N2O emission under fluidized bed combustion condition

N2O emission under fluidized bed combustion condition

Fuel Processing Technology 84 (2003) 13 – 21 www.elsevier.com/locate/fuproc N2O emission under fluidized bed combustion condition B.X. Shen a,*, T. M...

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Fuel Processing Technology 84 (2003) 13 – 21 www.elsevier.com/locate/fuproc

N2O emission under fluidized bed combustion condition B.X. Shen a,*, T. Mi b, D.C. Liu b, B. Feng b, Q. Yao a, Franz Winter c a

Department of Thermal Engineering, Tsinghua University, Beijing, 100084, China National Lab of Coal Combustion, Huazhong University of Science and Technology, Wuhan, 430074, China c Research Group Fluidized Bed Technology and Reaction Engineering, Technology University of Vienna, A-1060 Vienna, Getreidemarkt 9/159, Austria

b

Abstract In this paper, many rules about N2O and NOx emission under fluidized bed combustion conditions were found by experiments. The research results indicate that CaO, CaSO4, Fe2O3 and char have important influence on decomposition of N2O; co-combustion of coal and biomass are effective measures to low N2O and NOx emission. D 2002 Published by Elsevier Science B.V. Keywords: N2O decomposition; Influence of oxide and sulfate; Co-combustion of coal and biomass

1. Introduction N2O is a sharp greenhouse gas. It is responsible for the destruction of ozone in the atmosphere. The concentration of N2O in atmosphere increases at the rate of 0.2 –0.4% every year [1]. The emission of N2O in fluidized bed coal combustion is about 50 –200 ppmv with the highest rate of 400 ppmv [2]. In recent years, intensive researches that worked on the mechanism of formation and decomposition of N2O have been carried out at home and abroad. Moritomi et al. [3] and Amand and Andersson [4] correlated the N2O emission to the fuel ratio of various fuels. Boemer et al. [5] considered factors such as coal rank, excess air ratio and temperature as the most important parameters influencing N2O and NOx emission. N2O and NOx emissions decrease with increasing coal rank. When temperature rises, NOx emission increases and N2O emission decreases. NOx and N2O * Corresponding author. Current address: College of Environmental Science and Technology, Nankai University, Tianjin, 300071, P.R. China. E-mail address: [email protected] (B.X. Shen). 0378-3820/02/$ - see front matter D 2002 Published by Elsevier Science B.V. doi:10.1016/S0378-3820(02)00104-2

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Fig. 1. Schematic diagram of the laboratory-scale bubbling fluidized bed.

increase with the increasing excess air ratio. Leckner and Amand [6] showed that the present publications agree with each other concerning the influence of the main parameters except that of limestone feeding. Coal combustion can be separated into two phases: quickly releasing and combustion of volatiles; and gradually combustion of the char. It is known that N2O and NO can come from both volatile and char combustion. Char by itself can also reduce N2O emission in fluidized bed condition. This is found in our experiments. Fuel always contains minerals that are represented in ash as Fe2O3, Al2O3, SiO2, MgO, CaSO4 and MgSO4. How do they influence the emission of N2O and NOx? How does co-combustion of coal and biomass reduce the emission of N2O and NOx in fluidized bed? These questions will be discussed in this paper because of few literatures about these topics.

2. Experimental apparatus and measurement The experimental apparatus is presented in Fig. 1. The main body is a small bubbling fluidized bed. The bed body is made up of quartz glass tube of 20-mm inner diameter and 600-mm length. The quartz glass tube is set into an electric heating furnace. Air (or argon gas, Ar) is sent into the bed through a grid plate. The height of bed material is maintained at about 20 mm. The air (or Ar) is pre-heated at the lower part of the electric heating Table 1 The ultimate and proximate analyses of the coals and other fuels Fuels

GL coal CF coal SM coal SY coal XH coal Wood chip Rice husk

Ultimate analysis (%)

Proximate analysis (%)

C

H

N

S

O

Water

Volatile

Ash

Fixed C

88.9 83.7 82.1 84.5 89.1 48.9 45.2

4.2 5.4 5.3 5.2 4.8 10.61 15.6

1.4 1.3 0.9 1.8 1.2 0.21 0.2

1.3 1.0 0.8 1.8 1.1 0.15 0.09

4.2 8.6 10.9 6.7 3.8 40.13 38.91

1.1 4.9 4.5 3.2 2.8 6.49 6.64

10.9 16.3 30.1 23.6 13.9 81.13 55.12

16.9 32.3 25.4 19.2 14.6 3.86 18.87

71.1 46.5 40.0 54.0 68.7 8.52 19.37

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Table 2 Properties of char Char

