Slow pyrolysis of maize stover for biochar Production

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Biochar is a carbon-rich solid material produced during pyrol ysis, which is the thermo-chemical conversion of biomass in the absence of oxygen. Biochar can be used for soil and compost amendment to increase agronomic productivity. Biochar from agricultural residues has not found its application for soil and compost amendment towards agricultural improvement in Ghana. In Ghana, agricultural residues are mostl y burnt on the farm which destroys soil microbes and does not replenish the land, and further contribute to climate change. At times the residues are abandoned on the field to decay. During decay, greenhouse gases, such as carbon dioxide and methane, are released into the atmosphere imparting on global warming . The objective of this research was to explore the use of maize stover to produce biochar for compost amendment using a decentralised biochar reactor. The other aims of the project was to determine the temperature profile of the reactor, determine the temperature-time profile and its effects on yield of biochar, determine the chemical composition of the biochar, and determine the physiochemical impact of the biochar additions to compost. The maize stover was obtained from farm at Ayuom in the Bosomtwe district of Ashanti region. The feedstock was size reduced in sizes of 1.5cm and then air dried. The feedstock was fed into the biochar reactor through the biochar inlet and pyrolysis was conducted in the reactor to obtain biochar. The temperature profile during pyrolysis was determined using a K-type thermocouple. The biochar was grinded and sieved through a 1mm sieve to make it suitable for chemical analysis. Analyses of the biochar include proximate analysis, elemental analysis, pH and water holding capacity. To determine the physiochemical impact of biochar additions to compost, biochar was applied to compost at the following rate (volume percent); 0 vol. % biochar to 100 vol. % compost (control), 50 vol. % biochar to 50 vol. % compost, 75 vol. % biochar to 25 vol. % compost and then incubated for six (6) weeks. Elemental analysis (N, P, K), pH and water holding capacity (WHC) tests were completed on the biochar-compost mixtures at the start of the incubation and at the end of the six weeks incubation. Using the decentralised biochar reactor, biochar was obtained by the slow pyrolysis of maize stover. The decentralised biochar reactor achieved slow pyrolysis temperature within the range of 250 – 418 ºC, with a heating rate of approximately 6.8 ºC/min. Proximate analysis of biochar gave a moisture content of 7.5 wt. %, ash content 15.2 wt. %, volatile matter 18.0 wt. % and V fixed carbon 59.3 wt. %. Obtaining a fixed carbon content of greater than 50 wt. % suggest that biochar from maize stover is good for carbon sequestration. The pH of the biochar was 9.2 units, which implies that it can be used to improve the pH of acidic soils. The biochar had a relatively higher nutrient content, which followed N> K> P. Biochar amendment to compost increased the nutrient content of compost at the start of the experiment due to the high nutrient content of biochar. However at the end of the six weeks incubation, the 25 vol. % biochar application rate had a higher % increase in nutrients (NPK) than the 50 vol. % and 75 vol. % biochar application rates. The control compost had the highest percentage increase in nutrients at the end of the incubation period. This implies that the nutrient content of compost can be increased by incubation before soil application. .
A thesis submitted to the Department of Chemical Engineering, Kwame Nkrumah University of Science and Technology in partial fulfillment of the requirements for the degree of Master of Science (Chemical Eng.) .