NO Emission Characteristics of Low-Rank Pulverized Bituminous Coal in the Primary Combustion Zone of a Drop-Tube Furnace

Abstract

Experiments were conducted in a drop-tube furnace (DTF) to investigate NO emission characteristics of low-rank bituminous coal burning in the primary combustion zone. The combustion tests were performed under high-stoichiometric-ratio (SR) combustion conditions (SR g 1) and low-SR combustion conditions (SR<1). Along the length of the furnace, concentrations of flue gas species, such as NO, HCN, NH 3 , CH 4 , CO, CO 2 , and O 2 , were carefully measured during pyrolysis and different SR combustion conditions. The results for high-SR combustion conditions show that high concentrations of O 2 favor the conversion of volatile N and char N into NO, with extensive NO formation taking place in the initial stages of combustion. Reducing species, such as CH 4 , C 2 H 6 , and NH 3 , in volatile matter were mostly oxidized and generated little effect on NO elimination. Results from low-SR combustion conditions show that there is insufficient O 2 for oxidization of combustibles, allowing more hydrocarbons to react with NO. For the coal tested in these experiments, the conversion ratio of fuel N to NO, R fuel N , yielded the lowest minimum values around SR = 0.9 among all such ratios because of a larger fraction of fuel N being decomposed from coal particles through pyrolysis, as well as char oxidization being reduced to N 2 . For lower SR conditions, the total fixed nitrogen (TFN), which includes NO, NH 3 , and HCN, can be reduced to N 2 by homogeneous reactions, so that the remaining N in char becomes the major contributor of NO emissions in the burnout zone.

Experimental study on bituminous coal blending in a down‐fired boiler with anthracite combustion system under low load

“…As the bituminous coal mixing ratio increases by 10%, the NO x emission concentration reduces by 15–20 mg/m 3 on average. The low‐NO x combustion technology of the coal‐fired boiler reduces the generation of NO x in the combustion process by controlling the conversion of the volatile nitrogen to nitrogen oxides 41 . Increasing the bituminous coal mixing ratio will increase the volatile content of the blended coals in the furnace.…”

Experimental Study on Coal-Nitrogen Release and NOx Evolution in Oxygen-Enriched and Deep-Staging Combustion

“…They attributed the increase of NO x in the FL zone to the oxidation of char-N. The results obtained by Fei et al 6 also showed that NO and its precursors can be effectively converted to N 2 in the FR zone under the staging condition, and they speculated that char-N is the main source of NO x in the FL zone. Fan et al 1 also found that burnout air injection would result in a further increase in NO x concentration.…”

“…In air-staging combustion, the FR zone is the major area for NO x reduction due to the reducing atmosphere. Therefore, the effect of key factors on NO x formation in the FR zone has attracted much interest from scholars. ,− Bai et al reported that NO x could be remarkably decreased in the FR zone when the elevated temperature and strong reducing atmosphere condition was available in this zone. This phenomenon was attributed to the fact that the reduction reaction of NO x was strengthened by elevated temperature under the strong reducing atmosphere.…”

“…In the FR zone, nitrogen contained in char occupies a larger proportion due to the incomplete conversion of char. The results obtained by Thomas et al showed that nitrogen-contained in char was much harder to reduce than that in gas-phase by adjusting the operation parameters, and the evolution of char nitrogen (char-N) in the FR zone is critical to the ultimate NO x formation. − Fei et al also found that nitric oxide mainly came from nitrogen-containing gas species in higher stoichiometric ratio (SR) conditions, while in lower SR conditions, the nitrogen remaining in the char became the primary source of nitric oxide. Therefore, it is essential to explore the effects of key factors on the release of char-N in the FR zone in the staging condition.…”

“…In addition, Bi et al 5 NH 3 consumed the existing NO x in flue gas, which greatly improved the efficiency of NO x reduction. Therefore, the NO x existing in flue gas in the FR zone can be effectively reduced by the methods of increasing temperature, 7,10 lowering SR 6 , or adding reductive agents 5 to the FR zone. While previous investigations mainly focused on the conversion mechanism of gas phase nitrogen, the release mechanism of nitrogen contained in char was rarely concerned about.…”