NO Emission Control during the Decoupling Combustion of Industrial

May 6, 2013 - content were chosen as fuels to investigate the feasibility of applying a new combustion technology known as decoupling combustion (DC) ...
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NO Emission Control during the Decoupling Combustion of Industrial Biomass Wastes with a High Nitrogen Content Chen Hongfang,*,†,⊥ Zhao Peitao,*,†,‡,⊥ Wang Yin,§ Xu Guangwen,∥ and Yoshikawa Kunio† †

Department of Environmental Science and Technology, Tokyo Institute of Technology, G5-8, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan ‡ Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, People’s Republic of China § Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, People’s Republic of China ∥ Institute of Process Engineering, Chinese Academy of Sciences, No. 1 Zhongguancun North Second Street, Beijing 100190, People’s Republic of China ABSTRACT: Most industrial biomass wastes (IBW) are of high nitrogen content and likely to release high levels of NOx during the thermal utilization process. In the present study, two typical IBWs (sewage sludge and mycelia wastes) with a high nitrogen content were chosen as fuels to investigate the feasibility of applying a new combustion technology known as decoupling combustion (DC) to control NO emissions from the combustion of high nitrogen content IBW. A small-scale quartz dual-bed reactor was used to simulate DC, in which the combustion process was separated into pyrolysis gas and char combustion, and the pyrolysis gas was burned out when passing through the burning-char bed. The results indicated that except for one type of mycylial waste sample, DC could greatly reduce the NO emissions for other biomass wastes at a higher temperature (above 873 K) with an O2−fuel ratio of less than 11 L/g as compared to conventional combustion (CC). A high combustion temperature favored NO reduction in DC before the optimum temperature for NO reduction was reached. Moreover, the effects of the gas velocity and O2−fuel ratio on NO emissions and the reduction in DC were also discussed, and the results demonstrated that DC presented good stability versus the operating conditions.

1. INTRODUCTION At present, a large amount of industrial biomass wastes (IBW) is produced every year because of high-speed industrialization, causing many serious environmental problems. Until now, there have been no effective ways to dispose of such wastes. Most waste is dumped or landfilled in places, which is harmful to nearby ecosystems. Secondary pollution, such as air pollution resulting from bad odor, soil, and water pollution caused by the heavy metals, nutrients, and pathogenic organisms, is typically accompanied with random dumping. In contrast, IBW have been the second largest source for biomass energy, which is regarded as renewable energy because of their carbon-neutral nature. Therefore, thermal conversion technologies, such as pyrolysis, gasification, and combustion, have been developed recently to extract the potential energy contained in organic matters from IBW. These methods can not only recover energy from IBW but also relieve the environmental stress caused by IBW. However, most typical IBW, such as sewage sludge (3.0−5.0 wt %, dry basis (db)), mycelial wastes (2.0−9.0 wt %, db), and distilled spirit lees (3.4 wt %, db), have similar properties and a high nitrogen content. During the thermal utilization of industrial biomass wastes with a high nitrogen content, the nitrogen in biomass is converted into relatively high amounts of NO and a certain amount of N2O associated with the direct combustion of biomass or the combustion of the pyrolysis gas- or gasified gas-containing NOx precursors. Thermal utilization of these wastes also causes secondary pollution, such as photochemical smog, acid rain, the greenhouse effect, and ozone © 2013 American Chemical Society

depletion, which is a result of the high NOx (NO, NO2, and N2O) emissions.1−3 As a result, the viability of new technologies for typical IBW with a high nitrogen content to extract energy in the future4 will largely be determined by their NOx emission properties. Intensive studies on NOx emission and reduction during coal combustion have been carried out, and air- and fuel-staging technology, selective catalytic reduction (SCR), pressurized fluidized bed combustion (PFBC), the thermal DeNOx process, and decoupling combustion (DC) were typically used to reduce NOx emissions.5−12 DC as a new low-NOx-emission technology for coal was originally developed in a dual-bed model and then applied in a circulating fluidized bed (CFB) combustor.6,11,13 It has been verified as an effective NOx emission control technology for coal combustion. In DC, the fuel pyrolysis/gasification step and combustion processes are separated in different places, and the pyrolysis gas is combusted together with the char when passing through the hot char bed produced from the pyrolysis step or when recycling the pyrolysis gas from the CFB downer into the CFB riser to reduce NOx emissions. Correspondingly, the complex reactions in the combustion process are separated and reorganized to strengthen the beneficial interactions and inhibit the undesired interactions.14 Cai et al.12 studied NOx emissions for five different combustion modes: conventional combustion (CC), air-staged combustion, pyrolysis gas reburnReceived: December 5, 2012 Revised: May 5, 2013 Published: May 6, 2013 3186

