ARTICLE pubs.acs.org/EF
Process Evaluation and Detailed Characterization of Biomass Reburning in a Single-Burner Furnace Su Sheng, Xiang Jun,* Hu Song, and Sun Lushi State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China ABSTRACT: Reburning technology is considered to be one of the most promising and cost-effective NOx reduction strategies for coal combustion systems. Although natural gas and coal have been the most widely studied reburn fuels, use of biomass in the reburning process is also attractive because of the nearly CO2-neutral character as well as the lower NOx and SO2 emissions of biomass, with respect to fossil fuels. Further studies on process evaluation and detailed characterization of biomass reburning, especially on the semi-industrial pulverized-coal furnace, are required before large-scale biomass reburning implementation can be realized. In the present study, process evaluation for the sawdust, rice husk, and dry sludge reburning have been performed on a semiindustrial pulverized-coal single-burner furnace. The detailed O2, NO, CH4, HCN, and NH3 concentration distributions in the reburn zone have been obtained through in-furnace measurements. The effects of the reburn fuel ratio and biomass carrier gas on the NO reduction efficiency were also evaluated and analyzed. The experimental results showed that 4060% NO reduction efficiency has been achieved by different biomass reburning. Sawdust had the best performance for NO reduction; rice husk was in second place; and dry sludge was less effective. At least a 10% increase of the NO reduction efficiency could be obtained when the biomass carrier air was replaced by the recycled flue gas. The NO reduction efficiency increased with the increase of the reburn fuel ratio, but only slightly further NO reduction could be achieved with the further increase of the reburn fuel input. The results of in-furnace measurements showed that the major nitrogenous intermediate species observed in the reburn zone was HCN with the sawdust and rice husk reburning and was NH3 with the dry sludge reburning. The NO reduction process was greatly related to the concentration changes of O2, CH4, HCN, and NH3 in the reburn zone. These key gaseous species played important roles in NO reduction. It is expected that this study provided wider knowledge of the characterization of the biomass reburning process and a useful basis for further studies.
1. INTRODUCTION Nitrogen oxide (NOx) emissions from coal-fired boilers are significant contributors to atmospheric pollution. In recent years, a number of techniques and methods, stimulated by the need to control the NOx emissions from the combustion system, have been investigated.1 Among the alternatives, reburning is considered to be one of the most promising and cost-effective nitric oxide (NO) reduction strategies for coal combustion systems.2,3 Reburning is an in-furnace NOx control technology, involving combustion of the major fuel under the normal fuel-lean conditions in the primary zone of a furnace, followed by injection of the reburn fuel to establish a fuel-rich reburn zone, in which NOx formed in the primary zone is reduced to molecular nitrogen and other nitrogenous species, and finally injection of overfire air into the burnout zone to complete combustion. A number of studies have evaluated the potential of alternative fuels to be used as reburn fuels. Although gaseous fuels, such as natural gas and other hydrocarbons, have been the most widely studied reburn fuels because of the null or minimal content of nitrogen in their composition and their ability to produce easily active hydrocarbon radicals, which strengthen the NOx reduction,4,5 liquid fuels, such as residual oil,6 and solid fuels, such as coal710 and biomass,1216 are also available for the reburning process. In this respect, there has been an increasing interest in the use of biomass in the reburning process.17 Biomass is a good candidate for reburn fuel because of its nearly CO2-neutral r 2011 American Chemical Society
character as well as the lower NOx and SO2 emissions with respect to coal, granted by its smaller N and S contents.18 Because the reburn fuel is normally 520% of the total heat input, large quantities of biomass are not necessary. Biomass reburning in large-scale boilers appears to be a low-risk option and allows for wider operation flexibility in the case of limited or discontinuous biomass supplies, as compared to firing 100% biomass in largescale boilers.16,19 The substitution of 520% of the energy generated with coal by biomass according to a reburning configuration would provide similar values for the reduction of CO2 and SO2 emissions and could also afford substantial reductions of NOx emissions. Many studies and demonstrations on gaseous fuel reburning have been accomplished since the 1980s, and the fundamentals and technological aspects are quite well-established.2,20,21 However, industrial application of biomass reburning in power plant boilers has not been reported. Different facilities have been used in the past for the study of biomass reburning processes, ranging from bench-scale to pilot-scale rigs and enabling different degrees Special Issue: 2011 Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: July 29, 2011 Revised: October 25, 2011 Published: October 31, 2011 302
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Table 1. Characteristics of Experimental Fuels (on an Air Dry Basis) proximate analysis (wt %)
ultimate analysis (wt %)
moisture
ash
volatile matter
fixed carbon
C
H
SC
0.49
23.51
11.24
64.74
65.91
3.24
0.20
1.97
4.68
24.84
SF
3.79
2.61
33.18
60.42
74.44
4.85
13.11
0.92
0.28
28.27
fuel
O
N
S
Q LHV, ad (MJ/kg)
sawdust
12.38
0.81
69.41
17.40
45.78
6.06
34.60
0.11
0.26
17.55
rice husk
9.68
15.57
58.11
16.64
37.24
5.07
31.56
0.45
0.43
14.49
dry sludge
4.17
64.89
28.17
2.77
17.21
2.55
8.60
1.63
0.95
7.67
of similarity with the real systems.1216,2225 Although it seems that the reductions of NOx emissions around 5070% can be obtained by biomass reburning on these rigs, gaseous fuel, such as methane and propane, rather than coal was usually used as the primary fuel in most of the studies. Significant development studies, especially on the semi-industrial pulverized-coal furnace, are required before the large-scale biomass reburning implementation can be realized. In addition, most of the earlier studies concentrated on the effect of operational parameters, such as the reburn fuel fraction, the reburn zone stoichiometric ratio, and residence time, on NOx reduction based on flue-gas data, with little or no attention paid to the detailed characterization of the reburn zone, except the studies by Luan et al.,13 Casaca and Costa,15,17 Munir et al.,16 and Ballester et al.26 Flue-gas data do not give the detail required to interpret the more fundamental aspects of the biomass reburning process, and therefore, combustion data in the reburn zone are valuable. It has been indicated by some researchers that volatiles from biomass, including hydrocarbon species, are crucial for removing NOx, similar to the mechanism of the natural gas reburning process.27,28 Some nitrogencontained intermediate species play important roles in NOx reduction, and HCN and NH3 are believed to be the main intermediate reactants in the NOx reburning routine.29,30 However, taking into account the studies already existing in the literature, the detailed characterization of these key species in the biomass reburning process is absent. Better knowledge about the characterization of the biomass reburning process is required. Moreover, NOx control capabilities of different biomasses are various, in both qualitative and quantitative terms. Further evaluations on the different biomass reburning processes are also required for satisfactory application. The present study aims to evaluate the performance of different biomasses as the reburn fuel and to obtain a wider knowledge of the characterization of the biomass reburning process. A series of experiments have been carried out on a semi-industrial pulverizedcoal single-burner furnace using sawdust, rice husk, and dry sludge as the reburn fuel. The effect of crucial parameters, such as the biomass carrier gas and reburn fuel ratio, on the reburning process were inspected and analyzed. The detailed concentration distributions of the key gaseous species in the reburn zone were also accomplished through in-furnace measurements both to gain further insight into the biomass reburning process and to provide clues to understand the mechanism of the biomass reburning.
