Effect of co-combustion of chicken litter and coal on emissions in a laboratory-scale fluidized bed combustor

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1 FUEL PROCESSING TECHNOLOGY 89 (2008) Effect of co-combustion of chicken litter and coal on emissions in a laboratory-scale fluidized bed combustor Songgeng Li, Andy Wu, Shuang Deng, Wei-ping Pan Institute for Combustion Science and Environmental Technology, Western Kentucky University, Bowling Green, KY, USA ARTICLE INFO Article history: Received 1 October 2006 Received in revised form 20 May 2007 Accepted 20 June 2007 Keywords: Chicken litter Co-combustion Fluidized bed Emissions ABSTRACT Co-combustion of chicken litter (CL) with coal was performed in a laboratory-scale fluidized bed combustor to investigate the effect of CL combustion on pollutant emissions. The emissions of major gaseous pollutants including CO, SO 2,H 2 S and NO and temperature distribution along the combustor were measured during the tests. Effects of CL fraction and secondary air on combustion characteristics were studied. The experimental results show that CL introduction increases CO emissions and reduces the levels of SO 2. The ratio of H 2 S/ SO 2 increases with increasing fraction of CL. NO emissions either increase or decrease depending on the percentage of CL in the mixed fuels. The temperature in the freeboard region increases with increasing the fraction of CL while the reverse is true for the bed temperature Elsevier B.V. All rights reserved. 1. Introduction Litter from poultry farms is traditionally used in land applications as a fertilizer because it is rich in nutrients. But overapplication of this material could lead to an overabundance of water nutrients resulting in eutrophication of water bodies, the spread of pathogens, the production of phytotoxic substances, air pollution and emission of greenhouse gases [1 3]. One of the alternative disposal methods is direct combustion with the potential to provide a cost-effective, environmentally benign disposal route for the litter while providing for both space heating of poultry houses and large-scale schemes involved power generation or combined heat and power. However, the high moisture and ash contents as well as low heating value of the poultry litter could arouse problems on maintaining steady and complete combustion of poultry litter alone, as indicated in the literatures [1,4]. Therefore, co-firing poultry litter with coal is considered as a feasible means. Several investigations on this aspect have been reported [4 6]. These research efforts have shown that co-combustion could address the energy supply issues and aid in the solution of air pollution control problems. Compared with the conventional combustion technology, the fluidized bed combustion is considered as an optimal technology to dispose the animal waste with energy recovery due to its ability to accept fuels with a relatively high ash and moisture content, the low cost associated with fuel preparation, operational flexibility with regard to ash collection and easy control for pollutant emissions [7]. However, the use of poultry litter as a secondary fuel in a fluidized bed combustor has not been yet investigated extensively. The purpose of this paper is to present the results obtained from an experimental study on co-combustion of chicken litter (CL) and coal in an atmospheric bubbling fluidized bed combustor with respect to the emissions characteristics of Corresponding author. Current address: Department of Chemical and Biomolecular Engineering, Ohio State University, 140 W 19th Ave. Columbus, OH43210, USA. Tel.: ; fax: address: songgengli@hotmail.com (S. Li) /$ see front matter 2007 Elsevier B.V. All rights reserved. doi: /j.fuproc

2 8 FUEL PROCESSING TECHNOLOGY 89 (2008) 7 12 carried out on the gas stream exiting from the cyclone. Concentrations of SO 2, CO and H 2 S are measured with on-line infrared gas analyzers. NO x concentration is measured with an on-line chemiluminescence detector. O 2 concentration is monitored using a paramagnetic sensor. All data is displayed and logged on a PC via data acquisition unit. A K-type thermocouple is employed to measure the temperature within the combustor Experimental procedure To attain the steady-state combustion and stable bubbling mode, it is essential to heat up the inert bed material above the ignition temperature of the fuel. After the required temperature was reached, the fuel was slowly fed into the bed. The duration of the test run was about 4 h, out of which h period was used to reach the steady state condition. The steady state condition criteria were to have steady bed temperature and steady pressure drop. After steady state was reached, flue gas composition was measured. The cyclone were emptied and cleaned after each test, and fly ash samples were collected Characteristics of fuels Fig. 1 Schematic diagram of the laboratory-scale fluidizedbed combustor. gaseous pollutants including CO, SO 2,H 2 S and NO x. Specially, the influence of CL mass fraction in the mixture and secondary air is described. 