Contents
Contents 1
Abstract 2
1. Introduction 3
2. Experimental studies of the fate of char-N during combustion 6
2. 1 Experimental technique 6
2. 2 Effect of operation conditions on the formation of NO during char combustion 11
2. 3 The formation of other N-containing gas species during char oxidation 21
3. Mechanistic studies of char-N conversion 24
3.1 The mechanisms of the conversion of NO during char combustion 25
3.2 NO reduction over char with oxygen and effect of other reacting gases 29
4. Model studies of nitric oxides formation from char combustion 34
4.1 Single particle model 34
4.2 The SKIPPY (Surface Kinetics in Porous Particles) approach 42
5. Concluding remarks 45
References 46
The fate of fuel nitrogen during combustion of char: A review
Zhou H., Jensen A., Glarborg P.,
Abstract:
The conversion of char-N to NO is the main contribution of the formation of nitric oxides and is the most unsolved modeling problem during the char combustion. This literature review summarizes the current understanding of the fate of fuel-N during combustion of char. The review focused on the following three topics: (1) Experimental findings of the split of the fuel-N under single and batch char particle combustion conditions. Approximately 100 % of fuel-N was found to be converted to NO for fine particle under single particle conditions for fixed bed, fluidized bed, and pulverized combustion systems. The finding implies that the formation of minor N-containing species such as NH3, HNCO, and HCN may be caused by the secondary reaction after the release of NO. The operation conditions such as the reactor temperature, the particle size, and the coal rank may affect differently the formation and destruction of NO at different combustion systems; (2) The most useful mechanism for describing the formation and destruction of the NO and N2O during char combustion was proposed by De soete (1990). The effects of major and minor species such as O2 and H2 on the NO reduction of char have not been well elucidated both experimentally and theoretically; and (3) Different simplified models have been proposed at single particle level for describing the fuel-N conversion during char combustion.
Keywords: char, combustion, nitrogen oxides, emission, model
1. Introduction
Nitrogen oxides (NO, NO2, and N2O) emission has drawn much attention by researchers and public health authorities due to the serious environmental impact of NOx (Kraushaar and Ristinen, 1993). Combustion systems are one of the major sources of pollution due to NOx. Fossil fuels such as coal, natural gas, petroleum, as well as renewable bio-fuels and municipal solid waste for instances agricultural residuals (straw, cotton stalk), and forest debris are commonly used in the systems (Smoot, 1993, Saxena, 1994, Samuelsson et al. 2004, Zhou et al. 2006). The fuels, particularly solid fuels, as listed in Table 1, contain significant amounts of nitrogen and give rise to the NOx emission during combustion.
Table 1. Typical nitrogen contents of various fuels
Fuel / Nitrogen (% w/w)Natural gas
Crude oil
Heavy fuel oil
Light fuel oil
Coal
Biomass / May contain 1-5 % molecular N2
0.1 – 0.8
0.2 – 0.5
0.003 – 0.01
0.5 - 2.0
0.2 – 2.1
Three mechanisms have been identified for the production of NOx in combustion systems: (a) Thermal NO - significant NO may be formed by this route when the temperature is in excess of 1573 K. Therefore control of the thermal NO can be achieved by decreasing either the combustion temperature or the residence time at high temperature; (b) Prompt NO - hydrocarbon radicals produced when the fuels being burned react with N2 to produce HCN or CN which subsequently be oxidized to NO. Under most practical combustion conditions, the contribution of the prompt NO to total NO formation is small; and (c) Fuel NO - the fuel NO is formed by combustion of fuel nitrogen. In modern combustion systems, fuel NO contributes more than 80 % of the total emissions. N2O may also be produced during fuel nitrogen conversion particularly at low temperature as in fluidized bed combustors.
The nitrogen functionalities present in coal and in char are: pyrrolic (50-80 %), pyridinic (20-40 %), and quaternary nitrogen (0 – 20 %) (Thomas, 1997). The group may rearrange during pyrolysis and at higher temperature the formation of quaternary nitrogen in graphene layer has been postulated (Wojtowicz et al. 1995). The effect of the nitrogen functionalities on the rate of NO formation is believed to be small (Pels, et al. 1995, Stanczyk, 1999).