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ing, partial gasification gas reburning, and DC using five different types of coal. The results indicated that DC presented the highest NOx reduction (32−37%). However, because of the different nitrogen functionality in biomass, the NOx emissions during biomass combustion are expected to be different from that of coal, and the feasibility of DC technologies for biomass combustion should be verified.1,15 Thus, DC technology was also investigated for use in biomass and biomass−coal blends combustion.5,6,11 Decoupled CFB coal and rice husk cofiring tests were carried out by Yang et al.,11 and the results indicated that the NO and N2O emissions decreased as the proportion of rice husk increased. An experimental study of the NO emissions from biomass and biomass−coal blends in a DC dual-bed reactor was reported by Dong et al.5 The results indicated that DC exhibited less NO than CC for both the biomass and biomasscoal blends. However, the conversion of fuel−N to NOx depends on not only the specific characteristics of the fuel but also the nitrogen content of the fuel and the operating conditions.16 In the current literature, the nitrogen content of the biomass used in DC is relatively low (0.35−0.84 wt %, db) and even lower than that of coal (0.98 wt %, db). In the work of Dong et al.,5 for corn straw with a relatively high nitrogen content (0.84 wt %, db), little difference was detected in the NO emissions between DC and CC at 1173 K, whereas at 973 K, NO reduction by DC was clearly observed. Thus, it is reasonable to assume that the NOx emission from IBW with a high fuel-N content in DC is different from that with a low nitrogen biomass and that the reaction conditions will also affect the effectiveness of the NOx emission control within DC. Moreover, the high nitrogen content (>2.0 wt %) in those IBW likely leads to a much higher NOx emission or complex NOx emission behavior with various operating conditions. However, until now, there has been little research focusing on the NOx emissions from the combustion of IBW with a high nitrogen content, especially for DC technology. Therefore, the feasibility of the DC technology for those IBW with a high nitrogen content should be investigated, and the effects of some important reaction conditions on NOx emission control also must be illustrated. The present study investigates NO emission control during the DC of some typical IBW with a high nitrogen content using a small-scale quartz dual-bed reactor designed according to the DC technology concept. Two typical IBW (sewage sludge and mycelia waste) with a nitrogen content of 2.8−8.7 wt % (db) and similar organic matters were chosen as biomass fuels to conduct the experiments. The effects of the major reaction parameters, including the combustion temperature, the gas velocity of the mixed gas, and the O2−fuel ratio, on the NO emission control in DC were experimentally studied by comparing the NO emissions with that from CC. Furthermore, the results of a recent work17 on fuel−N behavior during the pyrolysis of the same types of industrial biomass wastes, which is the first step for DC, were also discussed to demonstrate the mechanism of NO formation and NO emission control in DC.

is similar and mainly exists in the nucleic acids and proteins of dead microorganism cells. However, this bond is quite different from that in coal. In addition, because of the health hazards caused by the bacteria contained in sewage sludge and mycelial waste, they should be disposed, and combustion technology is regarded as a suitable treatment for these wastes. Based on the details above, sewage sludge and mycelial waste were chosen as the representative IBW with a high nitrogen content to investigate the feasibility of the DC technology. Because the characteristics of the two wastes differ according to various production processes, sludge A (SA) and sludge B (SB), which were respectively obtained from wastewater plants in Thailand and China, and mycelial waste A (MWA) and mycelial waste B (MWB), which were respectively produced from antibiotic production companies in Harbin and Shijiazhuang, People’s Republic of China, were chosen as raw materials to conduct the experiments. All biomass fuels were ground and screened to the same size of 0.5−1.0 mm to eliminate the influence of the size of samples and then were dried at 105 °C in an electric oven before each experiment. The analysis results of the biomass wastes are presented in Table 1.

