Comparison of oil-tea shell and Douglas-fir sawdust for the production of bio-oils and chars in a fluidized-bed fast pyrolysis system

Abstract

Oil-tea shell, a fertilized agro-waste, was compared with Douglas-fir sawdust, a non-fertilized woody waste, for the production of bio-oils and chars in a fluidized-bed fast pyrolysis system. Comparison of the metal contents in biomass, derived bio-oil and bio-char indicated that most of the metals retained in the char after the fast pyrolysis process. The bio-oils were almost free of metals. The water contents of oils from Douglas-fir-sawdust and oil-tea-shell were as high as 23.8wt.% and 38.1wt.%, respectively. Combustion of water–fuel emulsion, formed by the high moisture content of Douglas-fir-sawdust oil (11wt%) mixed in the heavy fuel oil (89wt%), led to the lower NOx emission of 231ppm@6% O2 as compared to 265ppm@6% O2 of 100wt% heavy-fuel-oil combustion. The water–fuel-emulsion suppressed the peak flame temperature in boiler and hence reduced the thermal NOx emission. To elucidate the metal-concentrated effect from biomass to char, a measured Rc was defined as the ratio of the metal concentration in char to the metal concentration in biomass. The average measured Rc from oil-tea-shell and Douglas-fir-sawdust chars were 8.3 and 8.0, respectively. A simple predicted Rc was proposed based on the bio-char yield. The predicted Rc for oil-tea-shell and Douglas-fir-sawdust chars were 7.7 and 8.9, respectively which were in agreement with the results of average measured Rc. The predicted Rc will be useful for the provision of additional information to double checking the measured metals in bio-char obtained from the fast pyrolysis process when bio-oil contained very low metal contents. The oil-tea-shell char contained a very high potassium content of 12,700mgkg−1 due to the metal-concentrated effect from biomass, making it suitable for soil amendment but unqualified for energy application. Both the Douglas-fir-sawdust and oil-tea-shell oils with low sulfur content, almost free of metal and water–fuel-emulsion effect, can be considerable as potential fungible fuels for energy application, but necessary precautions should be taken to against the low pH value of the bio-oils which they may corrode the facility.