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To address the issues of ignition difficulty, incomplete burnout, and high NOx emissions during coal char combustion in industrial applications, a series of co-firing experiments involving bituminous coal and its medium-temperature pyrolysis char were conducted using a micro fluidized bed analysis system. The experiments explored co-firing behaviors and NOx emission characteristics for five different fuel blending ratios in an environment with 21 % oxygen concentration, across temperatures between 923 K and 1223 K. The study delved into the synergistic effects of coal and char co-firing, as well as the influence of combustion temperature and fuel blend ratio on the co-firing efficiency and NOx emissions. The results show a synergistic effect in the co-firing of bituminous coal and char, influenced by reaction temperature and coal-to-char blend ratio, which evolves as combustion progresses. In the 923–1073 K range, initial coal addition to the blend enhances reactivity. However, as the reaction advances or coal content exceeds 75 %, this synergy shifts to an inhibitory effect. At temperatures above 1023 K, increased proportions of coal in the blend are associated with improved burnout rates, especially when the coal content is at or above 50 %. Moreover, at a combustion temperature of 1123K, there is a marked decrease in nitrogen oxide emissions, with coal blend ratio of 25 % and 50 % showing particular efficacy in reducing these emissions. These results provide valuable insights for the efficient and clean utilization of coal char in industrial combustion processes. • The synergistic effect of co-firing depends on the temperature and the blend ratio, and changes with the reaction process. • Between 923 and 1073K, bituminous coal initially enhances fuel reactivity but later or above 75 % content, it hinders the reaction. • Temperatures over 1023 K combined with coal contents at or exceeding 50 % lead to increased burnout rates. • NOx emissions attain lowest value at 1123 K, with coal proportions of 25 % and 50 %. [ABSTRACT FROM AUTHOR]

Syngas tempering technology refers to highly reactive gases to consume excess O 2 in the circulating flue gas. An activation environment is created, and the reburning coal is rapidly activated into highly reactive char, increasing its ability to reduce NO. The syngas activation of reburning pulverized coal was studied by a plane flame-carrying flow reactor. Reduced intermediates such as CH i *, H*, and CO* are produced in the combustion reaction of the main components of the syngas (CH 4 , H 2 , and CO) with oxygen. The results indicate that after syngas tempering, the char's BET-specific surface area and total pore volume increase by 54.2% and 51.2%, respectively. The relative content of oxygen-containing groups is reduced by almost 5.7%, the relative content of alkyl groups increases by about 14.8%, and the degree of the disorder increases by about 20.68%. The syngas composition has a synergistic tempering effect with O 2. CH 4 has a better tempering effect than H 2 , while the effect of CO is not apparent. Among the reducing intermediates, CH i * has the most significant effect on char, followed by H*. It is instructive to solve the problem that the removal effect of existing pulverized coal reburning to reduce NO x emissions still needs to be improved. [Display omitted] • After quenching-tempering with syngas, the ability of coal to reduce NO has been enhanced. • Pore structure plays a prominent role in improving the reducing ability of coal-char. • The quenching-tempering effect of methane is the strongest among the syngas components. • Methane and hydrogen increased the number of mesopores and micropores, respectively. • Among the reducing intermediates, CH i * plays the most prominent role. [ABSTRACT FROM AUTHOR]

Reburning technology combining coal and biomass can effectively reduce NOx emissions and promote carbon neutrality in coal-fired systems. This work is based on the flat-flame burner test bench that decoupled the process of biomass-coal reburning effect on coal in the initial stage. The changes in surface functional groups and carbon skeleton structure of coal char were analyzed using FTIR, XPS, and Raman detection methods. The char's combustion reactivity and NO reduction ability were analyzed based on the micro-reactive fluidized bed test bench. The results showed that biochar and syngas would react with oxygen in the transport medium, and the resulting reactive species would activate the char. The activated effect of syngas is significantly better than that of biochar. The syngas reacts with oxygen while protecting the unnecessary consumption of the coal particle surface, and the generated active substances are enriched on the surface of the coal char to enhance its reactivity. The disordered structure of char was enhanced, and the number of small aromatic ring clusters increased by about 28%. The total alkane group content on the char surface increased by nearly 140%, and the total oxygen-containing group content decreased by nearly 17.8%. The combustion reactivity of char is increased by nearly 44.6%, and the reduced ability of NO is increased by nearly 23.4%. Theoretically, the reburned pulverized coal is reduced by 23.4%. [Display omitted] • Study on the interaction of biomass and coal co-reburning. • In co-reburning, biochar competes with biomass syngas and coal for O 2. • Biomass syngas has a better activation effect on coal than biochar. • Syngas enhances the combustion reactivity and NO reduction ability of coal char. • The reducing intermediates produced by syngas will be enriched on the char. [ABSTRACT FROM AUTHOR]