Table 2. Ash Chemical Composition of Experimental Fuels (wt %) fuel
Na2O K2O MgO Al2O3 SiO2 P2O5 TiO2 CaO Fe2O3
SC
1.44
0.83 4.57 30.48 42.82 1.83 2.21
SF
1.60
0.71 3.91 18.26 25.29 0.30 0.90 34.62 12.89
sawdust
3.43 13.11 5.82
5.45 26.91 3.27 0.73 34.70
1.48
rice husk
2.05
1.23 83.15 2.62 0.78
1.57
2.46
2.67 2.42 17.53 56.69 2.99 1.03
5.28
5.05
dry sludge 1.84
3.50 0.76
6.59
7.28
analysis or experiments, the coals were ground in a small ball-tube mill and the fraction 50100 μm was selected by pneumatic sieving. Three biomasses, sawdust, rice husk, and dry sludge, were served as the reburn fuels. The main properties of the biomasses are also shown in Table 1. The sludge was first mechanically dewatered and then dried in a drying oven at 40 °C. The dry sludge was also ground in the small balltube mill, and the fraction 50100 μm was selected. The sawdust and rice husk were very difficult to grind to fine particles in the laboratory. Therefore, the sawdust and rice husk were fragmented and classified under a nominal size of 1 mm. It is evident from Table 1 that biomass as a class is very much different from coal. Biomass generally contains a higher proportion of volatile matter and less carbon than an equivalent mass of coal, which reduces the heating value. Table 2 shows the results of the ash analysis of the experimental fuels. Chemical analysis of major elements in ashes of the experimental fuels was conducted using an X-ray fluorescence spectrometer (XRF), type EAGLE III of EDAX, Inc. in Mahwah, NJ. As shown in Table 2, the chemical compositions of biomass ashes were different from those of coal ashes. Ashes obtained from sawdust were enriched in Ca, Si, and K oxides. The rice husk ash had the highest amount of Si, while the lowest amount of Ca and Al among all tested fuels. Ash chemical composition of dry sludge was dominated by Si, Al, Ca, and Fe inorganic species. Chemical analysis of biomass ashes showed that they were richer in K and Na and poorer in Al and Fe oxides, in comparison to coal ashes. The fusion temperatures of the biomass ashes would be low for combustion processes because of their high alkali contents.31,32 In the biomass reburning process, ash-related problems should be considered. In the reburn zone with a reducing atmosphere, the tendency for slagging/fouling of the biomasses might be very high, especially for the rice husk and the dry sludge containing a high ash content. It would be a problem for using these biomasses in a practical reburning process. A feasible proposition to deal with this issue is to set the reburn fuel ratio reasonably because the impacts of ash-related problems can be minimized when the reburn fuel comprises only a small fraction of the total heat input. In addition, the atmosphere in the reburn zone should be controlled strictly. The stoichiometric ratio in the reburn zone could not be set too low to cause the slagging/fouling in the furnace, just as it was performed in this study. 2.2. Experimental Facilities and System. The schematic diagram of the experimental system is shown in Figure 1. It comprises a
2. EXPERIMENTAL SECTION 2.1. Fuel Characterization. Two typical Chinese coals, lean coal SC and bituminous coal SF, were used as the primary fuels in the present study. The characteristics of the coals are given in Table 1. Prior to any 303
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Figure 1. Schematic diagram of the experimental system: (1) centrifugal blower, (2) air preheater, (3) air distributor, (4) coal hopper, (5) packing auger, (6) primary air valve, (7) direct current motor, (8) secondary air valve, (9) central air valve, (10) swirl burner, (11) FTIR gas analyzer system (GASMET DX-4000), (12) sampling hole, (13) combustion chamber, (14) burnout air blower, (15) flow controller, (16) oxygen analyzer (KANE KM940), (17) cyclone separator, (18) draft fan, (19) smokestack, (20) recycled flue gas baffle plate, (21) recirculation fan, (22) reburn fuel feeder, and (23) test port. water-cooled probes from any of the available sampling holes along the furnace horizontal axis. A Fourier transform infrared (FTIR) analyzer system (GASMET DX-4000), produced by Gasmet Technologies, Inc. in Finland, was used for the measurements of NO, CH4, HCN, and NH3 concentrations in the furnace. This FTIR analyzer system was designed suitably to meet the specific requirements of flue gas measurements. The O2 concentration was analyzed with an individual online analyzer, type KM940 of KANE Corporation in U.K. The designed accuracy of the FTIR analyzer system for different gas species is (1% for NO and CH4 and (2% for HCN and NH3. The designed accuracy of the O2 analyzer is (0.2%. All of the instruments were suitable to measure the gaseous species concentrations in flue gas and calibrated before each run. The sampling of HCN and NH3 required specific conditions to avoid the modification of the sample because of condensation, solution in water, and reaction with other species. Two sample transport lines were connected to the probe outlet, each of them including the absorber to eliminate those components that can react with HCN and NH3, respectively. The temperature of the gas sample was kept above 180 °C along the transport line using an internal heating element in the transport pipe of the FTIR analyzer system. Condensation and solution in water of the samples were avoided. No attempt was made to quantify the probe flow disturbances. To obtain the more accurate data, the measurements were repeated until the measured variables did not drift from a constant average level for a period of up to 15 min. The direct readings showed that, in the regions of highest gaseous species concentrations, the “true” concentrations did not exceed the measured one by more than 10%. The uncertainty of measurements did not exceed 10%. The final data of the temperature and O2, NO, CH4, HCN, and NH3 concentrations presented in this study were the average values in each run. Considering the difficulty and uncertainty of HCN and NH3 measurements, the direct readings of HCN and NH3 concentrations from the FTIR analyzer may not be accurate enough. The FTIR analyzer has been specially calibrated before the experiments with certificated BOC special gas mixtures offered by the manufacturer. The compositions of the certificated BOC special gas mixtures were similar to the flue gas. The HCN and NH3 concentrations in the special gas mixtures were known. The FTIR analyzer has been checked carefully by the special gas mixtures with different HCN and NH3 concentrations. Therefore, the actual accuracy of the FTIR analyzer for HCN and NH3 measurements could be obtained. The final values of HCN and NH3 concentrations presented in this study have been corrected according to the direct readings of the HCN and NH3 concentrations in each run as well as the actual accuracy of the FTIR analyzer for HCN and NH3 measurements.
