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• Three typical fouling layers are proposed under (85/65/45 °C) heating temperatures. • Heat transfer performances of fouling layers are studied in two kinds of boilers. • Micro-topography and mineralogy of typical ash deposit samples are analyzed. • Acid-ash reaction mechanisms for three typical fouling layers are obtained. Comparative field experiments of typical low-temperature fouling layers are carried out in pulverized coal furnace (PCF) and circulating fluidized bed boiler (CFB). The reduction of heat transfer characteristics of test pipe is determined by the transformation of fouling layer, caused by the quantity of condensed acid solution exceeding the adsorption capacity of deposited ash particles. Based on the heat transfer performance of fouling layer covering the test pipe, as well as the appearance characteristics and degree of adhesion to the heating surface, three typical fouling layers are collected, i.e. dry loose ash deposits (85 °C), adhering ash deposits (65 °C) and viscous ash deposits (45 °C). Scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) are used to analyze the formation mechanisms of three typical fouling layers. The characteristic fouling layer is determined by the reaction degree of condensed acid solution and ash particles deposited on the heating surface. Furthermore, the formation mechanisms of unsaturated acid-ash action fouling layer, supersaturated acid-ash action fouling layer and mixed acid-ash action fouling layer are proposed, providing useful guidance for the safe and efficient operation of heat exchange equipment in coal-fired boilers. [ABSTRACT FROM AUTHOR]

• The LBV and AFT gradually increase with X H 2. • The hydrogen addition can increase the NO emissions and suppress the CO formation. • Reducing the equivalence ratio can significantly reduce the NO emission. • The most important reaction promoting the formation of NO is NNH + O = NH + NO. • The most important reaction that promotes CO consumption is H + CO = H + CO 2. The influence of hydrogen addition (0–100%) on the combustion and emission characteristics (NO and CO formation) of laminar-premixed methane flame under gas turbine relevant operating conditions was numerically studied. The PREMIX code of ANSYS CHEMKIN-PRO and the GRI-Mech 3.0 were utilized. The study of the effect of increasing hydrogen content in the methane-hydrogen mixtures on combustion and emissions at T = 298 K, P = 1 atm can provide a reference for our subsequent research on gas turbine conditions (T = 723 K, P = 16.5 atm). The LBV (laminar burning velocity) and AFT (adiabatic flame temperature) with different hydrogen contents (X H 2 =0–100%) and equivalence ratio (Φ = 0.6–1.4) were investigated. In addition, under gas turbine relevant lean burning conditions (Φ = 0.6, 0.8), the effect of hydrogen addition on the NO and CO emissions were investigated. The rate and sensitivity coefficients of NO/CO production were also examined. The results show that the LBV and AFT increase with the hydrogen content. Both the mole fraction of H, O and OH radicals and the flame temperature were observed to increase with the hydrogen content. Furthermore, the hydrogen addition can significantly increase the NO emissions and on the other hand suppress the CO formation. Reducing the equivalence ratio (from 1.2 to 0.6) can significantly reduce the NO emission. The addition of hydrogen can also increase the combustion stability of methane fuel under fuel-lean conditions. This is a good hint for gas turbine device to reduce NO emissions at fuel-lean side (Φ < 1). [ABSTRACT FROM AUTHOR]

• The reaction images for different equivalence ratio were shown. • The OH* imges and NO x production in the MILD combustion zone were analyzed. • The NO x reduction mechanism prevails over the NO x formation mechanism under MILD mode. The effect of equivalence ratio and staging ratio on the CH 4 MILD (Moderate or Intense Low-oxygen Dilution) combustion was examined in an axially staged MILD combustor. The results were presented on reactant temperature and dilution as well as the ignition delay time through CRN (chemical reaction network) calculation. Also, the data on the OH* radicals distribution and exhaust emissions were obtained using experiments. Higher staging ratio induces the high temperature low oxygen thermodynamic condition required for MILD scheme, yet reduces the ignition delay time. An expanded and downstream moved reaction region was observed at higher equivalence ratio. Reduced and even negative NO x emissions from the MILD combustion zone are acquired at high staging ratio, implying that the NO x reduction mechanism is superior to the NO x formation mechanism in the MILD combustion zone. The opposite effect of staging ratio on the CO emission for low equivalence ratio and high equivalence ratio is also observed. With primary air swirled in the primary combustion zone, the staged combustor exhibits an excellence performance with distributed reaction region and ultra-low NO x and CO emissions under gas turbine relevant lean operating condition. [ABSTRACT FROM AUTHOR]

