Biodiesel has been in commercial use for more than a decade with several known benefits: reducing the nation's reliance on petroleum import, significant reduction in the emission of air pollutants and green house gases (GHGs), and comparable fuel properties to the petroleum diesel.
However, expansion of the biodiesel industry has also resulted in some concerns. As an example, the ¿¿¿¿¿¿¿food vs. fuel¿¿¿¿¿¿¿ debate reflects the competition of biodiesel with food supply when edible seed-oils, such as soybean oil, are used for more profitable production. In order for the biodiesel industry to continue thriving in the future, various governments and organizations have imposed and suggested an array of sustainability factors for biodiesel supply chain, such as land use, water consumption, waste management, cost and availability of feedstocks, etc.
This study addressed two major sustainability aspects regarding biodiesel production: (1) water consumption, and (2) utilization of waste materials. In detail, the following topics were investigated: (1) characterization of water consumption by soybean-derived biodiesel in plant growth and fuel production; (2) parametric study of reducing FFA (free fatty acid) in waste cooking oil; and (3) a preliminary evaluation on the utilization of waste coffee ground as biodiesel feedstock and purification material.
Water consumption from biodiesel process was characterized as three stages: plant growth, soybean processing and biodiesel production. Result showed that the nationwide average irrigation accounted for 61.78 gallons of water per gallon of soybean biodiesel while soybean processing (0.17 gal/gal) and biodiesel production (0.36 gal/gal) stages consumed much less. A state-by-state analysis for irrigation water indicated that the water consumption was highly dependent on the location and climate. Overall, on a nationwide basis, the total water consumption for making biodiesel from soybean was approximately 808.7 million gallons water per year.
In general, feedstock can account for up to 80% of the total cost for biodiesel production. This offers potentials for low cost and even waste materials, such as animal fats, waste cooking oil (WCO), and trap grease (brown grease). However, the high FFA content (>1 wt%) in these waste materials requires pretreatment prior to transesterification . Therefore, a parametric study on FFA reduction in WCO was performed to study the optimum conditions for FFA pretreatment. WCO with FFA level of 5¿¿¿¿0.5 wt% was treated by acid catalytic esterificiation using sulfuric acid (H2SO4). The influence of temperature, methanol-to-FFA molar ratio, and catalyst concentration on the conversion rate was investigated. Results indicated that the optimal condition was 60¿¿¿¿5¿¿¿¿C, 40:1 methanol-to-FFA molar ratio, and 12.5 wt% H2SO4.
Thirdly, a preliminary study was performed to investigate the feasibility of using waste coffee grounds (WCG) as both an oil source and purification material for biodiesel production. Results showed that the oil content of WCG was around 10 wt%. In addition, the post-extraction WCGs were found to be effective in removing impurities from crude biodiesel, such as free glycerin, methanol and metal ions. Results suggested that WCG may be comparable in purification capability to commercial materials. The use of waste as feedstock and purification material can greatly promote the sustainability of biodiesel production by lowering overall production cost, reducing waste generation (less/no additional purification material needed) and minimizing life cycle environmental impact (recycling/reusing wastes in each stage of the production life cycle).