2. EXPERIMENTAL SECTION

Figure 1. Experimental apparatus.

2.1. Feedstock Material. Currently, large amounts of sewage sludge generated in wastewater treatment plants and mycelial wastes from antibiotic production companies are produced every year. Both of these wastes have a high nitrogen content (>2.0 wt %) and similar organic matters, which mainly include dead microorganisms used to remove organic matters from wastewater or for producing antibiotic medicines. Thus, the fuel−N bond in sewage sludge and mycelial waste

system, an online flue gas analysis system, and a dual-bed reactor with a different gas inlet/outlet to simulate various combustion processes. The dual-bed reactor made from quartz was composed of three parts, a top cover with an injection and a gas inlet, an outer tube (50 mm in diameter, 1130 mm in height) with a second gas inlet in the middle, and an inner tube (40 mm in diameter, 470 mm in height). A sintered quartz

Table 1. Properties of the Biomass Wastes mycelial waste A (Harbin) volatile FC ash C H S Cl N Oa a

mycelial waste B (Shijiazhuang)

sludge A (Thailand)

Proximate Analysis (wt %, dry basis) 37.7 80.1 53.3 1.6 11.5 10.6 60.7 8.4 36.1 Ultimate Analysis (wt %, dry basis) 15.6 45.4 32.5 2.7 6.4 4.7 1.6 0.9 1.0 0.3 0.0 0.1 2.8 8.7 4.8 77.0 38.6 56.9

sludge B (China) 47.3 5.5 47.2 24.0 3.0 0.6 0.0 3.7 68.7

Calculated by difference.

2.2. Experimental Apparatus. Figure 1 presents the schematic experimental apparatus, which consists of a gas supplying and mixing

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Table 2. Summary of the Operating Conditions of the Experiments gas velocity (10−2 m/s) temp. (K) biomass amount (g)

DC

CC

DC

CC

DC

CC

2.1 873−1173 0.5

2.1 873−1173 0.5

2.1 1073 0.2−0.8

2.1 1073 0.2−0.8

1.6−2.6 1073 0.5

1.6−2.6 1073 0.5

Figure 2. NO emissions as a function of the temperature in CC and DC.

Figure 3. NO conversions as a function of the temperature in CC and DC. porous plate was installed as the gas distributor in each tube to support the biomass sample or char. The reactor was surrounded and heated in the temperature range of 873−1173 K by a two-zone electric furnace. The synthetic gas, which was composed of Ar and O2 at a ratio of 79:21, was used as the reaction media to avoid thermal NOx and prompt NOx formation caused by the N2 in air. Different combustion processes can be simulated by adjusting the supply gas and gas inlet/outlet. The flue gas (O2, CO, NO, NO2, SO2, and CxHy) was analyzed by an online flue gas analyzer (Testo 350XL, Japan) connected to the computer. Because the value for the NO2 measured by the flue gas analyzer was typically lower than 10 ppm for all experiments in this study and N2O was also suggested to be negligible in similar systems,5,6 only the NO emission from combustion was considered. 2.3. Procedure. In this study, the NO emissions under several operating conditions were investigated in DC and CC for comparison. For all experiments, 10 g of silica sand was added to a biomass sample to maintain a certain height of the sample and to avoid agglomeration of the biomass ash during the combustion process. Before starting the experiments, the silica sand was combusted alone to confirm that it had no effects on the flue gas emission. The experiments of NO emission at different combustion temperatures (873−1173 K) were carried out as follows. For DC, 0.5 g of biomass was first pyrolyzed with Ar gas on the gas distributor of the outer tube to obtain char under the same temperature as the combustion temperature. When starting the DC tests, 0.5 g of biomass was placed on the gas distributor of the inner tube with a supply of Ar gas from inlet A. The mixture gas of O2 and Ar was supplied from inlet B to combust the pyrolysis gas and char obtained in the above pretests. The total ratio of Ar/O2 in the reactor was maintained at 79:21. Because the gas always flew downward from the top of the reactor, it is reasonable to assume that only the volatile from the 0.5 g of biomass in the inner tube was

