Effect of Operating Conditions on the Combustion Characteristics of

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Effect of operating conditions on the combustion characteristics of coal, rice husk, and co-firing of coal and rice husk in a circulating fluidized bed combustor Prasan Sathitruangsak, and Thanid Madhiyanon Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01513 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 25, 2017

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Effect of operating conditions on the combustion characteristics of coal, rice husk, and cofiring of coal and rice husk in a circulating fluidized bed combustor

P. Sathitruangsak a, T. Madhiyanon a,*

a

Department of Mechanical Engineering, Faculty of Engineering, Mahanakorn University of

Technology, Bangkok 10530, Thailand

Abstract This study was performed to investigate the combustion characteristics of coal and rice husk and their co-combustion in a 100 kW circulating fluidized bed (CFB) combustor (dia.: 150 mm, height: 6000 mm). Gaining insights into the effects that the primary and secondary excess air ratios (λPA and λSA), biomass fraction, and Secondary air (SA) position have on CO and NOx emissions was also an aim of this study. In contrast to coal combustion, and co-combustion of coal and rice husk, the combustion of rice husk could not be sustained without the aid of an external heat source, and the released volatiles were found to be fired chiefly above the bottom bed. Air-staging substantially decreased NOx (40-75% reduction), but with a high CO penalty, compared to no airstaging. However, the effect of the SA position which was profoundly engaged with the char inventory in the bed was pronounced for coal combustion but not for rice husk combustion. Increase in λPA and λSA tended to reduce CO levels but enhanced NOx formation. The devolatilization and combustion of small eluding rice husk particles were found in the upper zone, despite being fed in pellet form. For co-combustion, the use of more rice husk resulted in higher CO emissions. Although

___________ * Corresponding author. Tel.: +66(0)-2988-3666 Ext. 3107; fax: +66(0)-2988-3655 Ext. 3106. E-mail address: [email protected]; [email protected] (T. Madhiyanon).

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rice husk contains half the nitrogen content of coal, co-firing it with coal gave higher NOx emissions when a higher fraction of rice husk was included in the mixed fuel. Keywords: Air-staging; Biomass; Char reduction; CFB; Co-combustion; Emissions

1. Introduction To intensify the battle against global warming, the United Nations climate change conference (COP 21) held in 2015 in Paris reached an agreement to limit the temperature increase to less than 2°C in 2100 compared to pre-industrial levels. Since biomass fuels are renewable energy and CO2-neutral resources and because among the common thermochemical conversion technologies (i.e., combustion, gasification and pyrolysis), combustion has proven to be the most efficient technology for heat and power production1, the combustion of biomass alone or its co-combustion with coal can be used as a primary measure to address the climate change due to the greenhouse gas effect. Several studies have investigated the effect that the operating parameters have on gaseous emissions from both standalone combustion and co-combustion using circulating fluidized bed (CFB) combustion technology. Leckner and Karlsson2 investigated the emissions of NO, N2O, CO, and SO2 during the combustion of wood and its co-combustion with coal in a 12 MW (thermal) CFB boiler. They stated that the secondary air (SA) injected along the height of the riser had no influence on NO reduction during wood combustion, except when SA was supplied at the cyclone outlet. The measure known as “reverse air-staging” was adopted by Lyngfelt et al.3 for coal combustion in a 12 MW CFB boiler to reduce N2O without having an adverse effect on other emissions. This method was later referred to as “late-air staging” and was employed by Lyngfelt and Leckner4 to decrease NO during the combustion of woodchips over three loads in the same boiler. A NO reduction of 20-70% relative to normal air-staging could be achieved without high CO emissions. They also found that the CO emissions were strongly responsive to the temperature in the exit chamber. Leckner et al.5 studied gas emissions from mono-combustion of coal, wood, and sludge, and their co-combustion in both a


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lab-scale CFB combustor and a 12 MW CFB boiler. Advanced air-staging (similar to late air-staging) appeared to be effective for NO and N2O reduction during coal combustion but led to high CO emissions compared to normal air-staging. This method, however, did not show any clear impact on the emissions from wood and sludge. Tourunen et al.6 investigated the trends of NO emissions for the temperature and O2 concentration in the dense bed for bituminous coal combustion in a 40-60 kW CFB reactor. Their results showed that the NO emissions above the dense bed decreased with a decreasing temperature and O2 concentration. This could be reasoned by the heterogeneous reactions between NO formed and char in the bed. Varol et al.7 studied the effect of the excess air ratio on gas emissions for the co-combustion of Turkish lignite coal and woodchips in a lab-scale CFB combustor (height: 6 m, inside diameter: 108 mm). The same research group8 investigated the effect of the secondary air ratio (SAR) and its location on gas emissions. The location of SA was found to play an important role in NO emissions. They added SA in the riser at a height of 1.42, 2.33, 3.24, and 4.15 m. Abelha et al.9 studied the relative amounts of NH3 and HCN released from coals and different residues, i.e., sewage sludge, RDF, and sawdust, during pyrolysis in a lab-scale bubbling fluidized bed reactor. The sewage sludge and RDF released significantly higher NH3/HCN ratios than did coal. Most of the nitrogen oxide precursor liberated from coal is HCN. The NH3 released from sewage sludge could reduce the NO that formed upon co-firing with coal. A review of kinetics of NO and N2O with char particles was presented by Aarna and Suuberg10 and Li et al.11. The generation and decomposition of NOx during fluidized bed combustion were also reported by Madhiyanon et al.12-13 and Sathitruangsak et al.14. Xie et al.15 examined the effect on the gas emissions of parameters such as biomass share, first stage stoichiometry (air staging), and total excess air ratio in a 30 kW CFB combustor for coal combustion and the co-firing of coal and rice husk. Air staging had a significant effect on NO emission for coal combustion but not co-combustion. They also reported that changing the fuel feeding position from riser to downcomer could dramatically decrease the NO emissions


