NOx Emission of Fine- and Superfine- Pulverized Coal Combustion in O₂/CO₂Atmosphere

Abstract

The effects of particle size, stoichiometric ratio (λ), atmosphere, temperature, and recycled NO on the emissions of three fractional nitrogens, [N] N2O, [N] NO , and [N] NO2 , during the combustion of superfine pulverized coal in O 2 /CO 2 atmosphere were investigated in the present study. NO 2 has shown little contribution to NOx compared with N 2 O and NO in all the cases. As the stoichiometric ratio increases, the trend is much similar in CO 2 /O 2 and N 2 /O 2 . However, the conversion ratio from fuel-N to NOx in CO 2 /O 2 atmosphere is less than that in N 2 /O 2 atmosphere especially at λ >1.2.[N] NO makes up the largest portion of [N] NOx at λ>1, and [N] N2O dominates at λ<1 resulting from the presence of hydrocarbons and CO at low stoichiometric ratio. For the three coals, [N] N2O increases as the mean particle size increases while [N] NO shows the opposite trend because the evolution of volatile nitrogen was delayed and the reburninglike scheme happened. There exists a minimum for the conversion ratio from fuel-N to NOx at the particle size range of 15-25 μm under the combined effects of [N] N2O and [N] NO . As temperature goes up, [N] NO and the conversion ratio from fuel-N to NOx increase while the [N] N2O decreases obviously. The conversion ratio from fuel-N to NOx decreases while the reduction rate increases as recycled NO increases. Recycled NO is destroyed in the flame through its reactions with hydrocarbon radicals in the form of CHi, and reduction reactions occur between recycled NOx and fuel-N. The increase in NO concentration accelerates the formation reactions of N 2 O and also promotes the conversion of char-N to N 2 O.

A Model for Nitrogen Chemistry in Oxy-Fuel Combustion of Pulverized Coal

“…The EFR results from the present work are characterized by the initial segregation of a primary flow containing the coal/carrier gas and a secondary flow with the remaining oxidizer. In the EFR experiments from the literature, drawn from Hu et al 8 and Jiang et al, 11 the fuel and oxidizer flows were premixed. Table 1 lists the ultimate and proximate analyses of the coals used in the entrained-flow reactor experiments.…”

“…In the model, which was developed for high-temperature pulverized-fuel combustion, tar is assumed to crack to form soot and H 2 . Pyrolysis experiments show that the soot/tar yields are constant in the temperature range 1480À 1900 K; 39,40 however, the soot fraction decreases with decreasing The experimental results of Jiang et al 11 for the bituminous coal under simulated oxy-fuel conditions are compared with model predictions in Figure 7. Similar to the results from the present work and from Hu et al, the measurements show the NO conversion ratio to increase with the excess air ratio.…”

Experimental Investigation on NOx Reduction by Primary Measures in Biomass Combustion: Straw, Peat, Sewage Sludge, Forest Residues and Wood Pellets

“…The N 2 O concentration increased with primary excess air ratio. Unburnt species such as CO and CH 4 that are reductive agents for NO x , as discussed earlier, have proven to promote the fuel-N conversion to N 2 O in high excess air ratios [51]. Therefore, the conditions which are in general favorable for NO x reduction result in more N 2 O formation.…”

Oxy-fuel combustion of coal and biomass blends

“…Yoshiie et al [28] found that the NO emissions and conversions of fuel nitrogen to NO under oxy-fuel combustion conditions were lower than that under air combustion conditions without adding NO in the fed atmosphere. Also, Jiang et al [29] shown lower NO concentrations under oxy-fuel combustion atmospheres than in air without gas recirculation.…”