2. Experimental 2.1. Experimental set-up All the experiments were carried out in an atmospheric bubbling fluidized bed combustor, as illustrated in Fig. 1. The combustor is a stainless steel pipe of 76 mm I.D. and 1.2 m in height,placedinanelectricallyheatedovenconsistingof total four silica carbon rods that preheat and make up for the heat loss from the bed. Fluidization air is introduced into the bed through a distributor, which is 10 mm thick stainless steel perforated plate with openings of 1 mm. A secondary air nozzle, which is a series of small holes arranged on a ring of 1/4 inch stainless steel tube, is located above the bed surface in order to promote the mixing of air with fuel. The fuel mixture is fed into the bed through a screw feeder. A minor compressed air is introduced into the fuel silo in order to avoid the flue gas back flow. The bed height can be controlled by adjusting the inserted height of the discharge tube from the bottom. Bed materials including the fuel ash and inertial sand come out of the combustor through the discharge tube once the bed height exceeds the height of the discharge tube inside the combustor. The pressure right before the air distributor is measured with a pressure transducer to monitor the quality of fluidization. The air is regulated by mass flow controllers based on the thermal mass flow sensing technique. The analysis of the flue gas is The ultimate and proximate analysis of both CL and coal are presented in Table 1. CL includes sawdust, wood chip and fecal matter. As a fuel, it has lower heating value equivalent to low rank coal (on the order of 5000 BTU/lb) due to high moisture, oxygen and ash content. High level of volatile matter and very little of fixed carbon imply that most of combustion for the litter takes place in the gas phase. The coal used in this experiment is a bituminous coal with relatively higher sulfur and lower chlorine. Table 2 shows the ash composition of both fuels. Compared with the coal ash, the ash of CL contains relatively high Ca, K and Na and low Si. This probably indicates that chicken litter ash has relatively lower fusion temperature and higher sulfur-retention ability. 3. Results and discussion The experimental conditions of the co-combustion tests are shown in Table 3. In this experiment, baseline data was first Table 1 Characteristics of the fuels used Parameter CL Proximate analysis (%) Moisture Ash Volatile matter Fixed carbon Ultimate analysis (%,dry) Carbon Hydrogen Oxygen Nitrogen Sulfur Ash Miscellaneous analysis Chloride(ppm) Btu(1b)

3 FUEL PROCESSING TECHNOLOGY 89 (2008) Table 2 Ash compositions of the fuels used Sample name Ash compositions (%) SiO 2 Al 2 O 3 Fe 2 O 3 CaO MgO Na 2 O K 2 O P 2 O 5 TiO 2 SO CL obtained for firing the coal alone and then co-firing tests at CL mass fractions of 10%, 25% and 50% were performed. The total calorific input of the fuel was maintained almost constant by regulating the total feed rate, ensuring that similar combustion conditions existed within the combustor. The coal was physically mixed with the CL. The furnace temperature and combustion air stoichiometry were kept at 800 C and 1.3, respectively, for all the tests. The ratios of secondary air/total air were controlled at around 0.3. The test with no secondary air was also performed in the case of 50% CL to make comparison Temperature profile Fig. 2 shows the axial temperature distributions observed along the combustor height. It can be seen that the temperature in the bed region is maintained in the range of C, and the temperature in the freeboard region varies in the range of C. The temperature drops slightly in the secondary air injection area owing to the secondary air consuming energy and increases significantly above the area because the unburned particles and volatile matter are burned with the contribution