Figure 2. Schematic diagram of the burner (dimension in millimeters): (1) central air, (2) air primary, (3) inner secondary air, and (4) outer secondary air. single-burner furnace, fuel feeder, blower, flue gas exhaust treatment system, and gas sampling system. The pulverized-coal single-burner furnace is 0.5 m in height, 0.35 m in width, and 4 m in length. The rectangular walls of the furnace are refractory-lined, and the outer surfaces of the refractory walls are surrounded by ovenproof steel jackets. The pulverized coal is stored in a hopper, which automatically loads a packing auger with closed-loop control. The coal feeder can provide pulverized coal with a feeding rate up to 35 kg/h. A swirl burner is installed on the furnace. The configuration of the burner and the relevant dimensions are shown in Figure 2. Combustion air is supplied by a high-pressure centrifugal blower and preheated in an electrical heater. The pulverized coal and the preheated primary air are mixed with the preheated secondary air and injected into the furnace. Flow rates of the primary air and secondary air are regulated by the valves and metered continuously through the rotameters. The pulverized coal is ignited using an oil gun, and the oil gun exits after the coal flame is stabilized. This semi-industrial furnace affords a good compromise between realistic trials and a close control and wide evaluation possibilities for reburning processes. A detailed description of the experimental facility can also be found elsewhere.20 2.3. Experimental Cases and Measurement. Five utility test ports are positioned horizontally on one side of the furnace wall, as shown in Figure 1. The distances between the burner exit and each test port are summarized in Table 3. On the other side of the furnace wall, 11 sampling holes are distributed horizontally along the length of the furnace. The distances between the burner exit and each sampling hole are also shown in Table 3. These ports and holes have made the furnace very flexible for reburning and sampling. The gas temperature in the furnace was measured by bare-wire Pt/13% RhPt thermocouples, whose effective range is 01800 K and accuracy is (2.5%. Gas samples were drawn through portable stainless-steel 304
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Table 3. Test Port and Sampling Hole Distribution on the Furnace Wall number distance from the burner exit (m)
1
2
3
4
5
sampling hole
0.50
0.80
1.15
1.50
1.85
test port
0.10
0.35
0.60
0.95
1.30
6
7
8
9
10
11
1.65
2.00
2.35
2.80
3.30
3.80
Table 4. Furnace Operating Conditions Primary Zone coal
SC
mass flow rate of coal (kg/h)
SF 25
mass flow of primary air (kg/h)
63.80
primary air temperature (K)
473
mass flow of secondary air (kg/h)
135.40
secondary air temperature (K) stoichiometric ratio
149.10 523
1.16
1.14
Reburn Zone biomass carrier gas
recycled flue gas
air
carrier gas velocity at the reburn fuel nozzle exit (m/s)
8.7
9.0
carrier gas temperature (K)
423
reburn fuel ratio (%)
300 10, 15, and 20
stoichiometric ratioa
0.840.92
reburn zone residence time (s)
0.891.02 0.62
Burnout Zone burnout air temperature (K) stoichiometric ratio
300 ∼1.15
a
The values were varied with the reburn fuel ratio and calculated according to the ratio between the total air injected into the reburn zone (burner air flows and carrier gas for biomass) and the minimum amount required to convert the primary and reburn fuel into CO2 and H2O. It varied with the reburn fuel ratio. The fly ash generated from the mixtures of biomass and coal has been collected from the cyclone separator during experiments. The unburned carbon concentration in fly ash was determined from the measured weight loss as a result of heating in the electric muffle furnace. To evaluate the influence of the reburning process on the burnout characteristic of the fuels, the relative variance rate of the unburned carbon concentration in fly ash was defined as UBCreburning case UBCbase case 100% ð1Þ λ¼ UBCbase case
carrier gas for biomass) and the minimum amount required to convert the primary and reburn fuel into CO2 and H2O and was varied with the reburn fuel ratio. The locations of the reburn fuel injection were established with the aid of detailed in-flame measurements of major gaseous species, including NO and O2 concentrations obtained from the base cases. The detailed in-flame data gathered for the base cases were reported in the following section. It was evidenced by the data reported by Munir et al.16 and our previous studies20,21 that the initial NO concentration played a key role in determining the NO reduction effectiveness of the reburning process. The reburn fuel injection position, which was located in the region of the maximum NO concentration of the base case, was beneficial to the NO reduction reactions.2,23 On the basis of that, the reburn fuel was introduced into the furnace from both numbers 3 and 4 test ports in the present study, where the maximum NO concentration and lower O2 concentration were observed in the base cases. The burnout air was supplied by a centrifugal blower and injected into the furnace from the number 10 sampling hole to bring the final excess air in the burnout zone. The burnout air injection position has been optimized at the number 10 sampling hole, where the O2 concentration of the base cases began to level off and the adequate residence time of the reburn zone could be obtained. The flow rate of the burnout air could be adjusted by the flow controller to keep the burnout zone stoichiometric ratio (SRB) ≈ 1.15 in all of the reburning cases. The average reburn zone residence time was estimated and calculated assuming plug flow between the injections of reburn fuel and burnout air. The average reburn zone residence time in the single-burner furnace was about 0.62 s, as shown in Table 4. The recycled flue gas and air were used as the biomass carrier gas, separately. The carrier gas was mixed with the biomass and injected
