Laboratory Experiments to Characterise the Pyrolysis Behaviour of Selected Biomass Fuels

Abstract

The aim of this work was to create fundamental characterisation data on the pyrolysis behaviour of various biomass fuels needed, e.g., for the development of gasification processes. Pyrolysis (Pyroprobe 1000) - Gas Chromatograph - Atomic Emission Detector (Py-GC-AED) technique was used to study the rapid pyrolysis of seven different biomass feedstocks: Kenaf, Miscanthus, pine bark, reed canary grass, sweet sorghum, wheat straw, and willow. The following fractions formed in pyrolysis were composed: light fraction, i.e. volatile gases (benzene and lighter gases), heavy fraction, i.e., volatile tars (heavier than benzene), and char residue. Carbon, hydrogen and oxygen content of the volatile fractions was determined with the AED. Pyrolysis temperatures (i.e., set in the pyrolyser) were 600, 800 and 1000 °C. Pine bark gave the highest amount of char residue and the lowest amount of volatiles at each temperature. However, each feedstock formed a considerable amount of condensable tars that was not analysable. The carbon content of the heavy fraction of the volatilised products decreased while the temperature increased. The oxygen of the volatilised products was mainly in the light fraction, and it increased only slightly with temperature. The char amounts measured in the Pyroprobe were compared with those obtained from the gasification reactivity measurements carried out under pressure in the pressurised thermobalance (PTG). In some biomasses, the char amount measured in the Pyroprobe followed the amount measured in the PTG in spite of the differences in experimental conditions. Pressure seemed to have no significance on the char amount measured in PTG. The oxygen content of the biomasses correlated with that of the volatiles.

The experimental conditions of the pyrolyser were found to affect the pyrolysis results due to the condensation of the heaviest volatilising products. The actual temperatures measured with a thin thermocouple inside the pyrolysable sample were 7–12% lower than the set temperatures. The actual heating rate of a sample was also much slower than the set value of 1000°C/s.