of secondary air. In addition, in the freeboard region except for the secondary air injection area, the temperature for the cocombustion is higher than that for the coal combustion alone, while the reverse is true for the temperature in the bed region. In fact, the higher the fraction of CL is, the higher is the freeboard temperature. This temperature variation indicates that CL is burned mostly in the freeboard region. This observation can be explained by the fact that CL has high volatile matter and low fixed carbon content CO emissions The emissions of CO as a function of CL mass fraction are shown in Fig. 3. It is obvious that CO emissions increase with an increase of CL mass fraction. Three reasons can be invoked to account for this: (i) Higher amount of volatile matter released from CL boosts hydrocarbon concentration in the combustion atmosphere, which inhibits the further oxidation of CO. Normally, oxidation of CO occurs through reactions (1) and (2) with oxygenated free radicals HO and HO 2 [8]. These radicals react easier with hydrocarbons than with CO. CO þ OH CO 2 þ H CO þ HO 2 CO 2 þ HO (ii) CL contains high level of chlorine. The formation of HCl inhibits the oxidation of CO through consuming the radicals OH and HO 2 [9]: HO 2 þ Cl HCl þ O 2 H þ Cl þ M HCl þ M ð1þ ð2þ ð3þ ð4þ Table 3 Experimental conditions of co-combustion tests HCl þ OH H 2 O þ Cl ð5þ Run number Blend fuel 1 0% 100% 2 10% 90% 3 25% 75% 4 50% 50% 5 50% 50% Feed rate (Kg/h) Furnace temperature ( C) Excess air (%) Secondary air/total air (iii) Unburned volatile matter constitutes an additional CO source. Fig. 2 Temperature profile along the combustor height.

4 10 FUEL PROCESSING TECHNOLOGY 89 (2008) 7 12 Fig. 3 CO emissions with secondary air. Fig. 5 Fuel-S conversion rates with secondary air SO 2 and H 2 S emissions SO 2 emissions normally correlate strongly with sulfur content of the fuel. As mentioned above, CL has low sulfur content in comparison to the coal. CL addition actually dilutes the sulfur content of the fuel. Therefore, SO 2 emissions decrease with an increasing CL share as shown in Fig. 4. Fig. 5 presents the percentage of fuel-s conversion to SO 2 as a function of CL fraction. Obviously, fuel-s conversion decreases with increasing CL fraction. Two reasons could cause this fact. Firstly, CL ash has strong retention for sulfur due to relatively large amount of Ca and Mg present in CL ash. This is evidenced by the plot of the sulfur content in the collected fly ash with CL fraction in Fig. 6, where shows that there is an increase in sulfur content of fly ash as CL fraction increases. Secondly, high volatiles CL creates strongly reducing atmosphere above the bed that inhibits the oxidation of H 2 StoSO 2. The variation of H 2 S/ SO 2 ratiowithclfraction(showninfig. 7)stronglysupportsthis point. From Fig. 7, it was found that the ratio of H 2 S/SO 2 increases with a decrease in CL fraction. H 2 Semissionsincrease when a little chicken litter is introduced and decrease with further increasing CL mass fraction due to low fuel-s input NO emissions Under fluidized bed conditions, NO and N 2 O emissions originate from the nitrogen of the fuel, so the formation of thermal NO x is considered insignificant [10,11]. Nitrogen in CL is mainly released as NH 3 [1], which could reduce the NO x produced from within the bed to N 2 according to reaction (6), or further give NO x according to reaction (7) [12]. 2NO þ 4NH 3 þ 2O 2 3N 2 þ 6H 2 O 4NH 3 þ 5O 2 4NO þ 6H 2 O Fig. 8 presents NO emissions as a function of CL mass fraction. It was observed that NO emissions follow an increasing trend in the case of 10% and 25% CL because of higher nitrogen contained in CL. However, further increasing the percentage of CL (in the case of 50% CL), NO emissions reduce. The probable reason is that in low percentage of chicken litter, the volatile matter released in the freeboard could not create strongly reducing atmosphere to suppress the formation of NO x via reaction (7). With increasing chicken litter share up to a certain extent, volatiles evolved from the fuel produce instantaneous fuel-rich condition in the freeboard region, which leads to the reduction of NO x via reaction (6) Effect of secondary air In order to elucidate the effect of secondary air on pollutant emissions, the test without secondary air introduced into ð6þ ð7þ Fig. 4 SO 2 emissions with secondary air. Fig. 6 Sulfur content in the collected fly ash with secondary air.

