Sichone, K., Lay, M. C., & Leatherland, C. (2013). Pilot-scale continuous pyrolysis of Pinus Radiatta sawdust. Presented at the New Zealand 2013 Biochar Workshop – The Final Answer?, Held at Massey University, Palmerston North, New Zealand, 04-05 Jul 2013.
Abstract
Lakeland Steel Limited developed a pilot plant for pyrolysing sawdust which proved successful in recent trials with sawdust and organic waste processing. Lakeland Steel was approached by a client to perform a preliminary economic assessment of a mobile sawdust pyrolysis unit. Feedstock properties such as proximate composition, elemental composition, calorific content, thermal decomposition reaction kinetics, and drying properties were determined. Pilot plant trials investigated the effects of processing parameters on product yields and properties. The parameters were feedstock moisture content at 15, 30 and 60%, reaction temperature at 400, 450 and 500°C and reactor auger speeds at 15, 20 and 25 rpm. Experiments at 60% moisture could not be performed to completion as the auger blocked repeatedly. The other two moisture contents showed that moisture content enhanced heat exchange properties of the feedstock and this generally increased the amount of volatile organic matter released. It was observed that for 15% moisture sawdust increase of temperature did not consistently exhibit an increase in degree of devolatilisation of organic matter. However, the 30% moisture sawdust showed an increase in devolatilisation with increase in temperature. The effects of increasing reactor auger speed had the most consistent trend with which an increase in speed showed a decrease in degree of devolatilisation thus increasing char yield.
A preliminary economic feasibility assessment was performed for the base case scenario which processed 29,000 tonnes/per year of green sawdust pre-dried to 15% moisture content at 400°C. The facility produced 5,250 tons biochar, 2,140 tons oil and 6,290 tons syngas per year. The syngas was recycled for heating and excess heat was also recovered from product cooling operations. However an additional 170 kg/hr. of natural gas was required to produce the required processing temperatures during start-up. The base case assessment was uneconomic, operating costs, including labour, were $1,200,000 per year and sales were $780,000 per year, assuming biochar could be sold for $150 per tonne. A sensitivity analysis showed that the process economics could be favourable assuming lower capital costs by eliminating the need for a rotary dryer and 50% reduction of labour costs by process automation. Provided that the char is sold at $150 per ton a payback period of four years can be achieved.
