7. As shown indicate that at high level of the studied in the figure CO concentration slightly operating parameters (fluidization velocity, reduces with bed height increase. Increasing bed height, bed temperature and excess air), bed height improves slightly the combustion NOx emission rises with different degrees. It process due to the longer residence time appears that the concentrations of NOx have inside the bed. converse trend comparing to CO In figure 8, CO concentration is concentrations (except in the case of presented as a function of the bed fluidization velocity). In other words, the temperature. It is obvious that increasing the conditions that improve the combustion process results in higher level of NOx Figures 10b-10d demonstrate that the concentration. It is likely that carbon carbon loss slightly reduces with increasing monoxide reduces NO to elemental nitrogen bed height, bed temperature and excess air. via the following mechanism [15,16]: As discussed above, the high levels of these 2CO 2 NO 2CO 2 N 2 operating parameters intensify the Moreover, under the conditions that intensify combustion process. Accordingly, the the combustion process the concentration of concentration of chars lessens in the bed. carbon char in the bed lessens. Consequently, Moreover, entrained char fines have smaller the rate of reduction of NOx by carbon char size by virtue of higher combustion rate. decreases via the following process [15,16]. 2C 2NO 2CO N 2 Combustion Efficiency An important objective of this In figure 6, higher fluidization investigation is to assess the conditions under velocity results in rising CO concentration which rice straw could be burned efficiently that should ameliorate NOx reduction, in fluidized bed. Combustion efficiency is however, NOx emission increases slightly calculated on an energy basis assuming that with fluidization velocity. The results seem the fuel hydrogen is completely burned. It is to have some contradiction. The mathematically determined according to the contradiction can be elucidated when following expression. considering the role of gas residence time. At higher fluidization velocity, gases have M f (HV ) F M CO (12 / 28 )( HV ) CO CO 2 M cl (HV ) C CO 2 shorter residence time inside the combustor. M f (HV ) F Thus NOx have shorter time for reduction process. The obtained result is the sum of these two counteracting consequences. Where MF is the feeding rate of fuel, Excess air has been found to have the (HV)F is the heat value of fuel, MCO is the highest impact on NOx emission. Variation mass rate of carbon monoxide in flue gases, of excess air has two reinforced combined (HV)CO→CO2 is the heat value of carbon effects. Increasing the excess air multiplies monoxide when burned to carbon dioxide , the formation of NOx due to high level of Mcl is the mass rate of elutriated fixed carbon available oxygen. Moreover, the combustion and (HV)C→CO2 is the heat value of carbon process enhances and the reduction of NOx when completely burned to carbon dioxide via the two mechanism mentioned above Influences of operating parameters on decelerates. calculated combustion efficiency have been illustrated in figures 11. Among the studied Fixed Carbon Loss parameters fluidization velocity has the The particulates elutriated by flue greatest impact on combustion efficiency. gases have been collected using a cyclone. The drop in combustion efficiency at higher The collected materials have been weighed fluidization velocity is mainly attributed to and analyzed. The obtained data have been increasing carbon loss within the elutriated shown in figures 10 where the carbon loss char and partially to carbon monoxide, see ratio is plotted against operating parameters. figures 6 and 10a. Combustion efficiency The carbon losses ratio is calculated as the improves with increasing bed height, bed ratio between the rate of collected carbon in temperature and excess air, as shown in cyclone to the feed rate of fuel fixed carbon. figures 11b-11d. Increasing these parameters Fluidization velocity has the highest results in reducing carbon monoxide and impact on the carbon