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without an obvious change in SO2. However, the effect of SA location in the riser was not examined in their study. Rice husk and straw are the most important agricultural residues in terms of quantity. Rice is mostly cultivated in China, Vietnam and Thailand. In Thailand alone, 5 million tons of rice husk is annually generated, equivalent to 7.5×107 GJ. Although several studies have been conducted on coal, biomass, and the co-combustion of coal and biomass, studies on the mono-combustion of rice husk and the co-firing of rice husk with coal, particularly in CFB, have been relatively scarce. Therefore, the objectives of this study include clarifying the combustion characteristics via temperature profiles and the influence of operating parameters, i.e., primary and secondary excess air ratios as well as the rice husk shares on CO and NOx emissions in the mono-combustion of coal and rice husk and their co-combustion. The experiments were carried out in a 100 kW CFB combustor. In contrast to previous works, in an attempt to prevent rice husk particles from escaping the bottom bed to the upper zone, the mono-combustion of rice husk was performed with the rice husk in a pellet form. Additionally, this investigation aimed to explore the effect of SA injection position located along the dilute zone, where the combustion of volatiles primarily occurs.

2. Experiment section 2.1 Test facility The lab-scale CFB test system conducted in this study is schematically illustrated in Fig. 1. The test rig, shown in Fig. 1a, is composed of a circulating fluidized bed combustor with a thermal capacity of 100 kW, a fuel feeding system, an electrical air preheater, a secondary cyclone collector, two root blowers, and an induced blower. The combustor consists of a riser, with an 0.15 m inside diameter and 6 m height, connected to a high efficiency cyclone to recirculate the particles of unburned fuel and bed material back to the riser through a downcomer with an inside diameter of 0.10 m and an L-valve loop seal, respectively. The aeration tap was provided at the loop seal to purge


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the solid particles back to the riser. The combustion air was split into three streams: primary (fluidizing) air, secondary air, and aeration. The primary air (PA) was introduced through a nozzletype distributor plate to promote fluidization phenomena. As shown in Fig. 1b, the secondary air was selectively injected into the riser at 2.3, 3.3 or 4.5 m above the air distributor plate. The minimum fluidization velocity was 0.3 m/s, and the superficial gas velocity usually ranged from 5 to 7 m/s at a bed temperature (measured at 0.2 m height) generally between 850°C and 900°C. 2.2 Measurement The temperature distribution along the riser and downcomer was continuously measured by 9 shielded thermocouples of type K linked to a data acquisition unit with an accuracy of ±1°C. The flow rate of each air stream, including the aeration at the L-valve, was controlled by a valve and was measured by a calibrated Venturi meter co-operated with a differential pressure sensor (accuracy ±2%). The fuel feed rate was adjusted by a variable-speed inverter. The Testo 350XL multigas analyzer was used to measure the gas concentrations at the exit pipe. The measuring principle was based on electrochemical cells for CO, O2, NO, and NO2. To compare the experimental results with other references, all gas concentrations were corrected to a 6% O2 basis. The unburned carbon content in the fly ash sample was identified using a Leco C-H-N-S analyzer.

2.3 Fuel and ash characteristics The fuels employed in this study were bituminous coal and rice husk, and their proximate, ultimate and ash analyses are listed in Table 1. Coal, which was supplied by a domestic supplier in Thailand, was sized and sieved into the range of 3-5 mm. Pelletized rice husk with a cylindrical shape of 10 mm in diameter and approximately 20 mm in length was used in the mono-firing study. However, rice husk in a natural form was used to study its co-firing with coal. Silica sand with a


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mean diameter of 300 µm was employed as the bed material in the CFB combustor. The static bed height in the riser was approximately 40 cm above the distributor plate.

2.4 Experimental procedure The sand bed material of 22 kg was first loaded equally into the riser and downcomer. The system was then preheated by blowing the fluidizing air through the air preheater before introducing it into the riser. When the bed temperature (T1) reached approximately 400 °C, the fuel prepared in the hopper was supplied. The bed temperature increased gradually as a result of the self-ignition and combustion of in-bed fuel until reaching 700 °C; the desired feed rate was then adjusted. The duration of each test run was approximately 8 h, of which 3 h was spent to achieve the steady state condition. The fly ash collected at the secondary cyclone during a test was analyzed for the unburned carbon content. Experiments were performed to investigate the combustion of coal and rice husk and the co-firing of their mixtures. The effects of the primary air and secondary excess air ratios, secondary air injection positions, and rice husk shares in co-firing with coal were also studied. The experimental conditions and results are summarized in Tables 2-5.

