Bioethanol production from excess food crops in Nigeria: process design, optimization, and techno-economic analysis.

Abstract

The global drive for diversification of energy sources, particularly by focusing less on non-renewable fossil fuels and harnessing renewable energy resources like bioethanol, has motivated this research work. Before the last 10 years, bioethanol meant for use as fuel was produced from carbohydrate-rich crops such as cassava, yam, maize, millet, rice amongst others. Because the production of bioethanol from these food crops has been envisaged to jeopardize food security, the focus has been shifted to the production of bioethanol from the residues left behind after processing the food crops. These residues can be classified as lignocellulosic biomass. The major concentration of this study is the production of bioethanol from residues of food crops, namely, corn cobs, rice husks, sugarcane bagasse, cassava peels, and yam peels. The biomass used in this research were sourced from different locations in Nigeria, where they are found in abundance at certain seasons yearly. In the course of the work, the biomass were sieved into two mesh sizes of 300 and 425 microns, and also some of the biomass as well as all the five biomass were all mixed and firstly characterized to evaluate the effects of particle size as well as hybridized biomass mixtures on the end products and production efficiency of bioethanol. The effects of the adopted pretreatments in this study on the biomass were also investigated, as such, three types of pretreatments were adopted in this study namely; combined hydrothermal and acid pretreatment, combined hydrothermal and alkaline pre-treatment, and hydrothermal only pretreatment. The results of the characterization of the different biomass, including the hybridized biomass after pretreatment showed the pore features for hybridized corn cobs and rice husks biomass have the maximum specific surface area and pore volume of 1837 m2/g and 0.5570 cc/g respectively. Also, the values of the cellulose content improved slightly with the pretreatment and the value of the lignin content decreased considerably. The cellulose values range from 34.2 to 36.5 wt% for the acid, alkali and hot water pretreated hybridized biomass. Releases from the pretreatment process to air, soil, and water were measured with SimaPro. The environmental impact categories accessed include global warming potential (GWP)/climate change, and acidification (AP). With a mean value of 15.82 kg CO2 (eq), the alkaline pretreatment using sodium hydroxide shows the highest release of GHG emissions, while acid pretreatment employing dilute sulphuric acid generated a mean value of 8.68 kg CO2. Hybridized feedstocks of cassava peels plus yam peels, and corn cobs plus rice husks biomass, were optimized using the Response Surface Methodology (RSM) centred on the statistical design of experiments (DOE) of the Box-Behnken design (BBD), in the production of bioethanol. The BBD was harnessed using a 3-level, 3-factor process variables using pH, time, and particle size. The bioethanol yield from the two hybridized biomass feedstocks was predicted by the developed quadratic polynomial models from BBD. The hybridized rice husks plus corn cobs biomass with a maximum bioethanol yield of 160 ml/1500 g biomass gave a better prospect for bioethanol production when compared with hybridized cassava peels plus yam peels biomass with a maximum bioethanol yield of 125 ml/1500 g biomass. This reinforces the finding that hybridizing the feedstocks enhances the capacity for better bioethanol yield after fermentation. The economic analysis of the produced bioethanol gave a price of 0.41 USD/l, which is a good deal as it compares favorably well with the 0.45 USD/l price of ethanol in the Nigerian open market.