combusted and that the pyrolysis char was protected by the Ar gas. Consequently, only 0.5 g of the biomass was burned out during DC. The procedure for CC was simpler than that of DC for biomass. Half a gram of biomass with the mixture gas of Ar/O2 was supplied into the reactor and then combusted under the corresponding temperature. The operation procedures for the DC and CC tests were the same as those described above for the other operating conditions. The effects of the gas velocity and the O2−fuel ratio (defined as the amount of oxygen supplied during the complete combustion divided by the biomass amount) on the NO emissions in DC and CC were also investigated. The O2−fuel ratio was changed by adjusting the biomass amount supplied with the same gas velocity. The detailed conditions of all experiments in the present study are summarized in Table 2. 2.4. Calculation. In this study, the change in the NO concentration over time was directly measured by the flue gas analyzer, and then, the total NO emission from the biomass (expressed as [NO], mg/g) was calculated by eq 1. Moreover, the NO conversion from fuel−N (expressed as NOconversion, %) was estimated by eq 2. Given the NO conversion in CC, the NO reduction efficiency (expressed as NOreduction, %) in DC can be evaluated by eq 3.

[NO] =

MNO 22.4m

∫t

t2

1

qvC NO dt

(1)

where CNO is the NO concentration, ppm; qv is the gas flow rate, m3/s; m is the mass of biomass fuel, g; MNO is the molar mass of NO, g/mol; t1, t2 is the the time at the beginning and end, respectively, of the biomass combustion, s.

NOconversion = 3188

[NO]mol × 100 [fuel − N]mol

(2)

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Figure 4. Yields of char−N and char versus the pyrolysis temperature.

with the results reported by Kambara et al.,8 who demonstrated that the same profiles of retained nitrogen in the coal char may exhibit different NOx emissions profiles, possibly due to the different ratio of NH3/HCN. The high NH3/HCN ratio could reduce the NOx conversion rate because NH3 is an important reductant and will react with NO at temperatures of approximately 1073 K, as displayed by reaction R1.22 Therefore, the different behavior of the NO emission and conversion for a temperature of 873−1073 K for MWA in CC was mainly attributed to the fact that a high NH3/HCN ratio of the pyrolysis of MWA at 773−1073 K was measured in our recent research.17

where [NO]mol is the number of moles of NO from the combustion of 1 g of biomass fuel, mol/g, and [fuel−N]mol is the moles of fuel−N in 1 g of biomass fuel, mol/g. NOreduction = ([NO]CC − [NO]DC )/[NO]CC × 100

(3)

where [NO]CC is the total NO emission in the CC, mg/g, and [NO]DC is the total NO emission in the DC, mg/g.

3. RESULTS AND DISCUSSION 3.1. Effects of the Combustion Temperature. Generally, the combustion characteristics are affected by the fuel properties and process conditions, such as the fuel type, particle size, temperature, and air flow rate. As one of the most important operating conditions, the effects of the temperature on NO emissions (mg/g) and NO conversion from fuel−N (%) in DC and CC are shown in Figures 2 and 3, respectively. Figure 2 demonstrates that for CC, the NO emissions increased with temperature, except for MWA. For DC, the NO emissions first decreased before the temperature reached 1073 K and then increased for all biomass in the studied temperature range. Figure 3 demonstrates that the variation of the NO conversions with the combustion temperature was the same as that of the NO emissions in CC and DC. Although the higher nitrogen contained in MWB resulted in the highest amount of NO emissions, no significant differences were observed between the NO conversions of these samples. This result is reasonable because the high concentrations of fuel−N favor the reactions between the nitrogen-containing species that generate N2 and N2O, thus lowering the NO conversion rate.16,18 In CC, the increasing trend of the NO emissions with temperature for the three biomass wastes was also observed during the combustion of coal or other biomass.9,19,20 It was believed that a higher heating rate caused by a higher heat flux at a higher temperature would release more volatile and volatile−N. This hypothesis can be further verified by the results shown in Figure 4A, indicating that the fuel−N conversion to char−N largely decreased with temperature during the rapid pyrolysis of the same biomass wastes (SB and MWA) in a similar reactor. It was also reported that volatile-N was a major contributor to NO emission,16,21 particularly at high temperatures and under oxygen-rich conditions. For MWA, although the opposite trend of the NO emission against temperature was observed at 873−1073 K, the NO emission also began to increase after the temperature exceeded 1173 K. The opposite trend might have occurred because of the NO formation related to the amount of volatile−N and the gas-phase composition of volatile−N (such as HCN, NH3, and HNCO) and to the gas-phase reactions during volatile−N combustion.8,16 As Figure 4A indicates, at 973−1073 K, even the char−N yields of both SB and MWA were almost the same, and the NO emissions and conversions in CC for SB and MWA differed significantly. This result agrees well