where UBC is the unburned cabon concentration in fly ash. The furnace operating conditions for different experimental cases were summarized in Table 4. For the base cases without biomass reburning, the coal feeder provided pulverized coal with a feeding rate of 25 kg/h. The stoichiometric ratio for the coal combustion was 1.16 for lean coal SC and 1.14 for bituminous coal SF. For the reburning cases, the operating conditions in the primary zone were the same with the base cases. The reburn fuel feeder was a scraper feeder (IHT12PF70N), and the feeding rate could be regulated by the current controller, according to the reburn fuel ratio (the ratio of the reburn fuel heat input to the sum of the primary fuel and reburn fuel heat input). It was found during calibration tests that it was not possible to consistently feed the dry sludge at the lower desired 5% (reburn fuel ratio) feed rate because of the high density of the dry sludge and exceed 20% (reburn fuel ratio) feed rate because of the low density of the sawdust as well as the limitation of the feeder volume. Therefore, reburn fuel ratios were varied as 10, 15, and 20% in present work to evaluate the effects of the reburn fuel ratio on the NO reduction. The reburn zone stoichiometric ratio (SRR) was calculated as the ratio between the total air injected into the reburn zone (burner air flows and 305
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husk and dry sludge. After the biomass was injected into the reburn zone, thermal pyrolysis reactions happened and volatile matter released as a high proportion of C-containing radicals (HCCO, CH, CH2, and CH3).11 CHi and HCCO radicals play the main roles in NO reduction, through the following reactions:15,33,34 CHi þ NO f HCN þ :::
ð3Þ
HCCO þ NO f HCN þ CO2
ð4Þ
HCCO þ NO f HCNO þ CO Reaction 5 was followed by reaction 6. HCCO þ H f HCN Figure 3. Maximum NO reduction efficiency for different biomass reburning, with recycled flue gas as the carrier gas and a reburn fuel ratio of 20%.
½NOB ½NOR ½NOB
! 100%
ð6Þ
Hydrogen cyanide produced via the above reaction, depending upon the atmosphere conditions, could be converted to N2 through the reactions with oxidizing species (O and OH).15,28 The sequence of reactions 36 mostly accounts for the reduction of NO by hydrocarbons. This sequence of reactions was favored under the fuel-rich conditions encountered in the reburning zone. High volatile gas yields from biomass suggested high CHi and HCCO concentrations in the reburn zone. It greatly influenced the reburning process and contributed to high NO reduction efficiency. At the same time, the reactions between the volatile gases and O2 consumed large amounts of O2, which greatly reduced the O2 concentration and contributed to the formation of the strongly reducing atmosphere in the reburn zone. The conversion of nitrogen-containing species to NO through reacting with O2 was impeded under the fuel-rich condition, and the reactions shifted to NO reduction processes. As show in Figure 3, the NO reduction efficiency was found to be slightly higher in the reburning case with bituminous coal SF as the primary fuel, in comparison to those with lean coal SC as the primary fuel. Considering the obvious difference between the characteristics of the two coals, the results indicated that the selection of reburn fuel did not remarkably depend upon the primary fuel in the reburning process. 3.2. Evaluation of Carrier Gas Effects on NO Reduction Efficiency. In this study, the uses of both air and recycled flue gas as the biomass carrier gas were investigated. Figure 4 represents the comparisons of the NO reduction efficiencies when the recycled flue gas and air were used as the biomass carrier gas, separately. Figure 5 shows the NO reduction efficiency as a function of the reburn fuel ratio when the recycled flue gas and air were used as the rice husk carrier gas, separately. As shown in Figure 4, the NO reduction efficiency increased from 50 to 64% for sawdust reburning when the biomass carrier air was replaced by the recycled flue gas. It increased from 40 to 55% for rice husk reburning and from 32 to 43% for dry sludge reburning. The experimental results indicated that a higher NO reduction efficiency can be obtained when the biomass used the recycled flue gas as the carrier gas. At least a 10% increase of the NO reduction efficiency was achieved when the biomass carrier air was replaced by the recycled flue gas. The O2 concentration of the recycled flue gas (about 3%) was much lower than that of the air, which helped reduce the reburn zone stoichiometric ratio and reduce the chance of premature ignition in the reburn stream. It was beneficial to form the reducing atmosphere in the reburn zone and enhance the NO reduction reactions. Figure 5 also shows that the higher NO reduction efficiency was obtained when the recycled flue gas was used as the biomass
horizontally into the furnace from two 40 mm inner diameter refractory nozzles, which connected with the test ports. The carrier air was supplied by a pint-sized centrifugal blower without preheating. The recycled flue gas was transported by the recirculation fan, and the flow rate could be adjusted by the baffle plate. At the entrance of the recirculation fan, the fuels were almost burned out and the flue gas composition was nearly uniform. The measured O2 concentration in recycled flue gas was about 3%. The carrier gas velocities at the reburn fuel nozzle exit, as shown in Table 4, were almost kept constant throughout the reburning cases by adjusting the fans and the baffle plate. In the reburning cases, the NO reduction efficiency (%) was defined as NO reduction efficiency ð%Þ ¼
ð5Þ 28,34
ð2Þ
where [NO]B and [NO]R are the average NO concentration (dry, 0% O2) at the number 11 sampling hole for the base and reburning cases, respectively.
