Briquetting and torrefaction of agricultural residues for energy production

Abstract

Agricultural residues are a potential feedstock for renewable energy because they are abundant and CO2 neutral. Due to their low energy density and heterogeneity, there are key challenges in handling, storage, transportation and utilization, therefore pre-treatment is required. The aim of this study was to evaluate a range of pre-treatment options of agricultural residues for energy applications. The effect of moisture content (7.14-16.94%), particle size (˂2.36-˂4.00mm), compression temperature (20-80oC), pressure (100-250MPa), and type of agricultural residues (corn cob and bean straw) on briquette properties i.e. density, impact resistant and compressive strength was studied. Torrefaction of corn cob and bean straw were also investigated over a range of temperatures (200-300oC) and holding times (0-90 min) to study the impact of operating conditions on yields and properties of torrefaction products (char, liquid and gas). The results showed that density, impact resistance, and compressive strength significantly increased with increasing compacting temperature (20-80oC) and compacting pressure (100-250MPa) but decreased with increasing moisture content and particle size. Briquettes that satisfied the German Standard DIN 51731(density >1000kg m-3) and European Standard Committee CEN/TC 335 (durability >95%) standards for solid fuels were obtained with particle size ˂4 mm, compression temperature of 80oC and (i) moisture content of 10-12% with pressure of 100-250MPa for bean straw and (ii) low moisture content (<10 %) and high pressure (200-250 MPa) for maize cob. Briquettes derived from a bean straw:maize cob blend had high density and strength at low pressure and temperature compared to those derived from maize cobs due to enhanced bonding via mechanical interlocking, thereby reducing the costs of production. Torrefied solid products obtained at 300oC had properties comparable to coal with energy yields of 74.84-79.47% for maize cob and 90.08-92.93% for bean straw. The gaseous product (3.25-17.41% yield) was predominantly CO2 due to decomposition of hemicellulose within the temperature range studied. Briquettes that met the above certified standards were studied for pyrolysis and combustion in a fixed-bed reactor. The effects of pyrolysis temperature (410-650oC), heating rate (10-20oC min-1), carrier gas flow rate (40-60 cm3 min-1) and briquetting conditions (temperature (20-80oC), pressure (150-200MPa) and blend ratio) on the yields and properties of pyrolysis products from maize cob iv and bean straw briquettes were investigated. It was found that bio-oil and gas yields increased while, char yields from both biomass feedstocks decreased with increasing pyrolysis temperature due to an increase in decomposition of lignocellulosic components and secondary decomposition of primary char. Briquetting conditions, heating rate and carrier gas flow rate had negligible effect on product yields and properties. Increasing maize cob content in briquettes resulted in an increase in the yield of bio-oil from 48 to 51% at the expense of char yield, due to the low ash and fixed carbon content of the maize cob. Combustion and pyrolysis of raw/untreated and torrefied maize cob and bean straw in a thermogravimetric analyzer occurred through moisture release, devolatilization and char degradation. The kinetic study of raw maize cob and bean straw combustion/pyrolysis revealed that the average activation energies of maize cob and bean straw were 202.26 kJ mol-1 and 165.64 kJ mol-1 for combustion and 214.15 kJ mol-1 and 252.09 kJ mol-1 for pyrolysis. Modelled data of pyrolysis and combustion of bean straw and maize cob using the obtained kinetic parameters agreed well with the experimental data, which will be useful in reactor design for energy generation via pyrolysis and combustion from agricultural residues. The findings of this study could help in promoting the use of agricultural residues for energy generation which will potentially lessen the impacts of global warming, diversify and decentralize the energy supply through the improved management/utilisation of agricultural wastes. Briquette production, torrefaction and pyrolysis could provide opportunities for the local population to increase employment and income in rural areas. This study will also provide a reference for future research on densification and utilisation of agricultural residues for energy generation.