NH3 + NO → N2 + H 2O + H·

(R1)

In DC, the complex homogeneous reactions between the gaseous phase and the heterogeneous interactions between the gaseous phase and solid matrix were decoupled and rearranged. As such, the performance of the NO emission in DC is closely related to how the pyrolysis products interact with each other or react with the char phase. Many works have reported that the decoupling technology can reduce NOx emission by the simultaneous reduction activity of pyrolysis gas and char.6,11,23,24 Using a similarly designed dual-stage reactor in our study, He et al.6 demonstrated that NO reduction by burning char was the major contributor to NO reduction in DC, although pyrolysis gas also exhibited some reduction activity for NO. Many researchers18,25−27 reported that biomass char was more active than coal char for NOx reduction. In the current study, more NO is produced because the increase in the temperature enhances the volatile−N release during the pyrolysis part in DC, as discussed. However, as shown in Figure 4B, although char−N decreased significantly with the increase in temperature, the char yield at 873−1073 K was only slightly decreased.17 This result suggests that the release of volatile−N from biomass was faster than the weight loss of the char with increasing temperature during the pyrolysis process. Thus, the amount of char was still sufficient to reduce the NO emitted from the volatile−N combustion in that temperature range. Moreover, the NO reduction by the char was also favored to some extent by a higher reaction temperature in the presence of oxygen26 because more CO was released at a higher temperature, strengthening the NO reduction. However, the least amount of char was generated at a very high temperature (1173 K) in DC and was likely to be consumed in a shorter time in the combustion zone, whereas the NO emission from the volatile−N combustion was greatly increased. Moreover, the char generated from a higher pyrolysis temperature exhibited poorer activity. Therefore, in DC, the NO emissions and NO conversion increased again after the combustion temperature was higher than 1073 K. 3189

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continually increases, the weight loss of the char and the loss activity of the char caused by char production at a high pyrolysis temperature becomes severe, there is a decrease in the NO reduction. The optimum temperature for NO reduction is different because of the distinctive feature of biomass wastes. In previous work on DC, up to 40% of NO emissions could be reduced at 1073 K for coal, and approximately a 15% reduction in the NO emission could be achieved for some biomass with a low nitrogen content at 1173 K (rice husk and sawdust).5,6 The previous work indicated that DC has the great advantage of low NO emission for biomass wastes with a high nitrogen content at a suitable temperature, and it is advised that DC be used for complex IBW. However, the effects of the biomass properties on NO reduction and the suitable temperature for DC should be deliberated and further studied. Figure 5 illustrates that at a low temperature (873 K), MWB and SB present a negative NO reduction in DC because more NO was emitted than in CC. This abnormality must be comprehensively investigated based on the burning properties of biomass, the competition between char oxidization and NO− char reduction, and the figuration of the reactor, which were closely related to how the pyrolysis rate matched the char combustion rate in DC. Figure 6 illustrates the behavior of the NO emissions of biomass wastes at different temperatures in DC or CC. The NO emission for MWB during the combustion period (shown in Figure 6A) illustrates that in CC, the combustion period is much longer and that the NO concentration has two separated peaks at the low temperature of 873 K, whereas only one peak exists at the high temperature (973 K). A similar phenomenon was also observed in SB. Nevertheless, as shown in Figure 6B, the NO concentration for SA at 873 K in CC has only one peak, and the combustion period is almost same as that at 973 K. It was suggested that at the relatively low temperature of 873 K, the heating rate and heat transfer were still sufficient for the volatile release from SA to reach a higher burning rate, whereas for other biomass wastes, the volatile was released slowly at 873 K and a higher temperature was needed to accelerate the combustion process.28 Thus, the slow release rate of volatile from MWB and SB at 873 K in the pyrolysis zone may be the reason for a negative effect on the NO emission in DC at 873 K. Taking MWB as an example, at 873 K in CC, the lower release rate of the volatile increased the possibility of the interaction between the volatile and nascent char part resulting in some NO reduction as the supplying gas flew downward to the bottom of the reactor. In contrast, in DC, as discussed, the mechanism for low NO emission depends mainly on the acceleration of the NO reduction by char. In this study, before starting DC, the char was made and put in the bottom of the outer tube. After starting the experiments, biomass with Ar gas was immediately supplied into the reactor, and the Ar/O2 gas mixture was supplied from gas inlet B. At the initial stage at a low temperature, the production rate of the pyrolysis gas was so slow that the excess Ar/O2 level was relatively high, and the oxygen was likely to react with the char exposed on the gas distributor. As a consequence, the first peak of the NO concentration is much higher than that in CC for the DC of MWB, as shown in Figure 6A. Because of the consumption of the char in the early stage, the char amount was greatly reduced such that only a small amount of NO could be reduced. Moreover, at a low temperature, the activity of the char for NO reduction is low.26 He et al.6 reported that it was important to keep a high temperature char layer near the exit of the stove. According to the discussion, at a low temperature in DC, the low release rate of the volatile, char

