Numerical study of further NOx emission reduction for coal MILD combustion by combining fuel‐rich/lean technology

• Published in: International journal of energy research Vol. 43; no. 14; pp. 8492 - 8508
• Main Authors: Xu, Mingchen, Tu, Yaojie, Zeng, Guang, Wang, Qingxiang, Zhou, Anqi, Yang, Wenming
• Format: Journal Article
• Language: English
• Published: Bognor Regis Hindawi Limited 01.11.2019
• Subjects: Air pollution control; Air temperature; Ammonia; clean coal technologies; Coal; Combustion; Computer simulation; Configurations; Contraction; Dilution; Emission analysis; Emission standards; Emissions control; Equivalence ratio; Feasibility studies; Fuels; fuel‐rich/lean technology; High temperature; Industrial emissions; Jets; Mathematical models; MILD combustion; Neutralization; Nitrogen compounds; nitrogen evolution; Nitrogen oxides; NOx prediction model; Reaction mechanisms; Technology; Temperature
• ISSN: 0363-907X; 1099-114X
• DOI: 10.1002/er.4849

Summary This paper numerically examines the feasibility of further reducing NOx emission from a semi‐industrial scale coal MILD (moderate and intense low‐oxygen dilution) combustion furnace by adopting fuel‐rich/lean technology. The implementation is achieved by separating the original fuel jet into two parallel jets which will be used as rich and lean streams. An effort has been made to develop a 13‐step reaction mechanism and NOx evolution UDFs (user defined functions) for better understanding the interactions between MILD combustion and fuel‐rich/lean technology. The experiment of the reference case (Combustion and Flame 156.9 (2009): 1771‐1784) is well reproduced by the present numerical simulation, indicating high reliability of developed models. The validity of the further reduction of NOx emission is assessed by the comparison among inner‐fuel‐rich (IFR), outer‐fuel‐rich (OFR), and reference cases resulting from the adjustment of the fuel supply through the two fuel‐rich/lean jets. The results show that both IFR and OFR configurations succeed in achieving further reduction of NOx emission as compared with the reference case, which stems from both thermal and fuel paths. Specifically, the decrease of thermal‐NO emission originates from the contraction of high‐temperature regions (>1800 K), where nearly 94% reduction occurs within the temperature range of 1800 K and 1950 K while only 6% within 1950 K and 2030 K despite their high temperature sensitivity. The reduction of the fuel‐NO emission is mainly attributed to the promoted NO reduction on char surface and neutralization with HCN and NH3. Generally, the NOx emission can be minimized by enlarging the equivalence ratio difference between rich and lean jets, and the OFR configuration exhibits a higher potential than the IFR counterparts. However, since a relatively high temperature (1623 K) secondary air was used in the experiment, the maximum NOx reduction potential was limited to only 2.5%. Fuel‐rich/lean technology is numerically evaluated to further reduce NOx emission for coal in a MILD combustion furnace. A 13‐step reaction mechanism and a NOx prediction model have been developed to account for fuel chemistry and NOx evolution. Both OFR and IFR configurations can achieve lower NOx emissions from thermal and fuel paths. Specifically, thermal‐NO is mainly formed within temperatures between 1800 and 1950 K. Fuel‐NO mitigation is mainly achieved by reduction on char surface and neutralization with HCN and NH3.