Article pubs.acs.org/EF
Relationships between Gas-Phase Stoichiometric Ratios and Intermediate Species in High-Temperature Pulverized Coal Flames for Air and Oxy-Fuel Combustions Masayuki Taniguchi,†,* Yuki Kamikawa,† Tetsuma Tatsumi,† Kenji Yamamoto,† and Yuki Kondo‡ †
Hitachi Research Laboratory, Hitachi, Ltd. 7-1-1 Omika-cho, Hitachi-shi, Ibaraki-ken 319-1292, Japan Kure Division, Babcock Hitachi, K. K. 6-9 Takara-cho, Kure-shi, Hiroshima-ken 737-8508, Japan
‡
ABSTRACT: Stoichiometric ratio (SR) and gas-phase stoichiometric ratio (SRgas) are fundamental burning conditions needed to consider combustion phenomena. For oxy-fuel combustion, large quantities of CO2 and H2O are included in the feed gas. CO2 and H2O are not combustible species, but C and H are combustible elements. Two types of stoichiometric ratios can be defined, depending on whether or not C, H, and O from CO2 and H2O in the feed gas are included in evaluating the stoichiometric ratio. We examined which definition was easier to use when considering the reaction mechanism. We measured NOx and the intermediate species, hydrocarbons, NH3, HCN, and H2S, under the fuel-rich conditions and using a staged high-temperature drop-tube furnace. Burning temperatures were 1573−1873 K. We also measured NOx emission and unburned carbon in ash at the furnace exit of staged combustion. SR-all (or SRgas-all) was obtained by an evaluation including C, H, and O from CO2 and H2O in the feed gas. SRgas-all was easy to obtain by considering the reaction mechanisms for intermediate species, such as hydrocarbons, NH3, HCN, and H2S, in the fuel-rich zone. When the SRgas-all values were the same, the differences in concentrations of these intermediate species between air and oxy-fuel combustions were small. SR (or SRgas) was obtained by an evaluation excluding C, H, and O from CO2 and H2O in the feed gas. The association with SR (or SRgas) was stronger for NOx concentration or conversion. NOx conversion ratio at the furnace exit for oxy-fuel combustion was almost the same as that for air combustion at the furnace exit, when the SR value in the burner combustion zone was the same.
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INTRODUCTION In a previous paper,1 we developed a new-concept drop-tube furnace (DTF) in which two high-temperature electric furnaces were connected in series. Coal was burned under fuel-rich conditions in the first furnace, then, staged air was supplied at the connection between the two furnaces. The reaction temperature (1800−2100 K) and time (2−3 s) were similar to those used in actual boilers. Similar combustion performance values as for actual boilers were obtained regarding NOx emission and unburned carbon in ash (UBC). In the present investigation, we extended this study to oxy-fuel combustion. We also examined NOx performances in the fuel-rich region of a pilot-scale furnace. We compared our past results obtained with the DTF with the results2 obtained with the pilot-scale furnace. Watanabe et al.3 studied NOx reaction mechanisms of staged oxy-fuel combustion. They compared concentrations of NOx, HCN, and NH3 of air and oxy-fuel combustions for staged combustion of methane doped with ammonia and found that the concentrations of HCN and NH3, which are intermediate species of NOx formation, differed significantly between air and oxy-fuel combustions. In the present study, first we compared concentrations of NOx and intermediate species for air and oxy-fuel combustions of pulverized coal at high temperature. We examined common points and differences of gas and coal combustion. NOx concentrations of air and oxy-fuel combustions have been compared previously at low temperature.4,5 Next, we also examined the influences of burning temperature by comparing present and previous results. Concentrations of intermediate © 2012 American Chemical Society
species, such as HCN, NH3, and hydrocarbons have been measured previously.5,6 Then, fourth, we also examined concentrations of H2S, which is an important intermediate species that causes corrosion7 of the water wall of boilers. The stoichiometric ratio (SR) is a fundamental operating condition for combustion systems. Several studies have examined the relationships between the stoichiometric ratio and product concentration.5,6,8 For numerical calculations, databases representing relationships between stoichiometric ratio and species concentrations have usually been used in order to shorten the calculation time.9,10 We proposed the index of gas-phase stoichiometric ratio (SRgas), in order to examine the NOx reaction for pulverized coal combustion.5,6,10 SRgas is one of the local stoichiometric ratios. The stoichiometric ratio (SR) is evaluated by excluding the combustible species remaining in the solid phase.5 SRgas makes it easy to consider the reaction mechanism of pulverized coal combustion at high temperature, because NOx, under the high-temperature, fuel-rich conditions, has been found to be strongly influenced by the index.5,6,10 Then, fifth, we examined the relationships between the index and intermediate species. The stoichiometric ratio (or air ratio) is usually evaluated from the ratio of fuels and oxygen in feed gas (usually air). Elemental contents of C, H, and O included in the fuel and oxygen in the feed gas are mainly considered in the evaluation. On the other hand, elemental contents of C and H in the feed Received: March 1, 2012 Revised: May 17, 2012 Published: June 22, 2012 4712
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Figure 1. Material balances for (a) air combustion, (b) oxy-fuel combustion without flue gas recirculation, and (c) oxy-fuel combustion with flue gas recirculation. The fuel in all three examples was methane.