In summary, the major pathways of NO formation in the DC of our study can be described by reactions R2−R14)11,16,18,25−27 and the NO reduction by char by reactions R9−R14 Pyrolysis zone: fuel − N → volatiles − N(tar − N, HCN, NH3, HNCO , NO)

(R2)

Combustion zone: tar − N → HCN, NH3 → NO

(R3)

HCN → NCO·, NH · → NO

(R4)

NCO· , NH·+ NO → N2 + ...

(R5)

NH3 + O2 → NO + H 2O + H·

(R6)

NH3 + NO → N2 + H 2O + H·

(R7)

char − N + O2 → NO + ...

(R8)

2C() + O2 → C(O) + 2CO

(R9)

C(O) → C()

(R10)

3C(O) + NO → C(N) + 2CO2

(R11)

NO + 2C() → C(N) + C(O)

(R12)

C(N) + NO → N2 + C(O)

(R13)

2C(N) + O2 → N2 + 2CO

(R14)

where C(), C(N), and C(O) denote the char surface free site, surface nitrogen, and oxygen species, respectively. Figure 5 illustrates the NO reduction efficiency of DC compared to CC with the increase of temperature. Because DC

Figure 5. Effects of the temperature on NO reduction in DC compared with CC.

always exhibited a negative effect on the NO reduction for MWA in the studied temperature range, it was not shown in Figure 5. Figure 5 suggests that DC can effectively reduce NO emissions above 873 K for the three other IBW with a high nitrogen content. A high temperature in the range 873−1073 K during DC favors NO reduction for biomass wastes. By increasing the temperature to 1173 K, the NO reduction efficiency can reach 70.0% and 72.0% for MWB and SB, respectively, whereas the NO reduction for SA decreases from the maximum value to 52.2%. This result could be ascribed to the same reasons described in the NO emissions of DC. When the char is sufficiently active to react with the NO emitted from the volatile−N part, a higher temperature favors NO reduction. When the temperature 3190

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Figure 6. Time change of the NO emissions for biomass wastes in DC or CC.

Figure 7. Effects of the gas velocity on the NO emission and conversion in DC and CC.

consumption by char−O2 reactions and the low activity of the char for NO reduction may be the major reason why the interaction of the char and volatile causing the negative NO reduction is weak compared with CC. This explanation can also be confirmed by Figure 6C, which indicates that the NO concentration of MWA at 873−1073 K in CC always has two peaks and a longer combustion time. In addition, MWA has the least fixed carbon, as shown in Table 1, which is a major reductant for NO and supplies the reaction surface for NO reduction, leading to little NO reduced. The table also shows that MWA has the highest ash content, which contains high Ca and K, resulting in a low activity of the char. Therefore, DC is not effective in reducing the NO emission for MWA at all temperatures. To design a DC stove that can recover the energy from complex IBW with high nitrogen, it is important to supply sufficient char, maintain a high activity of the char, and promote the complete oxidization of volatile−N to NOx before the char is consumed by char−O2 reactions. 3.2. Effects of the Gas Velocity. Many previous research studies28−30 have reported that the air flow rate is essential for the combustion of biomass in the fixed bed. The effects of the total gas velocity on the NO emissions and NO conversions in DC and CC are presented in Figure 7. MWB and SB were taken as the representative mycelial waste and sewage sludge, respectively. Figure 7 illustrates that the NO emissions and NO conversions

