Mathematical models are developed for the description of solid fuel conversion in fixed-bed and entrained flow gasification reactors. The proposed models differ from the well-known models in the fact that the general problem is reduced to solving a number of less complex subproblems one by one for certain stages of the physicochemical process. Such an approach permits a considerable reduction in the computational efforts for the obtained algorithms and the maintenance of the physical content of the model owing to the possibility of simplifying the macrokinetic equations.

Downdraft gasification is a promising process of energy conversion of wood biomass. There are such fuel conversion conditions that differ favorably from conventional conditions. In such conditions, there is no pyrolysis zone in the fuel bed, which precedes the oxidation zone. Fuel is supplied into the oxidizing zone without charring, where it reacts with the intensive cold air flow from tuyeres. The study aims to replicate the conversion of particles in a gasifier close to tuyeres. For this purpose, the individual particles are burned in the muffle furnace space and the quartz channel replicating presence of other bed particles at a first approximation. In the experiment, the furnace temperature was varied, as well as the velocity of air supplied to the particle. Two-stage and single-stage mechanisms of particle combustion were identified. A two-stage process is observed in the range of tuyere velocities below 20 m s⁻¹. The two-stage mechanism is characterized by a stage of devolatilization and volatiles combustion, followed by a stage of char residue combustion. The stages are predominantly separate from each other, and their degree of overlapping is low, amounting to 24%. At the tuyere velocities above 125 m s⁻¹ combustion of particles is realized primarily as a single-stage process. The intensive air flow reaches the fuel particle surface and initiates combustion of the surface char layer. In this case, the stages of devolatilization and char residue combustion run concurrently for the most part. In the single-stage mechanism, the degree of stage overlapping is significantly higher and amounts to 60–95%. For the two-stage combustion mechanism, the effect of cyclic movement of the flame across the particle surface is evident. The number of cycles can reach eight. This effect is due to the change of conversion stages. At air velocity above 95 m s⁻¹, fragmentation of fuel particles commences. A layer of char formed at an initial stage of burning heats up in the intensive air flow and is separated from the particle surface. The heated walls of the quartz channel contribute to the intensification of particle combustion. This effect is probably due to the swirling of the flame between the wall and the particle surface.

Fixed-bed conversion of polymer-containing waste is one of the most common ways of their disposal. One of the problems with such conversion is the cross-sectional non-uniformity of the bed, the formation of agglomerates, and burnouts. The agglomeration process essentially depends on the weight fraction of melting components (plastic materials). Usually, the non-uniformity of the bed is associated with the random nature of the packing (fluctuations of local concentrations and percolation properties). In the present work, we investigate the agglomeration process caused by non-uniform conditions of heat transfer. To this end, combustion of model waste (mixtures of sawdust and polyethylene with a content of the latter of 20–80) is experimentally studied in a laboratory setup with a bed height of about 10 cm. Due to the heat supply through the wall, airflow, non-uniformities developed, associated with different gas permeability in different areas of the bed. The transition from intense thermal decomposition to smoldering is achieved in 5–15 min (the higher the polyethylene content, the shorter the transition time). When the polyethylene content is high, the permeable area decreases so that the air velocity becomes high enough to entrain sawdust particles. As the polymer decomposes, tarry substances are released and form dense deposits in the gas cooling and cleaning system.

There are many methods for obtaining kinetic data of thermochemical conversion of fuel particles. These methods to some extent reproduce the conditions of industrial processes. At the same time, there is a lack of methods reproducing the conditions of wood particles gasification in downdraft gasifier. In this process, raw fuel enters the hot reactor region with intense air jets blowing through the tuyeres. It is difficult to directly measure the weight of particles under these conditions due to the effect of gas flow. The authors proposed a research method in accordance with which the sample is introduced into a heated furnace and placed in the tuyere flow. The sample is combusted for a certain time then removed from the furnace and quickly quenched. The method allows reproducing the dynamics of changes in temperature, mass, volume, shape and structure of the inner layers of particles. It provides visual observations of the process, high heating rate of wood samples (750-5500 °C min⁻¹) and large amount of data sufficient for statistical processing.

Composites of sawdust and crushed polyethylene were obtained by pressing at 5–10 atm. The resulting pellets with a size of about 10–20 mm were then burned in airflow in a muffle furnace at a temperature of 800 °C. The combustion process was recorded, and obtained video data were analyzed. The data obtained made it possible to estimate the change in particle size at different stages of combustion. An increase in linear dimensions during conversion was achieved of up to 2 times. Particle swelling led to a decrease in mechanical strength and destruction of particles before complete burnout.