• The flow , flame and emission are presented under furnace operation condition. • Smooth thermal field and reduced emissions were observed for MILD mode. • Higer thermal input and excess air ratio anticipate the realization of MILD mode. • From nonpremixed to premixed MILD mode, the NO x formation is suppressed. • Heat balance under premixed MILD combustion was presented in the present work. To promote the application of MILD scheme in furnace, the present work was performed to evaluate the effect of thermal input, excess air coefficient and combustion mode on the MILD combustion at high thermal level (thermal input varies from 600 to 786 kW); meanwhile, the heat from the utilized furnace was extracted by an annular water jacket and a corundum tube was installed in the furnace tube to create the high temperature atmosphere. The results were presented on flow field using numerical simulations and on global flame signatures, temperature/species distribution and exhaust emissions using experiments. Better combustion performance of MILD mode than conventional swirling diffusion combustion mode was verified in the present work. The positive effect of increased thermal input and excess air coefficient on gas recirculation, thermal uniformity and exhaust emissions reduction was observed. In the MILD combustion regime, the switching from nonpremixed to premixed combustion resulted in lower oxygen level and reduced reaction temperature in the combustion region, also much lower NO x emissions and slightly higher CO emission were observed. At 786 kW thermal input, the reaction zone covers the entire combustion chamber; meanwhile, low NO x emission level of 32 ppm@3%O 2 are achieved for premixed MILD combustion mode. The MILD regime was established for natural gas fueled furnace with colorless distributed combustion and suppressed NO x and CO emissions. [ABSTRACT FROM AUTHOR]

• H 2 containing syngas fuels produce the NO emission mainly through the N 2 O-intermediate and NO-reburning mechanisms, whereas for pure H 2 MILD combustion, the NNH route plays a more important role. • The syngas fuel with higher N 2 and CO 2 content can achieve MILD combustion at leaner condition and with lower NO x emissions. • MILD regime can be established for four typical coal-derived syngas fuels, with single-digit NO x emissions under gas turbine relevant lean operating condition. The MILD (Moderate or Intense Low-oxygen Dilution) combustion of four typical syngas fuels was examined in CRN (chemical reaction network) model and in parallel jet forward flow combustor. The results were presented on the thermodynamics, ignition kinetics and NO chemistry using CRN calculation. The flow field mixing was obtained from numerical simulation, the global flame signatures and exhaust emissions from experiments. The syngas fuel with higher content of inert gas (N 2 and CO 2) and lower concentration of active components (H 2 and CO) exhibited higher mixture ignition temperature under MILD condition, as a result, greater gas recirculation ratio was required to establish the thermodynamic condition of MILD scheme. H 2 -enriched syngas and CO-enriched syngas showed different chemical kinetics in the ignition process of MILD mixture. In the NO chemistry under MILD condition, it was revealed that the H 2 containing syngas fuels produce the NO emission mainly through the N 2 O-intermediate and NO-reburning mechanisms, whereas for pure H 2 MILD combustion, the NNH route plays a more important role. The N 2 component in the syngas fuel had physical effect in delaying the interaction between the air and fuel stream and in favoring the turbulent mixing as well as the gas recirculation. The CO 2 component in the syngas fuel had chemical effect in lowering the reaction rate. The critical equivalence ratio above which MILD combustion occurs (with the reaction region covering the entire combustor) was identified for the four syngas fuels. It was observed that the syngas fuel with higher N 2 and CO 2 content can achieve MILD combustion at leaner condition and with lower NO x emissions. Reduced CO emission and single-digit NO x emissions were achieved for the four syngas fuels. [ABSTRACT FROM AUTHOR]