loss, see figure 10a. carbon losses as illustrated in figures (7-9) When fluidization velocity increases the drag and (10b-10d), respectively. force increases and coarser particulates can be entrained with flue gases. CONCLUSIONS The assistance of Ms. A. Silvestre, IRC- Rice straw has been successfully burned CNR, Naples, Italy, in chemical analysis is in a bubbling fluidized bed. Fuel feeding, greatly appreciated. fluidization behaviour and combustion processes are stisfactory. Based on the REFERENCES obtained experimental results the following 1. Werther, J., Saenger, M., Hartge, conclusions may be drawn: E.U., Ogada, T. and Siagi, Z., A simple technique has been utilized “Combustion of Agricultural to convert rice straw into form of Residues”, Prog. in Energy and pellets which is suitable for feeding Comb. Sci. 26, 2000, p. 1-27. and burning. The pellets have a bulk 2. Mory, A., Tauschnitz, J., “Co- density, at least, 10 times more Combustion of Biomass in Coal-Fired compared to its primary state. Power Stations”,VGB A considerable amount of heat is Kraftwerkstechnik, 79 (1), 1999, p. released in the freeboard zone due to 65-70. the post-combustion of volatiles. The 3. Adanez, J., de Diego, L. F., Gayan, peak temperature occurs in the P., Labiano F., L., Cabanillas, A. and freeboard zone. Its degree and Bahillo, A, “Co-Combustion of location are dependent on operating Biomass and Coal in Circulating conditions. Fluidized Bed: Modeling and Burning of rice straw has been Validation”, 17th Inten. FBC realized with combustion efficiency Conference, 2003, FBC2003-064, above 96% over a wide range of Florida, USA. operating conditions. Combustion 4. Hiltunen, M. and Vilokki, H, “Green efficiency is found to be improved Energy from Wood-Based Fuels with increasing bed height, bed Using Foster Wheeler CFB Boilers”, temperature and excess air. 17th International FBC Conference, Fluidization velocity has a negative 2003, FBC2003-136, Florida, USA. impact on carbon loss and 5. Hulkkonen, S., Fabritius, M and combustion efficiency. Enestam, S,”Application of BFB The NOx level is relatively low Technology for Biomass Fuel: ranging from 175 to 270 ppm. The Technical Discussion and studied operating parameters Experiences from Recent Projects”, (fluidization velocity, bed height, bed 17th Internat FBC Conference, 2003, temperature and excess air) somewhat FBC2003-132, Florida, USA. promote NOx emission. 6. Kokko, A. and Nickull, S., 2003, “The First Operational Experience of NOMENCLATURE World’s Largest Biofuel Fired CFB”, EA excess air factor 17th International FBC Conference, Hbs static bed height, (m) FBC2003-034, USA. Tb bed temperature, (K) 7. McCann, D., “Design Review of u fluidization velocity, (m/s) Biomass Bubbling Fluidized Bed Umf minimium fluidizition velocity, (m/s) Boilers”, 14th International FBC Conference, Vol. 1, 1997, pp. 29-37, Vancouver, Canada. ACKNOWLEDGEMENT 8. El-Raie, A. S. and El-Zahaby, A. M., This work is a part of a research project “Utilization of Some Field Crops under the title “Utilization of Biomass Waste Residues” Rationalization of Energy in Energy Production Using Fluidized Bed in Agric. Conference, 1999, Technology. The project is financially Mansoura, Egypt. support by Mansoura University. 9. Okasha F., El-Emam S. H. and Comparison between Pilot Scale Mostafa, H. K, "The Fluidized Bed Experiments and Model th Combustion of Heavy Liquid Fuel Simulations”, 17 International FBC (Mazut)", Experimental Thermal and Conference, 2003, FBC003-2003, Fluid Science Journal, Elsevier Florida, USA. Science Inc, 2003, Volume 27, Issue 14. Tillman, D.A., “The Combustion of 4, pp. 473-480. Solid Fuels and Wastes” Academic 10. Leckner, B., Andersson, B., Vijil, J., Press, 1991, San Diego, California. “Fluidized Bed Combustion of Coal f 15. Loِ fler, G., Andahazy, D., Wartha, and Biomass”, Eng. Foundation C., Winter, F., Hofbauer, H., “NOx Conference, 1984, Davos. and N2O Formation Mechanisms - a 11. Guanyi, C., Mengxiang, F, Detailed Chemical Kinetic Modeling Zhongyang, L., Xuantian, S, Kefa, C. Study on a Single Fuel Particle in a and Mingjiang, N, “Experimental Stationary