Process Optimisation and techno-economic assessment of the slow pyrolysis of corn-stover

Abstract

There is a growing need for the production and use of sustainable biofuels worldwide. One noteworthy approach is the production of biofuels via the pyrolysis of lignocellulose biomass. Benefits of such a process include carbon neutrality of the biofuels and alleviation of growing concerns on waste management. Moreover, the use of lignocellulose biomass does not upset the surrounding ecosystem through widespread deforestation. In South Africa, corn-stover is an abundant lignocellulosic biomass with an estimated 9 million metric tonnes produced annually. The production of this is however seasonal. This study assumes a constant supply of corn-stover feedstock. The vast amounts of this biomass resource validates its potential as a suitable candidate for biofuel valorisation processes. This investigated the production of char and volatile by-products through the slow pyrolysis of corn-stover with particular emphasis on char as a prospective supplement or replacement of coal in industrial processes. The char quality was assessed according to ASTM D388, which ranks coals according to their higher heating value (HHV), volatile matter and fixed carbon. Furthermore, an evaluation of the techno-economic feasibility of an industrial scale 30 t/day slow pyrolysis plant was conducted. The techno-economic study was conducted at a char baseline price of $100/ton. A two-level three factor central composite design (CCD) making use of response surface methodology (RSM) was used to study slow-pyrolysis process conditions. Optimisation experiments were conducted at bench-scale gram-level to study the influences of process condition of char higher heating value (HHV) and yield. The results showed process temperature had the most significant influence on HHV and yield. By heating char from room temperature to 300°C, a 3.44 MJ/kg (29.13%) improvement in char HHV was attained. A 1.76 MJ/kg (11.37%) improvement was attained from 350°C to 450°C. Statistical analysis showed negative quadratic coefficient of temperature (-18.59) indicating that an increase in temperature had a detrimental effect on char yield. Optimal conditions for char productions were reactor temperature of 453°C and 5°C/min and 29 min for heating rate and holding time respectively. Under these conditions char with HHV of 26.25±1.5 MJ/kg and yield of 34.5% were produced. These chars are comparable to sub-bituminous A coals. An economic study on a 30t/day slow pyrolysis process showed the process had a fixed capital investment requirement of $980 440, which was significantly less than other pyrolysis types. A profitability analysis that assessed the impact of feedstock availability and product prices on the feasibility of the process was conducted. Results showed the feasibility of the plant is highly sensitive to the price of char and cost of feedstock corn-stover. An economically viable process was attained when the price of corn-stover and char were $3/ton and $200/ton respectively.