3. RESULTS AND DISCUSSION 3.1. Evaluation of Reburn Fuel Effects on NO Reduction Efficiency. The results of reburning experiments showed that all
of the three biomasses obtained the maximum NO reduction efficiency under the same reburning conditions. That is, the recycled flue gas was used as the biomass carrier gas, and the reburn fuel ratio was 20%, as shown in Figure 3. When lean coal SC was used as the primary fuel, the maximum NO reduction efficiency of sawdust reburning was 64%, which was higher than those of the rice husk reburning (55%) and dry sludge reburning (43%). When bituminous coal SF served as the primary fuel, the maximum NO reduction efficiency of sawdust reburning (68%) was also much higher than those of rice husk reburning (58%) and dry sludge reburning (45%). The results of reburning experiments showed that there was an obviously difference between the NO reduction efficiencies with different biomass reburning. Throughout the series of reburning cases under the same reburning conditions, sawdust had the highest NO reduction efficiency, rice husk was moderate, and dry sludge had the lowest. It was found that the NO reduction efficiency of the biomass reburning was greatly related to the volatile matter content of the biomass. As listed in Table 1, the volatile content in sawdust is 69.41%, the volatile content in rice husk is 58.11%, and the volatile content in dry sludge is 28.17%. Sawdust has more volatile matter than rice 306
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Figure 4. NO reduction efficiency with different carrier gases, with coal SC and a reburn fuel ratio of 20%.
Figure 6. NO reduction efficiency with different reburn fuel ratios, with coal SC and recycled flue gas as the carrier gas.
Figure 5. NO reduction efficiency with different reburn fuel ratios, with coal SC and rice husk reburning.
Figure 7. NO reduction efficiency with different reburn fuel ratios, with coal SF and air as the carrier gas.
carrier gas. In Figure 5, when the reburn fuel ratio increased from 10 to 15%, the NO reduction efficiency increased 15% (from 15 to 30%) with the air and increased 10% (from 38 to 48%) with the recycled flue gas. With a further increase of the reburn fuel ratio to 20%, the NO reduction efficiency still increased 10% (from 30 to 40%) with the air but only increased 6% (from 49 to 55%) with the recycled flue gas. The experimental results revealed that the variance of the NO reduction efficiency was more sensitive to the changes of the reburn fuel ratio when the air was used as the carrier gas than that when the recycled flue gas was used as the carrier gas. In other words, the greater increase of the NO reduction efficiency could be obtained with the increase of the reburn fuel ratio when the air was used as the carrier gas, in comparison to that when the recycled flue gas was used as the carrier gas. 3.3. Evaluation of Reburn Fuel Ratio Effects on NO Reduction Efficiency. Figure 6 shows the NO reduction efficiency as a function of the reburn fuel ratios when the recycled flue gas was used as the carrier gas. Figure 7 shows the relationships of the NO reduction efficiency with the reburn fuel ratios when the air was used as the carrier gas. As shown in Figures 6 and 7, for all of the three biomasses, the NO reduction efficiency increased with the increase of the reburn fuel ratio. An increase of the reburn fuel heat input provided more volatile gases in the reburn zone, facilitating the exposure of
NO to CHi and HCCO. Moreover, quick consumption of oxygen by a large amount of reburn fuel also enhanced the low oxygen atmosphere in the reburn zone. When the reburn fuel ratio was 10%, the NO reduction efficiency was low, especially for the dry sludge reburning. Increasing the reburn fuel ratio from 10 to 15% resulted in an obvious NO reduction for all of the three biomasses. In present work, a larger reburn fuel ratio of more than 20% was not performed because of the limitation of the biomass feeder and to avoid the ash-related problems in the furnace. The optimal reburn fuel ratio, reported by Lu et al.,30 could not be observed in the biomass reburning process. However, both Figures 6 and 7 show that the further increase of the reburn fuel ratio from 15 to 20% improved the NO reduction efficiency relatively little, as compared to that with the increase of the reburn fuel ratio from 10 to 15%. The results indicated that the NO reduction efficiency increased with the increase of the reburn fuel input at first, but only a slight further NO reduction can be achieved with the further increase of the reburn fuel ratio. It is well-known that NO reduction reactions require a fuel-rich atmosphere (SRR < 1), in which most NO produced in the primary zone are chemically reduced to N2 through reactions 36. With the increase of the reburn fuel ratio, the reburn zone stoichiometric ratio decreased, resulting in the increase of the NO reduction efficiency. However, when the reburn fuel ratio is further increased, the incomplete combustion of both coal and 307
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Figure 9. Profiles of the temperature along the furnace centerline, with coal SC, air as the carrier gas, and a reburn fuel ratio of 20%.