Previous studies have shown that the effect of volatile and char nitrogen on the transformation of fuel-N to NO is significant. Volatile nitrogen has been identified to form HCN, soot-bound nitrogen, and NH3 as intermediate species during the combustion of pyrolysis products. A large fraction of char-N may form NO directly. It is believed that char nitrogen is a greater contributor to NO and N2O formation than the volatiles (Tullin et al., 1993, b). Technologically, the released volatile nitrogen is more amenable to control by modification of combustion zone aerodynamics than is char nitrogen (Wendt, 1993). The fate of the nitrogen that remains within the char is crucial when determining the ultimate NO emissions (Coda et al. 1998). Phong-Anant et al. (1985) identified that the contribution to total NOx from char-N in low NOx burners is greater than 60 %. Wendt (1993) further described that the formation of NOx from coal char as the most important unsolved NO modeling problems. An improved knowledge of the parameters that determine NOx formation from the coal char is imperative in order to generate efficient control methods as more stringent emission regulations to be applied.
Reviews of the understanding of nitrogen conversion are given by Miller and Bowman (1989), Johnsson (1994), Thomas (1997), Aarna and Suuberg (1997), Molina et al. (2000), and Glarborg et al. (2003). Different authors restricted to specific topics of NO formation and destruction during solid fuel combustion. Miller and Bowman (1989) discussed mechanisms and rate parameters for the gas-phase reactions of nitrogen compounds. Johnsson (1994) tabulated data on the fraction of char-N to NO and N2O in fluidized bed combustion. He concluded that the conversions of char-N to NO vary between 20-80 %, largely because of wide variations in experimental techniques and the influence of devolatilization temperature, combustion temperature, coal type, particle size, and nitrogen content of the char. Thomas (1997) reviewed the influence coal and char structural characteristics on the release of nitrogen oxides during the combustion of chars. Aarna and Suuberg (1997) elegantly summarized the kinetics of the carbon-NO reaction, and they pointed out that this reaction may involve the possible initial chemisorption of NO and also the reaction of surface complexes. The reduction reaction with pure NO is generally found to be first order with respect to the NO concentration. The reaction is known to be enhanced in the presence of CO and inhibited in the presence of water. Molina et al.(2000) focused their review on the mechanism and model of nitrogen release from the char to the homogeneous phase and its further oxidation to NO, and the reduction of NO on the surface of the char. Glarborg et al. (2003) gave a general review of solid fuel (coal and biomass) nitrogen conversion in fired systems. They emphasized on discussing the homogeneous and heterogeneous pathways in fuel NO formation and destruction and evaluating the effect of fuel characteristics, devolatilization conditions and combustion mode on the oxidation selectivity towards NO and N2.
To further elucidate the mechanism of NO formation during the char combustion, the following three topics are reviewed below: (1) The experimental findings of the split of fuel nitrogen at single particle condition and during batch combustion for different combustion systems; (2) The mechanisms of the conversion of fuel-N proposed by different researchers. We will focus on the review of the formation of HCN during the char combustion and the reduction of NO on the char surface in the presence of oxygen; (3) The modeling work of the fate of fuel-N during char combustion at single particle condition. The main purposes of the review are to present how the fuel-N is split during the char combustion at different reaction systems, and how to explain the experimental findings from the mechanisms and the model point of view.
2. Experimental studies of the fate of char-N during combustion
The heterogeneous char-oxygen reaction, the complex char structure, the surrounding gas species and particles, as well as the very uncertain fluid dynamics in the practical combustion system make it very difficult to elucidate the pathway for char-N conversion during the char combustion. Therefore, different experimental techniques for different combustion systems have been used by researchers in order to understand the conversion of fuel-N to NO during the char combustion.