under one set of burning conditions with those under other burning conditions. A standard is necessary for the comparison, and the equivalence ratio is often used for this purpose. For example, product concentrations at the same equivalence ratio have been compared with air and oxy-fuel combustions.3 However, the definition of the equivalence ratio for oxy-fuel combustion varies, according to the viewpoint of the observer, because large amounts of combustion flue gas are circulated in a furnace system. Material balances when methane was burned completely are shown for air combustion, oxy-fuel combustion without flue gas recirculation, and oxy-fuel combustion with flue gas recirculation in Figures 1a, 1b, and 1c, respectively. In the figures, f is the fuel supply rate, a is a constant, b is a coefficient to express the quantity of recirculation of the combustion flue gas, and b/(1 + b) becomes the recirculation ratio. The molar amounts of supplied fuel (CH4) and oxidizer (O2) are the same for all cases. When flue gas is not recirculated (Figures 1a and 1b), the definition of the equivalence ratio (φ) is clear. The value of φ is decided from the amounts of supplied oxygen and fuel. However, the definition of the equivalence ratio changes when taking the viewpoint that the flue gas is recirculated (Figure 1c). In Figure 1c, oxygen supplied from outside the furnace system is supplied as oxygen from an air separation unit. In this case, the flue gas recirculation system is included in the furnace system. The value of φ is decided from the amount of the supplied rate of oxygen and fuel from viewpoint 1 in Figure 1c. However, we can define φ from viewpoint 2 in Figure 1c as well. From viewpoint 2, the amount of supplied oxygen includes the oxygen in the recirculated flue gas. The value of φ from viewpoint 2 is smaller than that from viewpoint 1. All materials included in the recirculated flue gas come from oxygen supplied from the air separation unit and fuel. However, the value of φ varies, depending on the viewpoint. The air separation unit and the flue gas recirculation system have often been omitted in experimental equipment. O2, CO2, and H2O that would be in the recirculated flue gas have been simulated by being supplied from other facilities such as
gas increase significantly for oxy-fuel combustion, because a large quantity of CO2 and H2O is included in feed gas. CO2 and H2O are not combustible species, but C and H are combustible elements. Two types of stoichiometric ratios can be defined, depending on whether or not C, H, and O from CO2 and H2O in the feed gas are included. Finally, in this paper, we examined which definition was easier to use when considering the reaction mechanism. We defined SR (or SRgas) as the value obtained when excluding C, H, and O from CO2 and H2O in the feed gas, and SR-all (or SRgas-all) as the value obtained by including C, H, and O from CO2 and H2O in the feed gas.