Figure 8. Unburned carbon concentration in fly ash with different reburn fuel ratios, with coal SF and air as the carrier gas.
biomass will occur because of the lower reburn zone stoichiometric ratio and the limitation of the reburn zone residence time. A fraction of the hydrocarbon and nitrogenous radicals generated in the reburn zone will be oxidized in the burnout zone, which significantly increases the NO concentration in the final flue gas and restrains the further increase of the NO reduction efficiency. 3.4. Evaluation of the Unburned Carbon Concentration in Fly Ash. Figure 8 shows the relationships between the relative variance rate of the unburned carbon concentration in fly ash (λ) and the reburn fuel ratios when the air was used as the carrier gas. As shown in Figure 8, the unburned carbon concentration in fly ash for the reburning case was higher than that of the base case. With the reburn fuel ratio increasing from 10 to 20%, the values of λ increased from 5.3 to 11% for sawdust reburning, from 7.2 to 15.5% for rice husk reburning, and from 6.8 to 14.0% for dry sludge reburning. The results indicated that the fuels were more difficult to burn out thoroughly in the reburning cases, in comparison to the base cases. During the reburning cases, the reburn zone stoichiometric ratio decreased with the increase of the reburn fuel ratio. It resulted in the incomplete combustion of both coal and biomass. The higher carbon loss would decrease the combustion efficiency of the reburning process. When rice husk reburning was compared to dry sludge reburning, a similar level of λ was observed in Figure 8. Although the rice husk reburning case was easy to obtain a higher NO reduction efficiency, in comparison to the dry sludge reburning case, the obvious advantages in combustion efficiency have not been obtained by the rice husk reburning. The highest combustion efficiency was achieved by sawdust reburning because of its lowest ash content and highest volatile content. The objective on reburning process optimization was not only to achieve the high NO reduction efficiency but also to obtain the low carbon loss. The results revealed that high NO reduction efficiency with low carbon loss could be obtained simultaneously in the reburning process when the proper biomass was selected as the reburn fuel. 3.5. Characterization of the Reburn Zone. Taking into account the studies in the literature, only a few studies reported the detailed measurements inside the reburn zone.1317,26,35 This is probably due to the fact that the collection of such data takes a greater effort and they are of less immediate practical relevance than parametric studies based on final emissions. However, as pointed out by Smoot et al.,2 detailed in-furnace profile measurements of the relevant variables may provide the information needed to validate comprehensive modeling tools. Also, some information
Figure 10. Profiles of the O2 concentration along the furnace centerline, with coal SC, sawdust reburning, and recycled flue gas as the carrier gas.
in this respect would also benefit to further understand the mechanism of the biomass reburning and to optimize the biomass reburning technology. In this study, the semi-industrial singleburner furnace was rectangular and horizontally oriented. The measurements in height could not be realized because of the limitations of experimental conditions. However, the axial profiles for the temperature, O2, NO, CH4, HCN, and NH3 along the furnace centerline were more easily obtained through in-furnace measurements. It would present the essential information of the biomass reburning process and provide clues to understand the mechanism of the biomass reburning. Figure 9 shows the profiles of the measured temperature along the furnace centerline for the base and reburning cases. In this figure and the next figures, the arrows represent the locations of the biomass injection positions. As shown in Figure 9, there was a general decrease in the temperature in the reburn zone with reference to the base case. The average temperature levels in the reburn zone were about 100150 °C lower than that of the base case throughout the series of reburning experiments. A lower temperature in the reburn zone could further reduce the formation of thermal NOx. In the reburning cases, it was noted that a higher temperature level was observed with sawdust reburning because of the higher heating value, as listed in Table 1. Figure 10 shows the profiles of the measured O2 concentration along the furnace centerline for the base and sawdust reburning 308
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Figure 12. Profiles of the NO concentration along the furnace centerline, with coal SC, recycled flue gas as the carrier gas, and a reburn fuel ratio of 20%.
Figure 11. Profiles of the O2 concentration along the furnace centerline, with coal SF, sawdust reburning, and a reburn fuel ratio of 20%.
cases. Figure 11 shows the O2 concentration profiles for the sawdust reburning case when the recycled flue gas and air were used as the biomass carrier gas, separately. As shown in Figure 10, O2 was quickly consumed after the biomass was injected into the furnace. An obvious oxygen-lean region, as expected, was formed downstream of the biomass injection position. The lowest O2 concentration with sawdust reburning was 0.6% when the reburn fuel ratio was 10% and was 0% when the reburn fuel ratio was 15 and 20%. A lower O2 concentration could be achieved earlier in the reburn zone with the larger reburn fuel ratio. In Figure 11, because of the higher O2 concentration in the air, a much higher O2 concentration could be observed at the biomass injection locations, with the air as the biomass carrier gas. However, the O2 concentration gradually decreased with the reaction of reburn fuel with O2, and the lowest O2 concentration (O2 = 0%) was observed at a distance from the burner exit of 2.35 m, as shown in Figure 11. The oxygen-lean region could be finally formed in the reburn zone when the air was used as the carrier gas, even though this region was smaller than that when the recycled flue gas was used as the carrier gas. The smaller oxygen-lean region limited the residence time of NO in the reducing atmosphere and caused the lower NO reduction efficiency. Figures 1215 represent the profiles of the NO, CH4, HCN, and NH3 concentrations along the furnace centerline for the base and reburning cases with the recycled flue gas as the carrier gas. In comparison to the NO concentration level of the base case, the NO concentration in the reburn zone decreased remarkably after the biomass was injected into the furnace, as shown in Figure 12. It was noticed that the NO reduction has already occurred upstream of the reburn fuel injection locations, which may be due to the entrainment of the reburn fuel into the upstream by the mainstream flow. For rice husk reburning, a fast decrease of the NO concentration was observed nearby the location of the biomass injection, indicating that NO destruction took place in a very short time after mixing with the reburn fuel. After that, the NO concentration began to level off downstream of the reburn zone. For sawdust reburning and dry sludge reburning, the NO concentration changed with a similar pattern as rice husk reburning, although a lower NO concentration level for sawdust reburning and a much higher NO concentration level for dry sludge reburning were observed in the reburn zone. The different performance for NO reduction by different biomass reburning was greatly related to the reductive gaseous species, such as CHi and HCCO, in the reburn zone.
Figure 13. Profiles of the CH4 concentration along the furnace centerline, with coal SC, recycled flue gas as the carrier gas, and a reburn fuel ratio of 20%.