3. Results and discussion 3.1 Temperature profiles along the CFB height The axial temperature profiles for some experiments are plotted in Figs. 2a-d to show the phenomena of CFB combustion. In general, all of the experiments (including others that are not shown) were performed in a similar manner, although with some differences in detail, depending on the operating conditions. The interface that divided the riser into a dense phase and a dilute phase was determined by marking the onset of temperature deflection. For example, in Fig. 2a, the temperature uniformity is visible between the T1 and T2 positions as a result of intensified


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combustion and the high internal circulation of solids (a well-mixing regime) there. In contrast, from the T2 to T3 positions, a gradual decrease in temperature was observed. This indicates a transition into a dilute phase at a 1.65-m height (T2 position), where the heat gain from the lean suspending solids was so small that it could not counterbalance the heat loss via the combustor wall. This was also partly due to the cooling effect of SA at a 2.3 m height. This finding for the transition level is supported by the study of Varol et al.8. There was an increase in temperature between the T1 and T2 positions, as shown in Fig. 2b, and it was attributed primarily to the combustion of volatiles above the feeding point. A transition point at a 1.65-m height also appears for the co-firing results shown in Fig. 2c and the 100% coal combustion results in Fig. 2d. In Fig. 2d, the SA injection levels had no impact on the existence of the transition point because the zone to which SA was added was far beyond the intense solid zone dominated by PA flow; thus, the additional SA could not cooperate with PA to expand the boundaries of the dense zone. The significant decline in temperature (50100°C) between T4 (3.65 m) and T6 (5.2 m) in all cases is associated with the balance of heat gain from the least suspended solids there, heat loss through the combustor wall and, to some extent, the cooling effect of SA. The persistence of the transitional point at a 1.65-m height closely related to a slight variation of the fluidizing velocity from 4.5-5.5 m s-1. The combustion characteristics of biomass, which inherently has a high volatile content, are given in Fig. 2b. An increase in temperature of 20-50°C from the T1 to T2 positions was obviously caused by burning some of the volatiles that were released during pyrolysis. This is, however, less pronounced in the cases of coal combustion alone (Figs. 2a, d) or even in co-combustion (Fig. 2c). In addition, evidence of volatile combustion was visually observed from the lively flames over the bed surface, which further illuminated the bed and occurred when the new incoming rice husk fell onto the bed surface. It is interesting to note that in contrast to coal, the heat from rice husk combustion itself could not sustain the bed temperature of >800°C due to low char inventory. Thus, a PA


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preheater had to be turned on for the duration of the rice husk tests. The quantitative and qualitative results mentioned above reveal an important characteristic of biomass combustion in CFB: char combustion occurs mostly in the turbulent bed, whereas volatile combustion occurs in the regions above the bottom bed. This encourages an air-staging measure for combustion dominated by volatiles. In other words, instead of introducing all of the combustion air into the bottom bed, it should be divided into PA for char combustion within the bed and SA for the complete burnout of the combustible gases escaping the primary zone.

3.2 Effect of excess air ratio, biomass share and SA position on bed temperature Because the bed temperature is a principal factor in combustion efficiency, emission control, and even bed agglomeration (which is beyond the scope of this study), the relationships between the dense-bed temperature (1/2(T1+T2)) and two operating parameters, i.e., total excess air ratio (λtotal) and biomass share, are plotted in Figs. 3a-c. In Fig. 3a, for standalone coal combustion (C7-12 in Table 2), λtotal was varied from 1.15 to 2.14 by changing the secondary excess air ratio (λSA) from 0.05 to 1.14 while keeping the primary excess air ratio (λPA) constant at 1.05. Interestingly, since the λPA was kept constant and the location of SA injection (2.3 m height) was far beyond the measurement point of the dense-bed temperature, the dense-bed temperature was unaffected by the increased λtotal; however, it seemed to decline with an increased λtotal. This can be explained by the upward and downward movement sequence of solids in the riser. After sending some heat to the ascending gases in the dilute region, the solids cooled before falling back into the bottom bed. Once they came into contact with the higher-temperature solids in the bed, the heat exchange develops in a way to balance the energy to decrease the overall bed temperature. Thus, the higher the SA flow is, the lower the temperature of solids recycled to the bed is and, hence, the lower the overall bed temperature is.


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As also shown in Fig. 3a, an λtotal between 1.2 and 1.6 for coal combustion with a near stoichiometric λPA (1.05) is recommended from an optimum operation perspective for common CFBs. This range responds to a dense-bed temperature between 800 and 900°C and is considered effective for NOx control. This range, however, may shift-down if the system operates with the PA preheater. In the same figure, it can be seen that the impact of λtotal on the dense-bed temperature is not as strong in the cases of rice husk combustion (R1-8) because the dense-bed temperature relied not only on the combustion heat but also on the artificial heat gain from the air preheater during rice husk combustion. Fig. 3b presents the dependence between the dense-bed temperature and rice husk share in the case of co-combustion (CR1-5 in Table 3). A total fuel mixture feed rate was held constant at 9.69 kg h-1, and the entire air flow was fed through the bottom bed (no air-staging). The dense-bed temperature decreased with an increase in rice husk mass fraction. This was associated with the connection between the rice husk share and the excess air ratio. Further, the stoichiometric air-fuel ratio of rice husk, 4.32, is less than that of coal, 6.89; therefore, the more the rice husk is blended, the lower the stoichiometric air-fuel ratio of the mixture is. Consequently, the excess air ratio increases with an increase in the rice husk fraction. Another relevant factor is the lower char inventory in the bed when more rice husk is blended, which results in less energetic combustion in the bed. A decrease in the dense-bed temperature is, therefore, actually a result of an increase in the excess air ratio and a decrease in the char concentrations in the bed. The same figure suggests that in order to ensure a dense-bed temperature >800°C, the rice husk fraction should not exceed 40% (wt.). An investigation of the influence of SA injection positions (2.3, 3.3, 4.5 m height) was performed under 100% coal combustion (C13-16 in Table 4), as shown in Fig. 3c. The λtotal, λPA, and λSA were held constant at 1.32, 0.96, and 0.31, respectively. It was found that air-staging (C14-16) by diverting the air into the dilute zone raised the dense-bed temperature compared to no air-staging


