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Recently, the co-firing ammonia (NH3) with coal in boiler has been paid more and more attention. A computational Fluid Dynamics (CFD) simulation approach was built to investigate the NH3 co-firing in a pulverized coal combustion facility with a single swirling burner. The simulation accuracy was evaluated with the experimental data. The differences between coal firing and NH3 co-firing can be well reproduced with simulation. Then, the effects of NH3 co-firing ratio (CR) on the combustion characteristic and NOx emission were investigated. The flame shape is significantly affected by NH3 CR, due to the changes of momentum of NH3 jet from center of burner. When the NH3 CR exceeds 40 cal.%, the internal recirculation zone is completely penetrated by high velocity NH3 flow, which leads to a long flame and much unreacted NH3 in downstream. As the increase of NH3 CR, the overall trend is that the total heat absorbed slightly decreases due to the decrease of particle radiation, and unburnt carbon in fly ash obviously increases due to decrease of flame temperature. In the case with NH3 CR of 10 cal.%, the NO concentration at exit increases due to the more intense combustion and more fuel-NOx, compared with the case of coal-firing. When the NH3 co-firing ratio exceeds 10 cal.%, the NO concentration at exit decreases monotonously due to the DeNOx effect of unreacted NH3. However, once the NH3 co-firing ratio exceeds 40 cal.%, the unreacted NH3 concentration at exit increases rapidly, which requires careful designs of burner and furnace.

Computational fluid dynamics (CFD) simulations of the oxy-coal combustion in a large-scale furnace were performed by applying different weighted sum of gray-gases models (WSGGMs). Before simulation of the furnace, the different WSGGMs were tested in an enclosed rectangular box containing only gases inside. In both cases of air and oxy-fuel combustion, the calculated radiative sources and wall heat fluxes obtained with gray and non-gray WSGGMs were completely different. Only with the non-gray WSGGMs (i.e., non-gray implementation), the calculated results could well agree with the benchmark data. In the simulation of furnace, three WSGGMs were adopted: Smith’s gray WSGGM (i.e., gray implementation), Johansson’s non-gray-A WSGGM with uniform distribution of radiative energy of particles in each gray band, and Johansson’s non-gray-B WSGGM, in which the radiative energy of particles was distributed using weighting factors. The weighting factors for the particles can be calculated with the same function as that used for gas but using the particle temperature instead of gas temperature, i.e., wp,i = wg,i (Tp). Being different from the rectangular box without particle radiation, in the real furnace, the gas temperature and wall heat flux calculated by gray and non-gray-A WSGGMs almost show no obvious difference. This is attributed to the fact that particle radiation plays a more significant role than gas radiation in the real furnace. However, using the non-gray-B model instead of Smith’s gray model or non-gray-A model can obviously influence the distribution of wall heat flux and total amount of heat absorbed by the walls. This indicates that in the numerical simulation of oxy-fuel combustion, the non-gray WSGGMs should be used by combining with the weighting factors of wp,i = wg,i (Tp) for particles, in order to rigorously describe both the gas and particle radiation during oxy-fuel combustion of coal in real furnace.

The dual fluidised bed gasification technology is prospective because it produces high caloric product gas free of N 2 dilution even when air is used to generate the gasification-required endothermic heat via in situ combustion. This study is devoted to providing the necessary process fundamentals for development of a bubbling fluidized bed (BFB) biomass gasifier coupled to a pneumatic transported riser (PTR) char combustor. In a steam-blown fluidized bed of silica sand, gasification of 1.0 g biomass, a kind of dried coffee grounds containing about 10 wt.% water, in batch format clarified first the characteristics of fuel pyrolysis (at 1073 K) under the conditions simulating that prevailing in the gasifier intended to develop. The result shown that via pyrolysis more than 60% of fuel carbon and up to 75% of fuel mass could be converted into product gas, while the simultaneously formed char was about 22% of fuel mass. With all of these data as the known input, a process simulation using the software package ASPEN then revealed that the considered dual bed gasification plant, i.e. a BFB gasifier + a PTR combustor, is able to sustain its independent heat and mass balances to allow cold gas efficiencies higher than 75%, given that the fuel has suitable water contents and the heat carried with the product gas from the gasifier and with the flue gas from the char combustor is efficiently recovered inside the plant. In a dual fluidized bed pilot gasification facility simulating the gasification plant for development, the article finally demonstrated experimentally that the necessary reaction time for fuel, i.e. the explicit residence time of fuel particles inside the BFB gasifier computed according to a plug granular flow assumption, can be lower than 160 s. The results shown that varying the residence time from 160 to 1200 s only slightly increased the gasification efficiency, but the reaction time available in the PTR, say, about 3 s in our case, was too short to assure the finish even of fuel pyrolysis.