As shown in Figure 13, the CH4 concentrations of the reburning cases obviously increased in the reburn zone, as compared to that of the base case. Both the peaks of CH4 and the lowest levels of O2 after the reburn fuel injection were an indication of a wellestablished fuel-rich, oxygen-lean reburn zone. A greater release of CH4 as a result of biomass fragmentation in the reburn zone caused the drastic reduction in NO, as shown in Figure 12. CH4 was one of the main volatile gases, which reacted with O2 to greatly reduce the O2 concentration in the reburn zone and reacted with NO to convert it to molecular nitrogen. The CH4 concentration level in the reburn zone was greatly related to the volatile content of the biomass. With the same reburn fuel ratio, the highest CH4 concentration can be achieved with sawdust because of its highest volatile content, rice husk had the moderate CH4 production, and the dry sludge had the lowest CH4 concentration because of its relatively lower volatile content. Figure 13 shows that CH4 displayed a significant rise at the biomass injection position and then dropped quickly to a low level. This behavior was consistent with the changes of the NO concentration, as shown in Figure 12. A small quantity of CH4 was maintained at the end of the reburn zone, indicating that the amount of reburn fuel was sufficient for the NO reduction in the reburn zone. As shown in Figures 14 and 15, the measured values of HCN and NH3 concentrations in the furnace were generally very low, 309
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reburning, rice husk had moderate HCN and NH3 production, and the dry sludge reburning had the lowest total concentration of HCN and NH3. These observations were in agreement with the NO reduction levels observed in Figure 12 for the three biomasses. A high total concentration of HCN and NH3 suggested that the reactions of NO with CHi and HCCO were enhanced and the CHi and HCCO interaction with oxygen was minimized, and thus, the reburn mechanism would be more effective. As shown in Figures 14 and 15, the HCN concentration was generally higher than the NH3 concentration in the reburn zone. Because most nitrogen in biomass are usually in the amine form,26 volatile nitrogen in biomass was normally considered to form NH3 preferentially on pyrolysis16 and the higher NH3 concentration should be observed in the reburn zone. That was in contrast to the measured results of HCN and NH3 concentrations, as shown in Figures 14 and 15, especially for the sawdust and rice husk reburning cases. Such differences could not be traced to measurement failings because of the repeated measurements. It might be related to the fuel N form in different biomasses as well as the biomass decomposition and subsequent reactions in the reburn zone. At present, in the absence of information, such as the nitrogenous intermediate species during rapid pyrolysis of the three biomasses, a potential explanation for this phenomenon might be the faster production rate of NH3 as a pyrolysis product and gradual transformation of nitrogen-containing species in the biomass to form HCN. A large fraction of HCN probably originated from chemical reactions with other species, such as CHi and HCCO, reacting with NO through reactions 36 rather than directly generated as a pyrolysis product. This hypothesis was supported by the observation that the peaks of the HCN concentration occurred somewhat later than the peaks of the NH3 concentration in the reburn zone, by comparing Figure 14 to Figure 15. Nevertheless, more work would be needed to confirm the mechanism as well as to determine the precise causes. Despite this complicated phenomenon, the change patterns of the overall concentrations of the nitrogenous intermediate species (HCN and NH3) in the reburn zone indicated that these species have made great contributions to the NO reduction process. 3.6. Comparison to the Literature. A comparison between the present data and that from previous biomass reburning studies is helpful to better understand the discussion of the results presented in this study. Examination of the literature reveals that a number of experimental studies on biomass reburning have been carried out in different experimental rigs using a variety of biomasses as reburn fuels. Maly et al.11 studied the reburning performance of wood waste in a pilot-scale 300 kW radiant furnace and found that the wood waste reburning performance approached that of natural gas reburning, with over 70% NOx reduction achievable at reburn heat inputs above 20%, which corresponded to the sawdust reburning performance (6468% NO reduction) in this study. Harding and Adams23,24 investigated the reburning process of two different biomasses, a hardwood and a softwood, in a 38 kW laboratory combustor (U-furnace) and concluded that wood was an effective reburn fuel and, if properly used, could result in NOx reductions as high as 6070% with approximately 1015% reburn heat input. They also reported a computer simulation of a full-scale 265 MWe cyclone-fired boiler and evaluated the conditions that maximize the NOx reduction efficiency using wood as the reburn fuel. The results showed that NOx reductions as high as 5060% could be achieved within the constraints set by the boiler and operations and using flue gas instead of air as the wood carrier gas increased the NOx reduction from ∼45 to
Figure 14. Profiles of the HCN concentration along the furnace centerline, with coal SC, recycled flue gas as the carrier gas, and a reburn fuel ratio of 20%.
Figure 15. Profiles of the NH3 concentration along the furnace centerline, with coal SC, recycled flue gas as the carrier gas, and a reburn fuel ratio of 20%.
