MILD-oxy combustion under the wet flue gas recirculation mode is of potential in achieving a synergetic control of C/N emissions, whereas its practical realization is restricted by how to efficiently form an in-furnace H2O-rich atmosphere under MILD regime. Towards this issue, this work numerically evaluates the feasibility of applying the direct steam injection method by arranging the individual steam nozzles with different diameters (Dste) on both sides of the primary stream nozzle (i.e., the inner- and outer-steam injections). The effects of incident position and velocity on combustion behaviors are studied. Results show that compared with the advanced confluence of primary and secondary streams in the inner injection cases, outer-steam injection delays the confluence of inlet jets to enhance the in-furnace dilution level. Inner-steam injection is better in inhibiting temperature rise and fluctuation, which facilitates a more uniform distribution of temperature. MILD combustion with diffusion/kinetics-controlled regime is better maintained in the inner injection cases under a small Damkӧhler number. The share of gasification reaction on char consumption in the inner injection cases is higher than that in the outer injection cases, and the burnout time of char is also increased. The increased Dste further enhances the contribution of gasification reaction and prolongs char burnout. Inner-steam injection inhibits the oxidation of fuel-N and enhances NO-reburning in a large low-O2 region as Dste increases, which leads to the lower NO emission concentration. Meanwhile, interaction between NO and fuel-N intermediates is found to play an inevitable role in reducing the overall NO emission.

The oxy-fuel combustion technology has attracted widespread attention because of its relatively low-cost and large-scale CO2 capture. During the oxy-fuel combustion process, the low diffusivity of O2 in CO2 brings several difficulties on coal char combustion due to N2 replacement with CO2. Catalytic combustion by adding alkali and alkaline earth metals (AAEMs) is an effective way to improve the coal char combustion performance. In this work, the catalytic effects of AAEMs on coal char combustion characteristics under different conditions were studied. Besides, the kinetic parameters of char combustion were obtained by Kissinger-Akahira-Sunose, Starink, and Flynn-Wall-Ozawa methods. Results showed that compared with other AAEMs (Na, Ca, and Mg), K showed better catalytic effects on ignition, burnout, and comprehensive combustion characteristics of coal char. The promotion effect of K on the combustion performance was gradually saturated when addition ratios exceeded 1 wt%. At low O2 concentrations or heating rates, K exhibited a strong catalytic effect on coal char combustion. Besides, K could accelerate the decrease in activation energy (Ea) of coal char combustion as char conversion (α) increased. When α was 0.9, the reduced effect of K on the Ea value was 13%. These results are expected to provide new insights into the improvement of coal char oxy-fuel combustion catalyzed by AAEMs.

MILD-oxy combustion operates conventional oxy-combustion processes under moderate or intense low-oxygen dilution (MILD) condition, which offers a practical approach towards the synergetic control of CO2 and NOx emissions. This numerical work investigates the MILD combustion characteristics and NO emission of CH4/NH3 fuel blend under N2, CO2 and H2O dilutions, aiming at revealing the fuel-NO formation mechanism under MILD-oxy condition. It is found that relative to MILD-N2 combustion, the flame temperature is reduced under MILD-CO2 and MILD-H2O combustion, while the reaction zone is expanded and reduced, respectively, due to the altered co-flow properties. CO2 dilution lowers the radical pool level, while the formation of OH radical is boosted under H2O dilution. Overall NO emission (EINO) under MILD-N2 combustion is the largest while that under MILD-H2O generates the lowest value, showing its low-NOx emissions superiority. NH 3 is converted into NO via routes NH3 → NH2 → HNO → NO and NH3 → NH2 → NH → HNO → NO under MILD-CO2 combustion. By contrast, the importance of route NH3 → NH2 → NH → N → NO in producing NO is largely strengthened under MILD-H2O combustion, wherein the direct formation of NO via HNO is inhibited with the enhanced availability of reactive N-containing species, contributing to lower NO emission under MILD-H2O combustion over MILD-CO2 combustion.

Compared with conventional air combustion, the combustion mode and medium of unconventional combustion technologies (oxy-fuel combustion, moderate or intense low oxygen dilution (MILD) combustion, MILD-oxy combustion, pressurized oxy-fuel combustion (POC), and chemical looping combustion (CLC)) exhibit significant changes, and the gasification behavior of coal char also changes, which can influence the flame temperature, heat transfer, char combustion, NOx emissions, and even particulate matter (PM) formation. This study systemically reviews the current progress in coal char gasification reaction behavior under different unconventional combustion modes. First, the combustion principle and char gasification behavior during unconventional combustion modes of pulverized coal are summarized. Next, the effects of the operating conditions (such as temperature, bulk diffusivity, and species concentration, etc.) on the gasification behavior are discussed. The transformation between the combustion and gasification regimes during pulverized coal combustion can be realized by adjusting the operating parameters. In addition, the influencing mechanisms of the gasification reaction on pulverized coal combustion, including flame temperature, heat transfer, char burnout, NOx emissions and PM formation, are clarified. Especially, NOx emissions can be further reduced by enhancing char gasification with three aspects: the dilution oxygen effect, homogeneous reduction effect, and heterogeneous reduction effect. Besides, the current challenges of these unconventional combustion technologies are summarized, and their application scenarios are expected to achieve CO2 mitigation targets. Finally, this work concludes it necessary to explore the internal association between enhanced gasification and reduced NOx emissions, which is expected to provide valuable guidance for C/N synergetic reduced emissions in the wide applications of carbon-based solid fuels.

Energy consumption is one of the crucial factors in determining the suitability of the chemical absorbents for capturing carbon dioxide (CO2). Polyamines have attracted increasing attention in recent years because of the existence of several amino functional groups that could effectively capture CO2 in their molecules. In this work, the structures of seven polyamines with excellent CO2 capture performance were investigated in relation to reaction rate and the CO2 absorption heat. A high-precision microcalorimeter was applied to analyze the absorption heat of CO2 directly, and a parameter defined as the CO2 quasi-cycle capacity was employed to assess the cyclic capture capacity of CO2, absorption rate, and desorption rate. Experimental results revealed that for the selected polyamines, the increase in the ratio of the secondary amino functional group to the primary one in the molecule leads to a decrease in the CO2 absorption heat and an increase in the CO2 cyclic capacity.The ΔHabs of the eight amines give out the order of HMDA (92.22kJ/mol) >MAPA(90.91kJ/mol) >AEEA (87.62kJ/mol) >MEA (86.36kJ/mol) >TEPA (82.95 k/mol) >AEP (82.36 kJ/mol) >2MPRZ (77.08 kJ/mol) >PZ (74.41 kJ/mol). The CO2 quasi-cycle capacity of 2MPZR is about twice that of the MEA. In addition, 2MPRZ was found to be an absorbent with phase change potential. The effective phase change solvent system, 2MPRZ+ Triethylene glycol monobutyl ether (TGBE)+H2O, was developed in this work. Compared to the 5M MEA aqueous solution, its sensible heat was reduced by 56.3%, indicating that 2MPRZ is a promising absorbent.