2. 1 Experimental technique
Table 2 summarizes the experimental studies of char-N conversion during combustion. Test rigs include pulverized coal combustor (PC), fixed bed reactor (FB) including Thermogravimetric Analysis (TGA), and fluidized bed reactor (FBC). It can be observed that the percents (W/W) of fuel-N converted to NO, N2O, and HCN are in the ranges of 3-90% , 0.2-5.7 %, 0.5-10 % depending on coal type, char preparation, residual volatile matter content, temperature, reactor type, stoichiometry, and particle size. The NO is the major product of fuel-N converted and the ratio of fuel-N conversion to NO is remarkably different for different researchers.
Traditional experimental method, for instance, a steady-state technique (de soete, 1990), is commonly employed to determine the global rates of char oxidation, NO and N2O formation during char oxidation and heterogeneous NO and N2O reduction on the char surface.
Some new and promising techniques have been applied and are very helpful to improve the understanding of the pathway of fuel-N conversion. Winter et al. (1996) applied an approach called an iodine addition technique, in which the purpose of the addition of iodine is to reduce radical concentrations to equilibrium levels and therefore to suppress homogeneous reactions, but not affect the heterogeneous. Their experiments provided strong evidence that N2O may be only produced by homogeneous oxidation of HCN in fluidized bed combustion conditions. However, the iodine addition technique has not been used by other researchers mainly due to the influence of the iodine chemistry on the combustion process is not known yet. Miettinen (1996) used tracer technique to trace the path of fuel-N during the char combustion. The advantage of the technique is that it may distinguish the source of N. A 15N-isotope-marked NO was used in the inlet gas by to investigate N2O formation during fluidized bed char combustion. Her experiments confirmed three paths of the possibilities of N2O formation during char combustion, i.e. (1) heterogeneous formation of N2O (one nitrogen atom from the char and the other from NO); (2) homogeneous formation of N2O (two nitrogen atoms from the inlet gas containing NO), and (3) primary formation of a cyano compound (HCN or HNCO) then further reacts with O2 and / or NO to form N2O (, ).
The NO formed from a particle can be subsequently reduced by surrounding char particles or gas species which may make the reaction system very complicated and very hard to elucidate the NO formation during char combustion. To reduce the possible secondary reaction, a few special techniques were developed. To minimize the temperature fluctuation and the second reaction of NO with char, a pulse technique was adopted by Orikasa and Tomita (2003). In their experiments, a small amount of char sample (about 2.5 mg of char, roughly 10-15 particles) was used to minimize the heat generation and a series of a little O2 pulse was fed to the sample bed. Furusawa et al. (1982), Aihara et al. (2000), and Jensen et al. (2000) adopted a single particle technique, in which very small amount of char was placed into fixed bed reactors during char combustion. In this case, the char particles situated on the reactor distributor is only one layer. Therefore, the interactions between particles can be neglected. The experiments thus may be regarded as single particle experiments. Their experiments showed that most of fuel-N is converted to NO, and in some cases the conversion is near 100 %.