although some literature reported that the HCN and NH3 concentrations during the biomass reburning process were at the same level.26 Considering the difficulties of HCN and NH3 measurements, it must be stated that the measured values of the HCN and NH3 concentrations presented in this study were more qualitative than quantitative. For sawdust and rice husk reburning, relatively high HCN peaks (100200 ppm) appeared downstream of the reburn fuel injection position, and then the HCN concentration decreased quickly to nondetectable in the reburn zone. For dry sludge reburning, the HCN concentration level was maintained similarly to that of the base case. Only unremarkable increasing in the HCN concentration was observed in the reburn zone. In Figure 15, the NH3 concentration displayed an obvious rise at the biomass injection position with dry sludge reburning but a much lower NH3 concentration (less than 25 ppm) was measured in the reburn zone with sawdust and rice husk reburning. On the basis of the reburning mechanism, HCN and NH3 were the two major reaction intermediates before NO was chemically reduced to N2 in fuel-rich environments. The total concentration, summed by HCN and NH3, was an index reflecting the tendency of conversion from major nitrogen-containing species to N2. Viewing Figures 14 and 15 as a whole, the highest total concentration of HCN and NH3 can be achieved with sawdust 310
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Energy & Fuels ∼55%. About a 10% increase of the NO reduction efficiency was reported by them when the biomass carrier air was replaced by the flue gas. This result was also consistent with the results presented in this study. Ballester et al.26 studied the oak sawdust reburning process in both propane and pulverized coal flames. The tests were carried out in a semi-industrial-scale furnace, which was cylindrical and vertically oriented with a swirl burner at the top, and in-furnace measurements of local temperatures and concentrations of NOx, HCN, and NH3 inside the reburn zone have been performed. In the propane flame, a higher HCN concentration than the NH3 concentration in the sawdust reburning process was observed by them and the levels of HCN and NH3 concentrations were similar to those presented in this study. However, a further comparison cannot be performed because the HCN and NH3 concentration distributions in pulverized coal flames were not provided by them. More recently, Casaca and Costa14,15 evaluated the effectiveness of the rice husk reburning process in a down-fired laboratory furnace and concluded that the reburning performance of the rice husk was almost 60% NOx reduction achievable at reburn heat inputs above 20%. Their studies showed that there was an optimum particle size range of the rice husk for enhanced NOx reduction through reburning. In the same furnace, they performed a detailed experimental study on the pine sawdust reburning process.17 The flue-gas data revealed that above 60% NOx reduction could be obtained with the sawdust reburning at reburn heat inputs of 20%. The detailed in-furnace data of O2, CO, CO2, HC, and NOx concentrations in the reburn zone indicated that the reburning process remained active throughout the reburn zone in the case of sawdust reburning. Despite the difference of experimental rigs, the effectiveness of the biomass reburning process and the change trends of the infurnace data reported by Casaca and Costa were overall consistent with the results presented in this study. Luan et al.13 examined the reburning characteristics of corn straw and rice husk in a one-dimensional temperature-controlled drop-tube furnace, and the profiles of O2, CO, CxHy, HCN, and NH3 in reburn zone were measured by them. The results showed that the NO reduction efficiency with corn straw reburning was higher than that with rice husk reburning. For test reburn fuels, a relatively higher NH3 concentration was observed at a low reburn zone stoichiometric ratio compared to HCN, which was in contrast to the observations in this study. They regarded that the increased NH3 production was the result of HCN conversion in the ashcatalyzed reaction with biomass chars. Munir et al.16 evaluated the biomass reburning process in a 20 kW down-fired combustor. Shea meal and cotton stalk were selected as reburn fuel using different reburn fuel fractions of 5, 10, 15, and 20%. NO reductions of 83 and 84% were obtained in their studies with an optimum reburn fuel fraction of 15% for shea meal and cotton stalk, respectively. Those were much higher than the NO reduction efficiencies obtained in this study. The axial profiles of the temperature and NO and O2 concentrations were also presented in their studies. When those were compared to the results in this study, the change patterns of NO and O2 concentrations in the reburn zone were very similar, although a slightly higher O2 concentration in the reburn zone was observed by them. In addition, they reported a general decrease of the temperature in the reburn zone with reference to the base case, which was also in agreement with the observation in this study. When the studies referred to above were reviewed, there were appreciable differences between the results of these studies because of the differences between the various test facility and
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combustor configuration, fuel type and composition, and operating conditions. Therefore, the definitive conclusions were difficult to draw from such a comparison. Nonetheless, it is clear that biomass is a valid reburn fuel and some gaseous species, such as O2, CHi, HCN, and NH3, play key roles in determining the effectiveness of the biomass reburning process.
4. CONCLUSION Experimental studies have been performed on a semi-industrial pulverized-coal single-burner furnace to evaluate the reburning characteristics of sawdust, rice husk, and dry sludge. The effects of the biomass carrier gas and reburn fuel ratio on the NO reduction efficiency were investigated and analyzed. The detailed in-furnace measurements for O2, NO, CH4, HCN, and NH3 concentration distributions in the reburn zone were also accomplished to gain further insight into the biomass reburning process. The experimental results showed that the NO reduction efficiency for biomass reburning was greatly related to the biomass characteristic. The maximum NO reduction efficiencies of 68% with sawdust reburning, 58% with rice husk reburning, and 45% with dry sludge reburning could be achieved when the recycle flue gas was used as the biomass carrier gas. At least a 10% increase of the NO reduction efficiency could be obtained when the biomass carrier air was replaced by the recycled flue gas. The NO reduction efficiency increased with the increase of the reburn fuel ratio, but only a slight further NO reduction could be achieved with the further increase of the reburn fuel input. The results of in-furnace measurements showed that a higher HCN concentration than the NH 3 concentration was observed in the reburn zone with the sawdust and rice husk reburning, but the reverse measurement results were obtained with the dry sludge reburning. The profiles of the NO concentration in the furnace were greatly related to the concentration distributions of O2, CH4, HCN, and NH3 in the reburn zone. These key gaseous species played crucial roles in the NO reduction process. It is expected that this study provided the essential information of the biomass reburning process and, thereby, enhanced the understanding of the large number of relevant physical phenomena involved in the biomass reburning process. ’ AUTHOR INFORMATION Corresponding Author
*Telephone: +86-27-87542417-8206. Fax: +86-27-87545526. E-mail:
[email protected].
’ ACKNOWLEDGMENT This research was financially supported by the National Natural Science Foundation of China (50806025, 51076052, and 51021065) and the Fundamental Research Funds for the Central Universities (HUST 2010QN023). ’ REFERENCES (1) Bowman, C. T. Control of combustion-generated nitrogen oxide emissions: Technology driven by regulation. Proc. Combust. Inst. 1992, 24 (1), 859–878. (2) Smoot, L. D.; Hill, S. C.; Xu, H. NOx control through reburning. Prog. Energy Combust. Sci. 1998, 24 (5), 385–408. (3) McCahey, S.; McMullan, J. T.; Williams, B. C. Techno-economic analysis of NOx reduction technologies in p.f. boilers. Fuel 1999, 78 (14), 1771–1778. 311
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