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(C13). The minimum is 830°C for no air-staging, and the maximum is 880°C for injecting SA at a 4.5-m height. In addition, the dense-bed temperature increased for three consecutive SA levels. This result can be explained by the inconsistent suspended-solid concentrations along with the axial height. That is, these concentrations decrease with height, which has a profound consequence for a number of solids that are cooled after being exposed to SA flow. Thus, the lower the level is when SA is added, the more solids are cooled by SA flow. As a result, more energy is expended for heating the cooled solids when returning to the bottom bed, thereby decreasing the overall bed temperature to a greater degree.

3.3 Effect of SA injection position on CO and NOx emissions Sixteen experiments, including cases of 100% coal and 100% rice husk combustion, were performed to examine the effect of SA injection positions on the emissions of CO and NOx. The SA was added to the dilute zone at heights of 2.3, 3.3 and 4.5 m above the air distributor. 3.3.1 100% coal firing Four experiments of coal combustion (C13-16 in Table 4) were set up under the same total excess air ratio (λtotal=1.32), primary excess air ratio (λPA=0.96) and secondary excess air ratio (λSA=0.31). These included three experiments each for three SA levels in sequence and one without SA addition (no air-staging). All of the results are shown in Figs. 4a-c. The O2 concentrations in Fig. 4a illustrate comparable consumption levels with little difference, 800°C (see section 3.1). All of the results are presented in Figs. 8a-c. The combustion left more O2 in the flue gas when more SA was supplied, Fig. 8a. The CO decreased as the λSA (or λtotal) increased, Fig 8b. However, no inverse point was observed as in the coal combustion (compared with Fig. 7b). The continuum of the CO decrease instead of increase against the increasing λSA was due to compensating for the heat from the heater with a drop in temperature caused by the cooling effect of SA flow. The NOx evolution, as shown in Fig. 8c, resembles coal combustion; that is, an increase in NOx relies heavily on the increasing λSA. The fuel-N conversion to NOx ranged from 3.5-8.5% for λtotal=0.95-1.60.

3.6 Co-combustion This section highlights the resulting emissions from the co-combustion of coal and rice husk under no air-staging regime, as presented in Figs. 9a-c, and the operating conditions are summarized in Table 3 (CR1-5). In fact, an increase in the rice husk fraction without changing the total amount of the mixture and air flow resulted in an increased λtotal. Figs. 9a-c thus exhibit the influence that both the rice husk share and excess air ratio have on the emissions. The mass fraction of rice husk to coal increases from 0, 10, 20, 30 and 40% (wt.), which allows the corresponding λtotal to vary between 1.43 and 1.68. It should be noted that rice husk in natural form was used in this co-combustion series. The O2 concentrations measured at the cyclone outlet, as expected, escalated as the rice husk share heightened due to the subsequent increase in λtotal, Fig. 9a. Fascinatingly, the trend observed in


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Fig. 9b illustrates an increment in CO as more rice husk was blended into the mixture, even with a higher corresponding λtotal, which would enable the better conversion of CO. This can be explained by the large amount of volatiles from rice husk, particularly of CO, that were released and not allowed to react with the available O2 in the upper zone, possibly due to a poor mixing process. The incomplete mixing permitted over-stoichiometric condition in name only; indeed, some of the combustion zones actually become oxygen-deficient. Another relevant factor is an occasional devolatilization and incomplete combustion in the upper zone of the small eluding rice husk particles from the bottom zone, as reflected by a large variation in CO emissions with a high rice husk share. The results showed that CO reached the minimum, 15 ppm, for coal combustion alone (λtotal=1.43) and reached the maximum, 340 ppm, for 40 wt% rice husk (λtotal=1.68). In the same figure, rice husk could be mixed with coal up to 30 wt% (λtotal=1.61) with reasonable CO emissions, 110 ppm. Since rice husk has approximately half the nitrogen content of coal, it would be expected that NOx would decrease when more rice husk was blended into the mixture. There was, however, an opposite trend, as shown in Fig. 9c. Indeed, coal and biomass produce light nitrogen species, i.e., NH3 and HCN, which are the well-known precursors of NOx formation, in a quantity commensurate to some extent with fuel-volatile matter9,16, and rice husk contains volatile matter in quantities comparable to coal (see Table 1). Accordingly, rice husk would release the NOx precursors in a lower quantity than coal. Furthermore, the increase in NOx with the increasing rice husk share can also be explained by an ineffective NOx reduction by char according to reactions (R.1-3) because less char accumulated in the bed and there was the unfavorable condition of an oxygen-rich atmosphere. So, from the above mention for co-firing and the resultant NOx emission during pure rice husk firing described earlier (in section 3.3.2), it would be implied that if the share of rice husk increases during co-combustion with coal, then there will be a lower amount of char in the bed, and thus lower potential for NOx reduction. Another contributing factor is the NOx formed by oxidizing NH3 (mostly