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Table 2. Summary of experimental char-N conversion studies
Reactor / Parent coal / Char preparation / Nitrogen(N/C, w/w) / Fraction of char-N to NOx, N2O, and HCN / Operation conditions / Reference
1 / PC / FMC coal char / From FMC-COED coal gasification process / 0.0136 / / 150.0, stoichiometric 1.02-1.25, O2/Ar / Pershing and Wendt (1976)
2 / PC / Montana lignite / Char produced at temperatures of 1250 and 1750 K / 0.0072-0.0162 / / 21%O2/He, 1250 K / Song et al. (1982)
3 / FB / 2 coals / Char produced at 1273,1373,and 1473 K / 0.008-0.022 / / 65-1125, 0.02-5 g, 5%O2/Ar, 1073 and 1173K / Ninomiya et al. (1989)
4 / FB / 3 coals / Char produced in argon at 1450 K / 0.0145-0.0178 / / 35-500 , 50-100 mg, 2-21 %O2/Ar / De soete (1990)
5 / FB / 9 coals / Char produced in fixed and fluidized bed at 1173 K / 0.00758-0.02275 / / 500-590 , 5-10 mg, 1163 K, 3.15O2/Ar, single particle level / Shimizu et al. (1992)
6 / FBC / Kentucky No. 9 bituminous coal / Char produced in the FBC at 1223 K / 0.021 / / 1.02-4.48 mm, 0.2-1.2g, 10%O2, 753-1003 K / Yue et al. (1992)
7 / FB / 2 subbituminous coal 1 lignite / Char produced in the FB up to 1223 K / 0.0144-0.0188 / / 90-106,0.2 MPa, 6-20 % O2, 0-50 %CO2 / Croiset et al. (1995)
8 / FBC / 3 coals – Tilmanstone, Holditch, and Baddesley / coals were put into the FBC / Not available / / 1.0/2.5 g (1-2 cm), single particle level and
1.0g batches of small particles (1.4-1.7 or 2.0-2.4 mm)
1073 K/1173 K / Hayhurst and Lawrence (1996)
9 / FBC / Bituminous and sub-bituminous coal / coals were put into the FBC / 0,0126-0.018 /
/ 5,10,15 mm, 600-1173 K, 5-21%O2/N2, at a particle level / Winter et al. (1996)
10 / FB
(TGA) / 20 coals from anthracite to bituminous coal / Produced in entrained flow reactor at 1273 K / 0.0124-0.031 / / 37-75, 5 mg, 20%O2/Ar, 873-1323 K / Harding et al. (1996)
11 / FBC / Newland coal / Produced in fluidized bed at 1123 K / Not available /
/ 200 mg(4-5 particles),1092 K, 8% O2/He / Goel et al.(1996)
12 / Fluidized bed / Anthracite and bituminous coals / Produced by rapid pyrolysis at 1123 K / 0.0157-0.017 /
/ 0.25-1.5mm, 1123 K, 2.5-21%O2/N2, single or a few particles / Klein and Rotzoll (1999)
13 / FB
(TGA) / Bituminous coals / Produced in a quartz reactor up to 1123 K / / / 25 mg, 20%O2/Ar, 773-1473 K. / Arenillas et al. (1999)
14 / FB / Blair Athol and bituminous coals / Produced in fluidized bed at 1273 K / / / 70-150, 50-250 mg, 0.55-2.0 %O2/He, up to 1273 K / Aihara et al. (2000)
15 / FB
(TGA) / Bituminous coal / Produced in entrained flow reactor / /
/ 45-75, 60 mg, 2 % O2/He, 873 K / Ashman et al. (2000)
16 / FB / bituminous and anthracite coals / Produced in fixed bed at 1123 – 1423 K / / / 10-20, 0.1-20 mg, 10 % O2/N2, 1123, 1323, 1423 K, experiments were performed at particle level / Jensen et al. (2000)
17 / FB / bituminous and lignite coals / Produced in fluidized bed at 1273 K / / / 0.25-0.5 mm, 25 mg, 5 %O2/N2 / Lin et al. (2002)
18 / PC / 4 coals / Produced in an entrained flow reactor / 0,017-0.021 / / 45-75,CO2/H2O/O2, 1473 K / Nelson et al. (2002)
19 / PC / 5 coals of various rank – bituminous, subbituminous, and lignite coals / Produced in the pc combustor at temperatures of 1700-1820 K / / / ~200, 64%Ar/15%CO2/21%O2, 1800-2020 K / Spinti and Pershing (2003)
20 / FB
(TGA) / 9 coals / Produced in drop-tube reactors at 1273 and 1623 K / 0.0174-0.0206 / / Ar/20%O2,
750-1250 K / Jones et al. (2004)
21 / FB / Australian bituminous coal / Produced in an entrained flow reactor / 0.021 / ,
/ 0.1 ~ 1.0MPa, ,, 873 K, 2% O2 / Park et al. (2005)
when loading is close to zero / 0.1 MPa, 1-120 mg
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