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DEFINITION OF EQUIVALENCE RATIO AND STOICHIOMETRIC RATIO Equivalence ratio (φ) and stoichiometric ratio (SR) are important parameters used to consider combustion phenomenon. The equivalence ratio is defined in the following expression: φ≡
molar amount of supplied fuel molar amount of fuel for stoichiometric conditions
(1)
Here, the amount of fuel for the stoichiometric condition is evaluated from the molar amount of supplied oxygen and the chemical reaction formula of the combustion. For example, for methane combustion, the amount of fuel for the stoichiometric condition is half of the molar amount of supplied oxygen. The stoichiometric ratio (SR) is the reciprocal of the equivalence ratio. It is expressed as follows: SR =
1 φ
≡
molar amount of supplied air (or oxidizer) molar amount of air (or oxidizer) for stoichiometric conditions
=
molar amount of supplied fuel molar amount of fuel for stoichiometric conditions
(2)
Combustion reaction mechanisms are usually examined by comparing the amount of products and intermediate species 4713
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Table 1. Evaluation of Equivalence Ratio and Stoichiometric Ratio from the Two Viewpoints Oxy-Fuel Combustion air combustion
(without flue gas recirculation)
(with flue gas recirculation)
total supply rate (mol/s)
fuel (CH4) supply oxidizer supply
f 2af
f 2af
f 2af
viewpoint 1 (supply rates of each species from fuel and oxidizer) (mol/s)
fuel (CH4) O2 N2 equivalence ratio, φ stoichiometric ratio, SR
f 2af 2af(79/21) 1/a a
f 2af 0 1/a a
f 2af 0 1/a a
viewpoint 1 (supply rates of each elements from fuel and oxidizer) (mol/s)
C H O N equivalence ratio, φ-all stoichiometric ratio, SR-all
f 4f 4af 4af (79/21) 1/a a
f 4f 4af 0 1/a a
f 4f 4af 0 1/a a
viewpoint 2 (supply rates of each species from fuel, oxidizer, and circulated flue gas) (mol/s)
fuel (CH4)
f
O2 CO2 H2O equivalence ratio, φ stoichiometric ratio, SR
2f(a + ab − b) bf 2bf l/(a + ab + b) a + ab − b
viewpoint 2 C (supply rates of each elements from fuel, oxidizer and circulated flue H gas) (mol/s) O equivalence ratio, φ-all stoichiometric ratio, SR-all
cylinders. In such a case, φ was defined only from viewpoint 2.3 We proposed a technique to evaluate φ from viewpoint 1 in a small-scale experiment without the flue gas recirculation system. We proposed an index (φ-all), which we defined as φ‐all =
O‐supply (2C‐supply + 0.5H‐supply)
f(b + l) 4f (b + 1) 4af(b + l) 1/a a
controlling combustion performances when fuel supply is kept constant. SR is the reciprocal of the equivalence ratio (φ). The meaning of φ and SR is the same. Therefore, we used SR in the present paper. We examined the relationships between concentrations of product and intermediate species, and SR from viewpoint 2 and SR-all. In this paper, we also used an index SRgas5,6,10 when gasphase reaction was mainly examined. The parameter SRgas was defined as
(3)
Here, O-supply is the gross molar amount of oxygen element supplied in a furnace. C-supply is the gross molar amount of carbon element supplied in a furnace, and H-supply is the gross molar amount of hydrogen element supplied in a furnace. Osupply, C-supply, and H-supply include O, C, and H, respectively, in the fuel, O2, CO2, and H2O. For the systems in Figures 1a−c, we evaluated φ and φ-all values from viewpoints 1 and 2. Results are summarized in Table 1. When the equivalence ratio was evaluated from viewpoint 1, all φ and φ-all values were the same. When flue gas was recirculated, the value of φ from viewpoint 2 was different. These φ values varied with the recirculation ratio. All φ-all values with and without flue gas recirculation were the same and the parameter φ-all was not influenced by the recirculation ratio. The stoichiometric ratio (SR) is usually used for practical systems. This is because the air supply is usually varied for
SRgas ≡
molar amount of fuel for stoichiometric conditions molar amount of supplied and gasified fuel
(4)
Here, the molar amount of gasified fuel means the amount of fuel that has moved from the solid phase to the gas phase via pyrolysis, oxidation, and gasification reactions. We did not consider the fuel components that were left in the solid phase. For example, unburned UBC is one of the fuel components that are left in the solid phase. The molar amount of gasified fuel was provided from differences between the molar amount of supplied fuel and the molar amount of fuel that was left in the solid phase as unburned UBC. SRgas can also be obtained approximately as eq 5:5 4714
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Article
upper furnace was varied from 1573 K to 1873 K and that in the lower furnace was constant at 1673 K. The coal supply rate was ∼0.1 kg/h. The total amount of air (air for burner zone + staged air) was 0.96 Nm3/h. Concentrations of HCN and NH3 were obtained from the concentrations of NH4+ and CN− in the water in a trap and total gas flow rate.5 Concentrations of NOx, O2, and CO were measured with continuous analyzers. Concentrations O2, CO, CO2, N2, H2, H2S, and hydrocarbons (CH4, C6H6, and other hydrocarbon species) were measured via gas chromatography (GC). For oxy-fuel combustion, a mixture of O2 and CO2 was supplied as feed gas. NOx, SOx, and H2O were not included in the mixture. The O2 concentration in the inlet mixtures was 21 or 27 vol %. For air combustion, the coal feed rate was kept constant as the amount of feed gas was varied. These operating conditions simulated the retrofitting of actual boilers. Some properties of the two main types of coal (A and B) studied are shown in Table 2. The size distributions of the test coals were close to those for coals used with an actual system. The fraction of fine particles