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the N-species released from rice husk17) that is released during rice husk suspension and/or during rice husk moving to the upper zone. These NOx emissions, however, have no chance to reduce to N2 via char reactions in the bed. Furthermore, in contrast to previous works1,15 indicating that the NH3 liberated from biomass was exploited as a reducing agent to reduce NOx to N2 under an O2-depleted atmosphere (same as the thermal De-NOx process18), the NH3 released from rice husk was oxidized with the accessible O2 and eventually formed NOx, as described above. In the same figure, the cocombustion of up to 30 wt% rice husk yields NOx emissions < 190 ppm, complying with the amount of 385 ppm according to Thailand’s regulations. It was found that 5-12% of the fuel-N was converted into NOx, corresponding to a rice husk share ranging from 10% to 30% (wt.), whereas for coal combustion alone, the conversion was approximately 7%. Finally, a large variation in NOx emissions was apparent for a high rice husk share that could be explained similarly to that observed for CO emissions. The fuel-N conversion to NOx was between 7 and 12%. 4. Conclusions This work presents some new information about combustion as follows: (a) The devolatilization and combustion of small eluding rice husk particles were found in the upper zone when feeding rice husk in either pellet form or natural form, resulting in fluctuations in CO and NOX emissions. This result suggests that bulky (low density) biomass may not be suitable for over-feeding combustion unless it is densified prior to combustion or co-fired with a dense fuel such as coal. (b) The SA flow may cool the internal circulating bed solids and cause a decrease in the bed temperature. The following conclusions have been obtained: (a) A height of 1.65 m in the temperature profiles was found to be the transition level from the dilute phase to the dense phase. The combustion of volatiles was more pronounced for rice husk combustion, which led to a significant increase in the temperature above the bottom bed.


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The combustion of rice husk alone required external heat to sustain the bed temperature >800°C because there was less char inventory in the bed. (b) The λtotal, biomass share, and SA injection position all affect the dense-bed temperature. For coal combustion, a λtotal between 1.2 and 1.6 with a λPA of 1.05 was recommended for the optimal CFB bed-temperature, at 800-900°C. For co-combustion with a fixed total feeding rate, the rice husk fraction should not be > 40% (wt.) to reach a bed temperature > 800°C. (c) Air-staging could markedly suppress NOx formation compared to no air-staging; however, this occurred with a high CO penalty. The higher the SA level is, the lower the NOx emissions are, and a reduction of 40-75% can be achieved for coal combustion. However, its influence on NOx reduction does not clearly appear during rice husk combustion, indicating that char yield and ash composition affects NOx emissions much more than SA injection position. For CO emission perspective, a height of 3.3 m appeared to be the most suitable for SA addition. (d) The increasing λPA and λSA satisfied the CO reduction but seemed to enhance NOx immensely. However, increasing λSA too much resulted in an increase in CO due to the cooling effect. (e) For co-combustion, a higher rice husk share yielded higher CO emissions. Blending more rice husk also emitted higher NOx emissions, despite containing half the nitrogen of coal. A rice husk share no greater than 30% (wt.) is recommended with NOx emissions < 200 ppm and CO emissions < 120 ppm. (f) Generally, fuel-N conversion to NOx was approximately 3.5-8.5% and 5-8% for rice husk combustion and coal combustion, respectively. For co-combustion, it was 5-12%, depending on the rice husk share.


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(g) For further investigation, a study with smaller pieces of coal to improve CO emission is suggested.

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Nussbaumer, T. Energy Fuels 2003, 17, 1510-1521.

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Leckner, B.; Karlsson, M. Biomass Bioenergy 1993, 4, 379-389.

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Lyngfelt, A.; Åmand, L.E.; Leckner, B. Fuel 1998, 77, 953-959.

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Tourunen, A.; Saastamoinen, J.; Nevalainen, H. Fuel 2009, 88, 1333-1341.

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Varol, M.; Atimtay, A. T.; Olgen, H.; Atakü, H. Fuel 2014, 117, 792-800.