have the same trends against the gas velocity during combustion. The NO conversions for SB in DC and CC are slightly higher than that for MWB, particularly at low gas velocities, although the NO emissions from MWB are much higher than that from SB. For MWB, the NO emissions and conversions first increased and then decreased as the gas velocity increased in CC, whereas for SB, both the emissions and conversions decreased as the gas velocity increased. This difference in the behavior may have resulted from the different combustion stoichiometric ratio for different biomass wastes. In CC, the oxygen amount was increased with an increase in the gas velocity so that more volatile−N (such as NH3 and HCN) was oxidized to form NO. However, a much higher gas velocity would likely shorten the residence time of the reactions or lower the burning rate of combustion, leading to lower NO emission. Thus, the NO emissions for MWB first increased and then decreased. In the case of SB, the maximum NO emission occurred at a lower gas velocity because of its lower volatiles and fuel−N content. The oxygen amount was already sufficient to convert all of the combustible gas and char to NO at the lowest gas velocity. Thus, the NO emissions decreased by continually increasing the gas velocity. However, in DC, although the NO emissions and conversions decreased as a function of the gas velocity, they were not as sensitive to the flow rate as those in CC. This lower sensitivity 3191

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meant that DC has good stability versus the gas velocity, which can be attributed to the separated and complete combustion of the volatile gas. The volatile released from the pyrolysis zone was easy to ignite and rapidly combusted with a large amount of oxygen gas. The heat generated by the fierce combustion was sufficiently large to retrieve the heat loss caused by the increase of the gas velocity, creating stable combustion. In addition, as discussed, at 1073 K, the char reduced the NO emission. The NO emitted from the fierce combustion of volatile can promote NO reduction by a hot char bed. Figure 7 further confirms that a significantly lower NO amount was emitted at the gas velocity of 1.7−2.5 × 10−2 m/s in DC compared with CC. The change of the NO reduction by DC with the gas velocity is shown in Figure 8. As the gas velocity increases, the NO reduction by DC has a peak level at a gas velocity of 2.1 × 10−2 m/s for both types of biomass.

Figure 10. NO reduction efficiency as a function of the O2−fuel ratio.

increased to a high value, low NO emission was observed. This observation was in accordance with the results reported by Qian et al.,20 who found that the NO emission decreased with excess air for biomass with a high nitrogen content in their vortexing fluidized bed combustion. This decrease was attributed to the characteristics of the experimental apparatus and the sample properties, particularly the amino acid contained in the soybean. Moreover, the variation of the O2−fuel ratio in this study was realized by changing the sample amount under the same gas velocity in the batch-scale experimental setup. It was believed that the small amount of fuel−N within the samples could also be the main reason for the low NO emission at a higher O2−fuel ratio. In DC, as shown in Figure 9, the variation of the NO emissions and conversions with the O2−fuel ratio present no notable difference from that of CC. However, DC exhibits better combustion stability than that of CC against the O2−fuel ratio. The NO emission and conversion also exhibit an increasing trend at a higher O2−fuel ratio, which can be attributed mainly to the fact that more O2 prompts the oxidization of nitrogen species, such as HCN and NH3. However, because of the high reduction ability of char, the NO emissions and conversions did not change considerably with increasing O2−fuel ratio as they did in CC. Figure 10 indicates that increasing the O2−fuel ratio decreased the NO reduction efficiency for MWB, whereas the NO reduction efficiency for SB first increased and then decreased. In addition, at the highest O2−fuel ratio (at approximately 12.7 L/g for MWB and 11.8 L/g for SB), the NO reduction efficiency is negative. As discussed, the reactions of char−O2 and NO−char compete with each other during DC. In the case of a high O2− fuel ratio, the char produced at the bottom of the reactor was too small to be easily consumed before reacting with the NO emitted from the volatile phase. Therefore, the volatile NO might not have had the chance to interact with the char, or the interaction

Figure 8. NO reduction efficiency as a function of the gas velocity.