SM char

SY char

XH char

Nitrogen content (%) BET (m2 kg 1) Pore volume (m3 kg 1)

0.7 35.6  103 13.98  10 6

1.5 6.676  103 13.63  10 6

1.3 3.353  103 6.48  10 6

furnace. The upper part of the electric heating furnace produces heat to maintain and control the temperature of the bed. Fuel is fed into the bed by a fluidization feeder. The composition of the flue gas passes through a dust catcher first. Some of the gas is passed through NaOH solution and then silica in order to analyze the concentration of N2O. Concentration of N2O is measured by the gas chromatograph with electron capture detector (GC-ECD). Concentrations of NOx, CO/CO2 and O2 are measured with infrared technology apparatuses. The analysis results of coals and other fuels are presented in Tables 1 and 2. Coal samples marked as SM, SY, XH, GL and CF came from different places of China. Three chars: SM, SY and XH were prepared in the small bubbling fluidized bed at 860 jC for 1 min by argon gas passing through the bed. SM, SY and XH chars were made from SM, SY and XH coal, respectively. The characters of the three chars are presented in Table 2. The specific surface area and pore volume are measured by MAC-500. The experiments on the decomposition of N2O by char are carried out with argon gas passing through the bed. The experiments about the influences of limestone and co-combustion on N2O and NOx are carried out under combustion condition. The experiments about the influences of oxide and sulfate are carried out with argon gas passing through the bed.

3. Results and discussion 3.1. The influences of char The bed temperature is maintained between 677 and 977 jC with the char as the fluidized bed material. The concentration of N2O in inlet gas (argon gas) is 105 ppmv. The decomposition ratio N2O is shown in Fig. 2. Compared with the bare bed, the chars in the bed have decomposed the N2O but the decomposition ratios have no direct relations with the nitrogen content. From Fig. 2, we can also find that the decomposition ratios of N2O in three chars are SM > SY>XH. This might be due to the different specific surface area and pore volume of the three chars. The larger the specific surface (BET) area and pore volume, the greater the decomposition. The decomposition increases with the temperature increasing. It is known that volatiles and char from coal can produce N2O when they combust because of their nitrogen content. The relative importance of N2O formation maybe decided by the amount of volatiles and char in the coal and the fuel-N content [15]. During combustion of the char from coal with high-carbon content, some of the fuel-N will be transformed to N2O and NO on the surface of the char. From the experiments, it shows that some N2O may be absorbed by the char and then be decomposed by the char. The mechanism of decomposition of N2O is not clear now. It is assumed that there are

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Fig. 2. Relation of decomposition of N2O by char.

active carbon ( –C) and active radical (– CO) existing in the char, the absorbed N2O in the surface of the char may react with ( –C) and (– CO) as described by the following. N2 O þ ðCÞ ! N2 þ CO 2N2 O þ ðCÞ ! 2N2 þ CO2 N2 O þ ðCOÞ ! 2N2 þ CO2 Although the experiments have been carried out in oxygen-free condition, it is supposed that in combustion condition the char can both produce and decompose N2O. The production of N2O by char is due to the fuel-N in the char and the decomposition of N2O may be due to the active carbon (– C) and active radical (– CO) in the char. The amount of N2O is the result of the two competitive reactions. It may be concluded that the char in the upper part of fluidized contains less fuel-N and always with large porosity after combustion in the lower part of fluidized bed, this will help to reduce N2O. However, too much of the char concentration in the upper part of fluidized bed will increase CO emission. 3.2. The influence of oxides and sulfates The influence of limestone on N2O decomposition is shown in Fig. 3. In this experiment, the lean coal is used whose character: Wf = 1.1%, Af = 16.9%, Cf = 82.0%, C = 84.0%, H = 4.5%, O = 6.4%, N = 1.8%, S = 3.3%. The conversion of fuel-nitrogen to N2O decreases with the limestone being added, while the conversion of fuel-N to NO increases with the limestone. The conversion of nitrogen to N2O decreases about 3.5% at the temperature between 850 and 940 jC and NO increases about 10%. The results of Shimizu and Inagaki [19] and Hiltunen et al. [7] are similar: the addition of limestone in

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Fig. 3. The influence of limestone on N2O and NO. (1) N2O no limestone, (2) N2O Ca/S = 2, (3) NO no limestone, (4) NO Ca/S = 2.