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(10) Aarna, I.; Suuberg, E. M. Fuel 1997, 76, 475-491. (11) Li, Y. H.; Lu, G. Q.; Rudolph, V. Chem. Eng. Sci. 1998, 53, 1-26. (12) Madhiyanon, T.; Sathitruangsak, P.; Soponronnarit, S. Appl. Therm. Eng. 2010, 30, 347-353. (13) Madhiyanon, T.; Sathitruangsak, P.; Soponronnarit, S. Fuel Process. Technol. 2011, 92, 462470. (14) Sathitruangsak, P.; Madhiyanon, T.; Soponronnarit, S. Fuel 2009, 88, 1394-1402. (15) Xie, J.J.; Yang, X.M.; Zhang, L.; Ding, T.L.; Song, W.L.; Lin, W.G. J. Environ Sci-China. 2007, 19, 109-116. (16) Werther, J.; Saenger, M.; Hartge, E. U.; Ogada, T.; Siagi, Z. Prog. Energy Combust. Sci. 2000, 26, 1-27. (17) Glarborg, P.; Jensen, A. D.; Johnsson J. E. Prog. Energy Combust. Sci. 2003, 29, 89-113. (18) Leckner, B.; Karlsson, M.; Damjohansen, K. Ind. Eng. Chem. Res. 1991, 30, 2396-2404. (19) Wu, Z.; Sugimoto, Y.; Kawashima, H. Energy Fuels 2000, 14, 1119-1120. (20) Wu, Z.; Ohtsuka, Y. Energy Fuels 1997, 11, 477-482. (21) Wu, Z.; Ohtsuka, Y. Energy Fuels 1997, 11, 902-908. (22) Tsubouchi, N.; Ohtsuka, Y. Fuel Process. Technol. 2008, 89, 379-390.


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(a) Photo and schematic diagram of the CFB system.

(b) Temperature and gas emissions measuring positions. (T, temperature; G, gas concentrations) Fig. 1. Schematic diagram and data measuring position of the CFB.


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(a) 100% coal firing λSA series (λPA = 1.05)

(b) 100% rice husk firing λSA series (λPA = 0.90)

(c) co-firing of coal and rice husk total feeding rate = constant (9.69 kg h-1) No SA supply

(d) 100% coal firing SA position series (λtotal = 1.32)

Fig. 2. Temperature profiles within the CFB (a) 100% coal firing, λPA = const.= 1.05, λSA = 0 – 1.14, SA position = 2.3 m. (b)100% rice husk firing, λPA = const. = 0.90, λSA = 0 – 0.65, SA position = 2.3 m. (c) Co-firing between coal and rice husk, rice husk share = 10, 20, 30, 40% (by weight), λtotal = 1.47 – 2.31, λPA = 1.37 – 2.14, λSA = 0. (d)100% coal firing, λPA = const.= 1.32, λSA = const.= 0.32 (except C-13), SA position = 2.3, 3.3, 4.5 m.


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Fig. 3. (a) Influence of excess air ratio on dense bed temperatures ((T1+T2)/2): 100% coal firing, λPA = 1.05, λSA = 0.05 – 1.14; 100% rice husk firing, λPA = 0.9, λSA = 0.07 – 0.65. (b) Influence of rice husk share on dense bed temperature for co-firing cases, primary air supply only, fuel mixture feed rate and primary air flow rate both were held constant. (c) Influence of SA injection position (2.3, 3.3, 4.5 m) on dense bed temperatures, λtotal = 1.32, λPA = 0.96, λSA = 0.31.


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Fig. 4. Effect of secondary air injection location on gaseous emission for 100% coal firing, λPA = 0.96, λSA = 0.31.


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Fig. 5. Effect of secondary excess air ratio (λSA) and its location on gaseous emission for pure rice husk combustion with λPA = const. = 0.90.


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Fig. 6. Effect of primary excess air ratio (λPA) on gaseous emission for 100% coal combustion with no air-staging (λSA = 0).


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Fig. 7. Effect of secondary excess air ratio (λSA) on gaseous emission for 100% coal combustion with primary excess air ratio (λPA) = 1.05.


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Fig. 8. Effect of secondary excess air ratio (λSA) on gaseous emission for pure rice husk burning with primary excess air ratio (λPA) = 0.90.


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Fig. 9. Effect of rice husk share on gaseous emission for co-firing with coal with no air-staging.


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Table 1 Chemical analysis of bituminous coal and rice husk Parameter

Bituminous

Rice husk

Proximate analysis (%, as received) Fixed carbon Volatile matter Moisture Ash

34.6 53.5 10.1 1.8

10.6 64.5 8.5 16.4

Ultimate analysis (%, as received) C H N S O (by difference) Higher heating value (MJ/kg)

56.82 4.23 0.81 0.36 25.89

35.41 4.64 0.37 0.02 34.64

23.95

15.12

Ash chemical analysis (%wt) SiO2 Al2O3 Fe2O3 MgO K2O Na2O CaO P2O5 SO3 TiO2 Cl

23.90 14.40 20.10 10.30 0.92 1.06 13.20 0.20 13.20 0.70 0.01

90.50 0.11 0.18 0.58 2.32 0.08 0.58 0.93 0.30 0.01 0.31


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Table 2 Summary of experimental conditions and results for 100% coal firing: effect of primary and secondary air ratios 100% coal firing case

Item Coal feeding rate (kg h-1) Secondary air position (m) Fluidizing velocity (m s-1) Superficial velocity (m s-1) Total excess air ratio, λtotal Primary excess air ratio, λPA Secondary excess air ratio, λSA L-valve excess air ratio, λLV T1 (°C) (H = 0.2 m) T2 (°C) (H = 1.7 m) T3 (°C) (H = 2.7 m) T4 (°C) (H = 3.7 m) T5 (°C) (H = 4.2 m) T6 (°C) (H = 5.2 m) T7 (°C) (H = 4.2 m) T8 (°C) (H = 0.8 m) T9 (°C) (H = 0.4 m) O2 (%) CO2 (%) CO at 6% O2 (ppm) NOx at 6% O2 (ppm) Carbon conversion eff. (%)

