Through biomass gasification, biomass can be converted at high temperature to a product gas rich in carbon monoxide, hydrogen, and methane. After cleaning and upgrading, the product gas can be converted to biofuels such as hydrogen; methanol; dimethyl ether; and synthetic diesel, gasoline, and natural gas. Particulate matter (PM) is formed as a contaminant in the gasification process, and the aim of this work was to develop and apply a method for sampling and characterization of PM in the hot product gas.

A particle measurement system consisting of a dilution probe combined in series with a bed of granular activated carbon for tar adsorption was developed, with the aim of extracting a sample of the hot product gas without changing the size distribution and composition of the PM. The mass size distribution and concentration, as well as the morphology and elementary composition, of PM in the size range 10 nm to 10 µm in the product gas from a bubbling fluidized bed (BFB) gasifier, a circulating fluidized bed (CFB) gasifier and an indirect BFB gasifier using various types of biomass as fuel were determined.

All gasifiers and fuels displayed a bimodal particle mass size distribution with a fine mode in the <0.5 µm size range and a coarse mode in the >0.5 µm size range. Compared with the mass concentration of the coarse mode the mass concentration of the fine mode was low from all gasifiers. The evaluation of the results for the fine-mode PM was complicated by condensing potassium chloride for the CFB gasifier when using miscanthus as fuel and by condensing tars for the indirect BFB gasifier when using wood C as fuel. The mass concentration of the coarse-mode PM was higher from the CFB gasifier than from the two BFB gasifiers. The coarse-mode PM from the BFB gasifier when using wood A as fuel was dominated by char. In the CFB gasifier the coarse-mode PM was mainly ash and bed material when using all fuels. The coarse-mode PM from the indirect BFB gasifier when using wood C as fuel was mainly ash.

Biomass gasification is a thermochemical conversion by partial oxidation at elevated temperature of solid biomass into a gaseous energy carrier. The product gas contains the major components CO, H₂, CO₂, CH₄, as well as some tar and inorganic impurities and solid particles such as ash, bed material, soot and char. The aim of this work is to develop an aerosol-based method to investigate on-line the reactivity of the suspended biomass char particles at high temperatures, and to apply aerosol measurement systems for sampling and characterizing particulate matter in the hot product gas from gasifiers.

An aerosol-based method including the steps for generating, transporting, and oxidizing suspended char particles (0.5–10 µm) was proposed and developed for investigation of char reactivity at high temperatures. An aerodynamic particle sizer (APS) spectrometer was used to measure the particle size distributions. A tapered element oscillating microbalance (TEOM) was used to measure the change of mass concentrations of particles in the carrier gas, before and after conversion. The intrinsic kinetics of various biomass (wood, straw, miscanthus) char particles have been experimentally established in a wide temperature range for both combustion (in air/oxygen) and gasification (in 33 vol% CO₂ or 33 vol% steam), up to 800°C and 1300°C, respectively, by combining the aerosol method with thermogravimetric analysis (TGA). The general CO₂ or steam gasification reactivity of chars from different biomass could be ranked as wood > miscanthus > straw. In CO₂ or steam gasification, the reactivity of char samples measured by the aerosol method at 1300°C would vary by a factor of 4-9 comparing with the extrapolated estimation from the TGA results at the low temperatures. This indicates that high-temperature reactivity estimation by extrapolation should be used with care. Variations of the morphology and the effective density of char particles during conversion indicated that in the initial stage of char conversion (either combustion or gasification), pore growth was dominant up to a certain conversion, and shrinkage or fusing would occur in the later stage. The aerosol-based method presents a set of benefits which are advantageous compared to previously established techniques: no mass transfer limitation at high temperatures; the flexibility to switch to different gas agent combined with continuous feeding of char sample; and the on-line measurement of particle mass and size. The aerosol method is not applicable under the conditions where the reaction rate is slow, since longer residence time will increase the probability of particle losses. In addition to laboratory applications, the aerosol method has potentials for on-line investigation of concentration and reactivity of suspended char fragments sampled directly from the product gas in different types of gasifiers.

Particulate matter (10 nm–10 µm) in the product gas was characterized for the size distribution, morphology and elemental composition by both on-line and off-line techniques. An aerosol particle measurement system including a dilution probe connected in series with a packed activated carbon bed was applied to extract aerosol from the hot product gas produced in the gasifiers using wood as feedstock: an indirect bubbling fluidized bed gasifier and a circulating fluidized bed (CFB) gasifier. The fine and coarse particles from the CFB gasifier contained calcium and magnesium, indicating the contributions from the ash and the magnesite bed material. From the indirect gasifier, the fine-mode (<0.5 µm) particles were dominated by potassium and chlorine whereas the coarse-mode (>0.5 µm) particles were dominated by calcium and silicon, probably from the ash and the bed material. Char fragments were identified in the hot product gas and contribute to the coarse-mode particles in both gasifiers.