coal combustion reduces the emission of N2O while increases the emission of NO. The reduction of N2O is mainly due to the catalyzing of limestone directly [20]. 2N2 O ! 2N2 þ O2 N2 O þ CO ! N2 þ CO2 At the same time, limestone can react with HCN and NH3, the precursors for the emission of N2O and NO, to form NO. So the emission of NO increases greatly, and the emission of N2O decreases [21]. The influences of Fe2O3, Al2O3, SiO2, MgO, CaSO4 and MgSO4 on the conversion of nitrogen to N2O are presented in Fig. 4. These experiments are carried out with argon gas and N2O (200 ppmv) passing through the bed. From Fig. 4, it is found that the influences are different for different oxides. At temperatures between 650 and 960 jC, the influences by Fe2O3 and CaSO4 are the most drastic. The influences by MgSO4 and MgO are weak.

Fig. 4. The influence of oxide and sulfate. (1) Fe2O3, (2) CaSO4, (3) Al2O3, (4) SiO2, (5) MgSO4, (6) MgO, (7) bare bed.

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Fig. 5. Co-combustion of GL and wood chip. (1) no W, (2) W/C = 5%, (3) W/C = 10%, (4) W/C = 15%, (5) W/ C = 25%, W denotes wood chip, C denotes coal.

3.3. The influence of co-combustion of coal and biomass The influences of co-combustion of coal GL and wood chip and co-combustion of coal CF and rice husks are presented in Figs. 5 –8. From the experiments, we know that both co-combustion of coal and wood chip and co-combustion of coal and rice husk can decrease the emission of N2O and NOx. The reduction of N2O emission by co-combustion decreases as the biomass coal ratio increases. As for NOx emission, the reduction for NOx increases as the ratio increases. N2O decreases with temperature increasing while NOx increases with temperature increasing. The reduction of N2O and NOx emissions may have several explanations. From Table 1, it is known that wood chip and rice husk contains very high volatiles. Adding wood chip or rice husk to coal results in a larger release of volatiles in the lower part of the fluidized bed. The volatiles will consume most of the oxygen and forms oxygen lean zone in this area to restrict N2O and NOx formation, N2O and NOx concentration decrease.

Fig. 6. Co-combustion of CF and rice husk. (1) no R, (2) R/C = 5%, (3) R/C = 10%, (4) R/C = 15%, R denotes rice husk, C denotes coal.

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Fig. 7. Co-combustion of GL and wood chip. (1) no W, (2) W/C = 5%, (3) W/C = 10%, (4) W/C = 15%, (5) W/ C = 25%, W denotes wood chip, C denotes coal.

Volatiles from biomass contain above 60% in weight, whose composition is CO, H2, CH4, C2H4 and C3H6 mostly [9,10]. In high temperature, pyrolysis gas will decompose into a lot of reductive radicals such as hydrocarbon, hydrogen and oxyhydrogen. The radicals will reduce N2O and NOx by reaction as follows [11,12]: N2 O þ H ! N2 þ OH N2 O þ OH ! N2 þ HO2 NO þ 3CH2 ¼ H þ HNCO NO þ CH ¼ HCN þ O The nitrogen content in biomass is lower than that of coal. The emission for cocombustion of coal and biomass is lower than that of coal combustion.

Fig. 8. Co-combustion of CF and rice husk. (1) no R, (2) R/C = 5%, (3) R/C = 10%, (4) R/C = 15%, R denotes rice husk, C denotes coal.

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The quick devolatilization of wood chip and rice husk results in the formation of char with higher porosity and higher reactivity compared with that from coal [13]. This encourages the decomposition of N2O and NOx [17]. Li [16] found that the pyrolysis of biomass produces NH3 and HCN at the same time, but the formation of HCN went to completion much more rapidly than that of NH3. HCN can interact with the nascent char significantly to form soot or N2 [17]. It indicates that in the lower part of the fluidized bed, the pyrolysis of biomass mostly produces HCN, which can be reduced by the char quickly. Because HCN is the main precursor of nitrogen oxide [18], N2O and NOx emission from co-combustion of coal and biomass maybe less than that from coal combustion.

4. Conclusions 1. Char from coal can decompose N2O by itself. 2. Catalysts such as limestone, CaO and Fe2O3 can promote the decomposition of N2O. 3. For high-nitrogen content coal, co-combustion of coal and biomass can be adopted to decrease the emission of N2O.

5. Uncited references [8] [14]

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