Fluidized Bed”, 16th Research on Rice Husk Combustion International FBC Conference, 2001, in CFB Boiler and the Design of a 35 FBC01-0068, Reno Nevada. T/H Rice Husk Fired Boiler”, 14th 16. Kilpinen, P., Kallio, S. and Hupa, M., International FBC Conference, 1997, “Advanced Modeling of Nitrogen Vol. 1, pp. 175-181, Vancouver, Oxide Emissions in Circulating Canada Fluidized Bed Combustors: 12. Preto, F., “Combustion of Wood Parametric Study Of Coal Processing Residues in a Circulating Combustion And Nitrogen Fluidized Bed”, 17th International Compound Chemistries”, 15th FBC Conference, 2003, FBC2003- International FBC Conference, 1999, 171, Florida, USA. FBC99-0155, Savannah, Georgia. 13. Miccio, F., Scala, F. and Chirone, R, “FB Combustion of a Biomass Fuel: Table 1. Analysis results of rice straw Proximate Analysis (as received) Moisture, % 8.9 Volatile matter, % 63.13 Fixed carbon, % 18.1 Ash, % 9.87 Ultimate Analysis (dry basis) Carbon, % 42.04 Hydrogen, % 6.26 Nitrogen, % 1.23 Sulphur, % 0.64 Oxygen, % 39 Ash 10.83 Lower calorific value, kJ/kg 19441 1. fluidizing column 2. gas distributor Fuel sand 3. compressor 4. cyclone to chimney 5. air tank 6. flow meter 9 4 7. control valve to gas 8. balance analyzer 9. paddle shaft 1 cooling 10. blower water 7 2 6 6 water 5 bed drain 3 10 LPG Figure 1. Schematic of test rig Figure 2. A photograph of rice straw pellets 950 950 900 900 Temperature, C Temperature, oC o 850 850 800 800 = Static bed height 20 cm Fluidization velocity =0.7 m/s = Static bed height 30 cm 750 750 Fluidization velocity =0.5 m/s = Static bed height 40 cm Fluidization velocity =0.3 m/s 700 700 0 50 100 150 200 250 300 0 50 100 150 200 250 300 Height above distributor, cm Height above distributor, cm Figure 3. Influence of fluidization velocity Figure 4. Influence of bed height on axial temperature profile on axial temperature profile (Hbs=30 cm & EA=1.2) (u=0.5 m/s, Tb=850 oC & EA=1.2) 950 700 600 CO 900 NOx CO & NOx, ppm 500 temperature, oC 850 400 300 800 Excess air factor=1.1 200 Excess air factor=1.2 750 Excess air factor=1.3 100 700 0 0 50 100 150 200 250 300 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Height above distributor, cm Fluidization velocity, m /s Figure 5. Influence of excess air on axial Figure 6. Influence of Fluidization temperature profile velocity on CO and NOx emissions. (u=0.5 m/s, Hbs=30 cm &Tb=850 oC) (Hbs=30 cm, Tb= 850 oC & EA=1.2) 350 1400 300 CO 1200 NOx 250 CO&NOx, ppm 1000 CO & NOx, ppm 200 800 150 600 100 400 CO NOx 50 200 0 0 15 20 25 30 35 40 45 700 750 800 850 900 950 Static bed height, cm Bed Temperature, oC Figure 7. Influence of static bed height Figure 8. Influence of bed temperature on CO and NOx emissions. on CO and NOx emissions. (u=0.5 m/s, Tb= 850 oC and EA=1.2) (u=0.5 m/s, Hbs= 30 cm & EA=1.2) 1800 CO 1500 NOx CO & NOx, ppm 1200 900 600 300 0 1.05 1.1 1.15 1.2 1.25 1.3 1.35 Excess air factor Figure 9. Influence of excess air on CO and NOx emissions. (u=0.5 m/s, Hbs= 30 cm & Tb= 850 oC ) 14 14 12 u=0.5 ms-1,Tb=850 oC & EA=1.2 12 Hbs=30 cm Tb=850 oC 10 Carbon loss, % 10 EA=1.2 Carbon loss, % 8 8 6 6 4 4 2 2 0 0 0.2 0.4 0.6 0.8 15 25 35 45 Fluidization velocity, m /s Static bed height, cm (a) (b) 14 14 -1 -1 o 12 u=0.5 ms , Hbs=30 cm &EA=1.2 12 u=0.5 ms , Hbs=30 cm & Tb=850 C 10 10 Carbon loss, % carbon loss, % 8 8 6 6 4 4 2 2 0 0 700 750 800 850 900 950 1.05 1.1 1.15 1.2 1.25 1.3 1.35 o Excess air factor Bed temperature, C (c) (d) Figure 10. Influence of operating parameters on carbon loss 100 100 -1 99.5 u=0.5 ms , Tb=850 oC & EA=1.2 99.5 Hbs=30 cm, Tb=850 oC & EA=1.2 Combustion efficiency, % Combustion efficiency, % 99 99 98.5 98.5 98 98 97.5 97.5 97 97 96.5 96.5 96 96 0.2 0.4 0.6 0.8 15 25 35 45 Fluidization velocity, m /s Static Bed Height, cm (a) (b) 100 100 -1 o 99.5 -1 u=0.5 ms , Hbs=30 cm & Tb=850 C u=0.5 ms , Hbs = 30 cm & EA=1.2 99.5 Combustion efficiency % Combustion efficiency, % 99 99 98.5 98.5 98 98 97.5 97.5 97 97 , 96.5 96.5 96 700 750 800 850 900 950 96 Bed temperature oC , 1.05 1.1 1.15 1.2 1.25 1.3 1.35 Excess air factor (c) (d) Figure 11. Influence of operating parameters on combustion efficiency