The experiments were performed on "model fuels" that differed greatly in their thermal and kinetic properties. Low-ash high-reactivity bio fuels of medium (wood) and high (dates seeds) density and products of their treatment (charcoal) were used to study the affect of pyrolysis kinetics, material density on formation of coke-ash residue reaction structure. Charcoal as oxydizing pyrolysis product entering the reaction zone of gas generators was used for investigation of gasification modes with different blow conditions. Fuels characteristics are given in Table 1.

Conditions of porous structure formation and porosity during preheating (devolatilization) were studied based on biomass particles with equivalent size dp « 10 mm. The particles were heated by two methods: fast heating by placing the particle in muffle furnace preheated up to preset temperature (100, 200, … 800 °С with accuracy +20 °С) and slow heating simultaneously with muffle heating under conditions of limited oxidizing agent supply. This allowed to simulate real conditions of thermal processes i. e. fast heating (for instance, particle pyrolysis in

fluid flow — or fluidized bed-type carbonizer) and slow heating (when the particle enters a cold fluidized bed and gets warmed gradually with the fluidized bed). After cooling the porosity was measured (mercury porometry: volume and sizing the pores with d > 5.7 nm) and specific surface area (nitrogen adsorption: surface area of pores with diameter d > 0.3 nm).

NA — not available.

Table 1. Model fuels characteristics

Kinetics of conversion in combustion mode was studied on individual particles with equivalent diameter dp = 3-75 mm. The range of diameters examined corresponds with values showed in (Tillman D. A., 2000) as allowed for individual and co-combustion (gasification) of biofuels. Test sample placed (centered) on thermocouple junction (Ch-A type) was brought into the muffle which was preheated up to preset temperature (100, 200, … 800 °С with accuracy +20 °С). The tests were performed with air flow rate 0-3.5 m3/h (upstream velocity of the flow is 0-0.5 m/s in normal conditions). Average effective burning velocity for coke-ash residue was calculated as loss of coke-ash residue estimated weight per surface unit of equivalent sphere (based on original size) during coke-ash residue burning out: j = AM / (Azcar — F). Coke-ash residue (CAR) burn-out time (Azcar) was estimated by thermograms (fig. 1.) as time interval between points C and D. The length of A"-B segment was not accounted for.

In some aspects, individual particle burning, combustion in fluidized bed and in flame, may be assessed on the same basis. Both in flame and in fluidized bed the fuel particles are spaced at quite a distance from each other and are usually considered as individual particles. The intensity of heat-mass-exchange of particles burning in FB inert medium is comparatively close to individual particle intensity. Application of experimental data for individual particle burning to calculation and assessment of thermo chemical pretreatment of large-size particles in furnaces with dense bed is justified by the fact that heat-mass — exchange processes in its large size elements are the same as for individual particle, within the statement of the problem. Therefore the experimental data on individual particle burning are usually used in calculations to assess thermo chemical pretreatment of large particles in furnaces of various types.

Experimental data was compared with other researches’ data on thermal pretreatment of low-grade fuel particles in the air showed in Table 2.

Kinetics of conversion in gasification conditions was studied at the plant consisting of quartz retort with inner diameter 37 mm, length 650 mm, located in cylinder-shape muffle

furnace (Nel = 2.5 kW, Tmax = 1250оС), air blower (Qmax = 6 m3/h, Hmax = 0.6 m), electric heater, steam generator, rotameter, a set of thermocouples, carbon dioxide cylinder and thermocouple polling and temperature recording system. Combustible gas components (СО, Н2, СН4) were determined by gas chromatograph, air flow coefficient was determined by effluent gas composition.

The experiments were performed in dense bed which provides for the most strict fulfillment of fuel thermochemical pretreatment as stratified process, stepwise and in compliance with temperature and concentration conditions, without flow disturbances and fluid mechanics problems. Gasification was based on downdraft process. Particles with initial diameter, varying from 3 to 20 mm in different experiments, were placed in retort having a tube welded to its bottom for gas release and sampling for analysis. Fuel bed was heated in muffle furnace up to 600-1000оС (the temperature depended on experiment). Gasifying agent (air, air and water vapor, water vapor, or carbon dioxide) was fed via furnace tuyere inside the bed to a different depth. Blown fluid was heated by electric heater up to 700- 750оС. The experiment was considered to be completed at the moment when СО and Н2 content in gas lowered by less than 1% of volume.