C-1 11.27 4.60 4.80 1.09 1.04 0.05 955 946 893 842 818 739 462 181 104 1.26 12.90 1515 59 96.6

C-2 11.27 4.71 4.92 1.17 1.12 0.05 898 897 858 867 864 802 530 506 109 2.27 12.41 1111 75 98.3

λPA series (λSA = 0) C-3 C-4 11.27 11.27 5.40 6.32 5.60 6.51 1.34 1.61 1.29 1.56 0.05 0.05 897 856 887 839 859 813 867 826 867 827 812 785 569 559 548 551 180 176 4.18 6.39 11.12 9.52 283 189 104 184 98.8 99.4

C-5 11.27 6.64 6.82 1.76 1.71 0.05 814 801 772 783 785 758 552 544 162 7.42 8.90 75 193 99.3

C-6 11.27 7.56 7.75 1.98 1.93 0.05 820 812 793 810 807 786 600 595 219 8.36 7.78 74 269 99.6

C-7 11.27 2.3 4.66 4.88 1.15 1.05 0.05 0.05 910 918 891 911 905 852 618 534 158 2.10 12.30 802 94 98.6


λSA series (λPA = 1.05) C-8 C-9 C-10 C-11 11.27 11.27 11.27 11.27 2.3 2.3 2.3 2.3 4.57 4.44 4.41 4.07 5.25 5.41 6.58 7.15 1.26 1.34 1.64 1.93 1.05 1.05 1.05 1.05 0.16 0.24 0.54 0.83 0.05 0.05 0.05 0.05 886 854 846 760 899 875 851 767 872 856 837 757 886 872 850 767 887 870 858 775 829 819 822 753 599 568 598 556 537 561 615 554 151 133 172 173 3.33 4.08 6.42 8.45 11.78 11.03 9.14 8.25 306 54 62 143 101 114 153 199 99.0 99.0 99.5 98.6

C-12 11.27 2.3 3.99 8.14 2.24 1.05 1.14 0.05 739 754 752 784 786 765 577 574 193 10.28 7.48 280 337 98.3

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Table 3 Summary of experimental conditions and results for co-firing between rice husk and coal with no SA injection Item Coal feeding rate (kg h-1)a Rice husk feeding rate ( kg h-1)a Rice husk share (% wt.) Fluidizing velocity (m s-1) Superficial velocity (m s-1) Total excess air ratio, λtotal Primary excess air ratio, λPAb L-valve excess air ratio, λLV T1 (°C) (H = 0.2 m) T2 (°C) (H = 1.7 m) T3 (°C) (H = 2.7 m) T4 (°C) (H = 3.7 m) T5 (°C) (H = 4.2 m) T6 (°C) (H = 5.2 m) T7 (°C) (H = 4.2 m) T8 (°C) (H = 0.8 m) T9 (°C) (H = 0.4 m) O2 (%) CO2 (%) CO at 6% O2 (ppm) NOx at 6% O2 (ppm) Carbon conversion eff. (%) a

Fuel mixture feed rate = 9.69 kg h-1

Co-firing case (coal and rice husk) Rice husk share series (Total feeding rate = 9.69 CR-1 CR-2 CR-3 CR-4 9.69 8.72 7.75 6.78 0.97 1.94 2.91 10 20 30 4.96 4.94 4.81 4.74 5.33 5.31 5.17 5.09 1.43 1.49 1.55 1.61 1.33 1.38 1.44 1.50 0.10 0.10 0.11 0.11 931 922 899 888 933 931 890 870 918 925 892 865 935 952 911 884 916 917 863 843 857 874 810 796 663 714 595 598 636 655 548 462 163 157 113 142 5.33 2.34 3.58 7.19 10.31 12.27 11.41 9.07 16 57 35 110 138 104 140 190 99.3 99.1 99.2 98.9 b

PA flow rate was constant


kg h-1) CR-5 5.81 3.88 40 4.54 4.87 1.68 1.56 0.12 834 826 818 831 774 735 506 458 135 10.22 7.12 342 229 98.6

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Table 4 Summary of experimental conditions and results for studying effect of secondary air injection levels for 100% coal firing, and effect of secondary air ratio for mono rice husk combustion

Item Coal feeding rate (kg h-1) Rice husk feeding rate (kg h-1) Rice husk share (%) Secondary air position (m) Fluidizing velocity (m s-1) Superficial velocity (m s-1) Total excess air ratio, λtotal Primary excess air ratio, λPA Secondary excess air ratio, λSA L-valve excess air ratio, λLV T1 (°C) (H = 0.2 m) T2 (°C) (H = 1.7 m) T3 (°C) (H = 2.7 m) T4 (°C) (H = 3.7 m) T5 (°C) (H = 4.2 m) T6 (°C) (H = 5.2 m) T7 (°C) (H = 4.2 m) T8 (°C) (H = 0.8 m) T9 (°C) (H = 0.4 m) O2 (%) CO2 (%) CO at 6% O2 (ppm) NOx at 6% O2 (ppm) Carbon conversion eff. (%)

