In the present study analysis of co-firing microwave (MW) pre-treated biomass pellets of different origins (wood and wheat straw), with raw pellets (wood, straw, and peat), to control and improve thermochemical conversion of biomass blends and achieve a sustainable use of local energy resources in energy production has been carried out. Effects of MW pre-treatment regimes and composition of blends were studied experimentally using measurements of the weight loss of blends, the yield of volatiles, flame temperature, total heat output from the device, and composition of products. It was found that co-firing MW pre-treated and raw biomass pellets promotes synergistic interaction between components of blends by increasing mass loss rate, the intensity of which depends on the proximate composition of pellets, MW pre-treatment regime and mass fraction of pre-treated pellets in the blend. The most effective synergistic interaction was found when co-firing pre-treated straw or wood pellets with raw peat, which increased the yield of combustible volatiles and heat output from the device as well as improved the composition of emissions. The least effective synergistic interaction was observed when co-firing pre-treated straw with raw wood pellets. Main factors that influenced the thermal and chemical conversion of MW pre-treated blends are discussed considering the effects of MW pre-treatment on the structural changes, elemental and chemical composition, and heating value of pre-treated pellets.

Possibilities of more efficient use of regional lignocellulosic resources (wood, wheat straw, peat) of different origin for an environmentally friendly energy production using selectively MW pre-treated blends of commercial wood or wheat straw pellets with raw peat pellets are studied. A hypothesis is proposed and tested that selective MW pre-treatment of wood or wheat straw pellets at the frequency 2.45 GHz and blending of MW pre-treated pellets with raw peat pellets can be used to enhance and control the thermo-chemical conversion of biomass blends. To test this hypothesis, a combined experimental study and mathematical modelling of the processes were performed. The thermo-chemical conversion of selectively activated blends was experimentally studied using a batch-size pilot device, which consists of a biomass gasifier and a combustor. To evaluate the effect of selective MW pre-treatment of biomass pellets on the thermo-chemical conversion of pre-treated blends, measurements of the kinetics of weight loss, yield of combustible volatiles, flame temperature, heat output of the device, and composition of emissions were made at different MW pre-treatment regimes of wheat straw and wood pellets and different mass fractions of pre-treated pellets in biomass blends. The developed novel 2D numerical model of thermo-chemical conversion of MW pre-treated straw confirmed that the pre-treatment of wheat straw pellets increases the generated heat and significantly affects the temperature distribution in the flame/bed zones. It was confirmed that MW pre-treatment leads to a faster thermal decomposition of biomass pellets, synergistically activating the non-treated parts of blends. The overall improved yield of combustible volatiles and their complete combustion provide a surplus of heat production by limiting the formation of GHG emissions, which allows promoting MW pre-treated biomass of different origin as efficient regional bioenergy resources for energy production.

The paper deals with a porous network model based simulation of effects of microwave pre-treatment of biomass pellets. Different heating regimesrapid and sloware compared; it is shown that rapid heating regime results in pressure build-up reaching values that cause breakage of the biomass material. Slow heating regime results in much lower maximum pressure values. As a second stage, ignition of pre-treated and non pretreated granules is compared. It is demonstrated that the pre-treated granule ignites considerably faster. The simulation considers intra-particle processes. The pellet is modelled as a porous material. The transport of volatiles is calculated using a nonlinear porous media equation. Thermal decomposition of the pellet is modelled using Arrhenius kinetics for three principal components of the biomass. An exponential rule for calculating the permeability of the material as a function of conversion rate is implemented. The model has been implemented in Octave. The result is a numerically cheap model that can be implemented and used to control the biomass gasification process. The model is versatile and can be extended to incorporate other physical and chemical processes.