5 FUEL PROCESSING TECHNOLOGY 89 (2008) Fig. 7 H 2 S emissions and ratio of H 2 S/SO 2 with secondary air. Fig. 9 Effect of secondary air on pollutant emissions in the case of 50% CL. the freeboard region was conducted in comparison with above-mentioned experimental results. This means that all the air is introduced as fluidizing air. The obtained results are shown in Fig. 9. As expected, CO emissions significantly increase owing to the insufficient mixing of fuel with air. The increment of SO 2 emissionsisattributedtothelessgas solid contact in the freeboard region that makes against the retention of SO 2 by ash. There is no reducing zone formed in the bed region since all the air is introduced as fluidizing air. Hence, the level of NO with no secondary air introduction increases. According to these results, it can be concluded that secondary air should be used and effectively organized to improve the combustion efficiency and decrease the pollutant emissions. 4. Conclusions Co-combustion tests of coal with CL have been performed in a laboratory-scale fluidized bed combustor to study the effect of CL mass fraction and secondary air on the combustion and emission characteristics of the major gaseous pollutants. Major conclusions from the test results can be summarized as follows: As CL mass fraction increases, the bed temperature decreases and the temperature in the freeboard region increases. This is attributed to low fixed carbon and high volatile matter contained in CL. The introduction of high volatile matter CL causes CO emissions to be increased. SO 2 emissions are lowered by addition of CL as a result of fuel-s dilution and CL ash derived natural desulfurization. Ratio of H 2 StoSO 2 increases with a decrease of CL mass fraction because high volatile matter released from CL creates a strong reducing atmosphere that suppresses the oxidation of H 2 S. Introduction of CL at low concentrations causes higher NO emissions because more fuel-n is introduced. However, high levels of CL may reduce NO emissions due to the larger amount of released volatile matter, which suppresses the formation of NO. Secondary air introduction contributes to lower pollutant emissions. Acknowledgement The authors wish to thank the United State Department of Energy (No. DE-FC26-03NT41840) and the United State Department of Agriculture (No S) for their financial support of this project. REFERENCES Fig. 8 NO emissions as a function of CL mass fraction with secondary air. [1] B.P. Kelleher, J.J. Leahy, A.M. Henihan, T.F. O'Dwyer, D. Sutton, M.J. Leahy, Bioresource Technology 83 (2002) 27. [2] H.L. Brodie, L.E. Carr, P. Cardon, Biocycle 41 (2000) 36. [3] R.L. Siefert, J.R. Scudlark, A.G. Potter, K.A. Simonsen, K.B. Savadge, Environmental Science and Technology 38 (2004) [4] J. Sweeten, K. Annamalai, B. Thien, L. McDonald, Fuel 82 (2003) 1167.

6 12 FUEL PROCESSING TECHNOLOGY 89 (2008) 7 12 [5] K. Annamalai, B. Thien, J. Sweeten, Fuel 82 (2003) [6] S. Zhu, S.W. Lee, Waste Management 25 (2005) 511. [7] J. Nelson, Renewable Energy 5 (1994) 5. [8] L.A.C. Tarelho, M.A.A.A. Matos, F.J.M.A. Pereira, Fuel 84 (2005) [9] E. Desroches-Ducarne, E. Marty, G. Martin, L. Delfosse, Fuel 77 (1998) [10] T. Furusawa, T. Honda, J. Takano, D. Kunii, Journal of Chemical Engineering of Japan 11 (1978) 377. [11] J.M. Beer, A.F. Sarofim, Y.Y. Lee, Journal of the Institute of Energy 54 (1981) 38. [12] W.G. Lin, K.D. Johansen, Proceedings of the Fourth International Conference on Technologies and Combustion for a Clean Environment, Lisbon, Portugal, 1997, pp