100% coal firing case SA position series C-13 C-14 C-15 C-16 11.70 11.70 11.70 11.70 2.3 3.3 4.5 5.42 4.28 4.35 4.36 5.42 5.62 5.70 5.72 1.32 1.32 1.32 1.32 1.28 0.96 0.96 0.96 0.31 0.31 0.31 0.05 0.05 0.05 0.05 827 866 883 886 838 856 871 870 837 832 863 853 815 772 810 799 815 760 777 769 797 720 742 679 626 516 552 455 620 513 540 462 215 161 153 165 5.74 5.53 4.58 4.66 13.35 13.57 14.37 14.32 281 4110 2177 18502 179 107 82 44 98.6 98.1 98.2 98.1

R-1 25.6 100 5.50 5.50 0.95 0.90 0.05 845 888 875 866 864 853 757 718 298 0.40 13.11 38326 96 97.6

R-2 25.6 100 2.3 5.46 5.87 1.02 0.90 0.07 0.05 838 874 870 859 858 847 765 727 309 0.33 13.13 43932 92 98.2

100% rice husk firing case λSA series (λPA = 0.90) R-3 R-4 R-5 R-6 25.6 25.6 25.6 25.6 100 100 100 100 2.3 2.3 2.3 2.3 5.41 5.53 5.67 5.49 5.88 6.19 6.70 6.61 1.03 1.06 1.12 1.14 0.90 0.90 0.90 0.90 0.08 0.11 0.17 0.19 0.05 0.05 0.05 0.05 827 852 880 844 879 873 914 868 864 865 903 857 850 857 889 847 845 857 894 848 828 846 883 839 707 757 825 778 669 693 810 748 216 366 281 302 0.66 0.99 1.90 2.47 13.15 13.14 13.14 13.23 34574 7385 6198 2461 99 93 120 133 96.6 98.4 96.5 98.8


R-7 25.6 100 2.3 5.43 7.10 1.24 0.90 0.29 0.05 832 862 854 834 839 836 778 784 217 3.82 13.13 614 155 98.7

R-8 25.6 100 2.3 5.49 9.26 1.60 0.90 0.65 0.05 843 866 861 842 805 807 760 752 340 7.48 9.64 322 179 991

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Table 5 Summary of experimental conditions and results for pelletized rice husk combustion: effects of secondary air injection levels and excess secondary air ratio (λSA)

Item Rice husk feeding rate (kg h-1) Secondary air position (m) Fluidizing velocity (m s-1) Superficial velocity (m s-1) Total excess air ratio, λtotal Primary excess air ratio, λPA Secondary excess air ratio, λSA L-valve excess air ratio, λLV T1 (°C) (H = 0.2 m) T2 (°C) (H = 1.7 m) T3 (°C) (H = 2.7 m) T4 (°C) (H = 3.7 m) T5 (°C) (H = 4.2 m) T6 (°C) (H = 5.2 m) T7 (°C) (H = 4.2 m) T8 (°C) (H = 0.8 m) T9 (°C) (H = 0.4 m) O2 (%) CO2 (%) CO at 6% O2 (ppm) NOx at 6% O2 (ppm) Carbon conversion eff. (%)

R-2 25.6 2.3 5.46 5.87 1.02 0.90 0.07 0.05 838 874 870 859 858 847 765 727 309 0.33 13.13 43932 92 98.2

λSA = 0.07 R-9 R-10 25.6 25.6 3.3 4.5 5.28 5.50 5.68 5.91 1.02 1.02 0.90 0.90 0.07 0.07 0.05 0.05 801 845 830 882 814 871 802 857 794 856 791 841 694 771 650 740 212 241 0.48 0.43 13.14 13.15 42111 39473 100 105 91.0 97.0

100% rice husk firing case SA position series (λPA = 0.90) λSA = 0.08 λSA = 0.11 R-3 R-11 R-12 R-4 R-13 R-14 25.6 25.6 25.6 25.6 25.6 25.6 2.3 3.3 4.5 2.3 3.3 4.5 5.41 5.57 5.54 5.53 5.61 5.55 5.88 6.06 6.02 6.19 6.28 6.21 1.03 1.03 1.03 1.06 1.06 1.06 0.90 0.90 0.90 0.90 0.90 0.90 0.08 0.08 0.08 0.11 0.11 0.11 0.05 0.05 0.05 0.05 0.05 0.05 827 860 853 852 869 857 879 895 879 873 880 877 864 881 873 865 880 876 850 866 861 857 867 863 845 864 859 857 860 861 828 858 844 846 857 841 707 764 720 757 782 784 669 742 670 693 733 773 216 234 220 366 448 369 0.66 0.62 0.49 0.99 1.13 1.09 13.15 13.16 13.14 13.14 13.15 13.15 34574 29948 32477 7385 6490 7714 99 116 99 93 87 93 96.6 97.7 98.0 98.4 97.1 97.7


R-5 25.6 2.3 5.67 6.77 1.13 0.90 0.18 0.05 880 914 903 889 894 883 825 810 281 1.90 13.14 6198 120 96.5

λSA = 0.18 R-15 R-16 25.6 25.6 3.3 4.5 5.58 5.69 6.66 6.80 1.13 1.13 0.90 0.90 0.18 0.18 0.05 0.05 862 885 897 879 885 868 868 850 860 848 860 818 805 767 787 747 300 214 2.36 2.88 13.16 13.21 4013 7842 134 152 97.3 96.6

Effect of Operating Conditions on the Combustion Characteristics of

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