3.3. Effects of the O2−Fuel Ratio. The highest NO reduction efficiency of DC was observed at a gas velocity of 2.1 × 10−2 m/s, as previously mentioned. Figure 9 presents the effects of the O2−fuel ratio on the NO emissions and conversions for MWB and SB within DC and CC at a gas velocity of 2.1 × 10−2 m/s. Furthermore, the NO reduction efficiency by DC as a function of the O2−fuel ratio is displayed in Figure 10. Figure 9 illustrates that the NO emissions and NO conversions still exhibit similar trends against the O2−fuel ratio during the combustion processes. In CC, the emissions and conversions first increased and then decreased with the O2−fuel ratio for these two samples. At a relatively low O2−fuel ratio, namely, fuel-rich combustion, some part of the nitrogen-containing species, such as NH3, HCN, and other radical matters, were prone to be NO reductant, leading to a relatively low NO emission through reactions R5 and R7, as described. With the increase in the O2− fuel ratio, more O2 was supplied to convert the nitrogencontaining species to NO, resulting in an increase in the NO emission. However, because the O2−fuel ratio continuously

Figure 9. Effects of the O2−fuel ratio on the NO emission and conversion in DC and CC. 3192

dx.doi.org/10.1021/ef301994q | Energy Fuels 2013, 27, 3186−3193

Energy & Fuels

Article

Agency (No. 21161140329) and also partly supported by the State Scholarship Fund of China under Grant No. 2011609050.

period was shorter than that in CC, resulting in the negative NO reduction in DC.

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4. CONCLUSIONS In this study, two typical types of IBW (mycelial waste and sewage sludge) with a high nitrogen content were combusted using CC and the new low NOx emission technology known as DC. The NO emission, NO conversion from fuel−N, and NO reduction efficiency with DC were experimentally investigated under different operating conditions using a dual-bed reactor to detect the feasibility of applying the DC technology to the abstract energy from complex IBW with a high nitrogen content. It can be concluded that DC is effective in controlling the NO emission from the combustion of high-nitrogen-content biomass wastes at suitable conditions. The NO reductions with DC were considered based on the properties of biomass wastes, combustion temperature, total gas velocity, and O2−fuel ratio. The NO emission from the combustion of these samples with a high nitrogen content can be effectively reduced at a higher temperature (above 873 K) with the certain O2−fuel ratio (below 11 L/g), except for MWA. The main conclusions are summarized as follows: (1). The NO emissions from the combustion of the biomass wastes used in this study could be effectively reduced at temperatures above 873 K in DC, except for MWA. The negative performance of the NO emission in MWA could be attributed to the slow burning rate and little reductant of the biomass wastes. The optimum temperature for NO reduction in DC depended on the properties of the biomass wastes. The increase in the combustion temperature favored NO reduction. (2). Compared with CC, DC has good stability for NO emission versus the total gas velocity and O2−fuel ratio at 1073 K. Under a gas velocity of 2.1 × 10−2 m/s, both types of biomass wastes exhibited the best NO reduction in DC. A moderate O2−fuel ratio was beneficial for NO reduction with DC. (3). To lower the NOx emission in DC from industrial biomass wastes with a high nitrogen content, it is important to provide sufficient amounts of char, maintain a high activity of char, and increase the burning rate of biomass to promote the NO conversion from the volatile part before char combustion.

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +81-45-924-5586. Fax: +81-45-924-5586. E-mail: pt. [email protected] (Peitao Zhao), *[email protected] (Hongfang Chen). Author Contributions

⊥ These authors contributed equally. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research work was supported by the Strategic China-Japan Cooperative Program on “Science and Technology (S&T) for Environmental Conservation and Construction of a Society with Less Environmental Burden” of the National Nature Science Foundation of China and the Japan Science and Technology 3193

dx.doi.org/10.1021/ef301994q | Energy Fuels 2013, 27, 3186−3193