Resource Magazine — September/October 2013
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Closed-Loop, Energy-Efficient Biofuel Production
Pratap Pullammanappallil

Fast-diminishing fossil fuel reserves, along with the high prices of crude oil imported into the United States and concerns about climate change, have led researchers and large-scale industries to investigate sustainable, low-cost biomass feedstocks for production of renewable biofuels like ethanol, butanol, and biodiesel. Currently, in the United States and Brazil, ethanol is primarily produced from food crops like corn and sugarcane, respectively. The use of food crops for biofuel production raises ethical concerns about diverting food to fuel production. A reasonable alternative is bioethanol production from lignocellulosic biomass originating from non-food sources.

The Agricultural and Biological Engineering Department (ABE) at the University of Florida (UF) is developing an integrated process that recovers energy, nutrients, and water from a celluosic ethanol distillation process. The cellulosic ethanol process developed by Lonnie Ingram, a distinguished professor of Microbiology and Cell Science at UF, uses a genetically engineered E. coli bacteria to produce fuel ethanol from inedible plant biomass, such as sugarcane residue (called bagasse), municipal green waste, and agricultural and forest residues. The cellulosic ethanol process is currently being demonstrated at the Stan Mayfield Biorefinery Pilot Plant in Perry, Fla.

The recalcitrant nature of lignin, a natural polymer that constitutes a large portion of plant biomass, makes it difficult for microorganisms to access the sugars that make up the complex carbohydrates of plants. Therefore, a number of pretreatment options are being explored to make these sugars more readily available for subsequent fermentation.

Meanwhile, at the downstream end of the bioethanol production process, following the distillation of ethanol from the fermented liquor, a wastewater stream is produced. This distillery wastewater is a high-strength, dark-colored, nutrientrich, acidic liquid that presents significant disposal problems. Discharge of such nutrient-rich wastewater into water bodies can cause eutrophication, which has deleterious effects on aquatic life. With the EPA tightening the standards for industrial effluent discharge, accompanied by the decreasing availability of land for waste disposal, more intense treatment approaches must be applied to the wastewater.

Closing the loop effectively

To address these issues, we are developing an integrated downstream process that recovers resources from the distillation waste stream which would otherwise be wasted. The residual fermentation broth after distillation is referred to as stillage. Our integrated system includes a process for anaerobic digestion of the stillage to produce biogas (which is a biofuel), struvite precipitation for phosphorous recovery, and finally cleaning the wastewater using an advanced oxidation process. Each component of the integrated system has been the subject of research by Gayathri Ram Mohan, a graduate student in the ABE department.

• Continuous anaerobic digestion of the stillage was successfully carried out in a fluidized bed reactor. Long-term, bench-scale digestion of the stillage was useful in determining the feasibility of the process, the biochemical methane potential of the stillage, and the various parameters required to design a large-scale digester. Methane yield from anaerobic digestion of stillage was more than 12 v/v (volume of methane at 0°C and 1 atm pressure to volume of stillage).

• Following energy recovery in the form of biogas, the stillage effluent, which contains nitrogen and phosphorous, is subjected to struvite precipitation. Struvite is a slow-release phosphate fertilizer, and its precipitation paves the way to recover and reuse plant nutrients from stillage. The phosphorous concentrations can be reduced to less than 2 ppm using this process.

• Next, the remaining effluent is passed through a TiO2/UV photoreactor for final cleaning. This photocatalytic treatment allows recovery of water from the process, which can be recycled in the plant.

Mass and energy balances for a biofuel plant producing 3.8 million L (1 million gal) of ethanol per year showed that the raw bagasse requirement would be about ten times the mass of ethanol produced, and a significant amount of water would be required to make a pumpable slurry. Therefore, the mass of stillage produced would be about 37% more than that of the raw bagasse feed. By anaerobic digestion, the organic content of stillage is converted to biogas (60% CH4, 40% CO2) with a heating value of 46 MMBtu d-1. Struvite precipitation from the digested effluent yields 615 kg d-1 of struvitecontaining processed sludge. The remaining dark-colored, nutrient-deprived stream, after exposure to advanced oxidation via TiO2-mediated photocatalysis, would decolorize due to degradation of the color-causing compounds and recalcitrant organics. This treatment would help with reuse of the recovered water in the plant.

If the amount of energy required for distillation is calculated assuming that 15 lbs (approximately 16,000 Btu) of steam is required to distill 1 gal of ethanol from a mixture containing 10% ethanol, then this energy requirement can be mostly (>90%) supplied by the biogas produced from the anaerobic digestion of stillage. Studies conducted by the USDA on the energy balance of a corn ethanol process showed that about 13,679 Btu L-1 (51,779 Btu gal-1) of energy is used in the ethanol conversion process. If this figure is used as a basis for the cellulosic ethanol process, then the biogas produced by anaerobically digesting the stillage can be used to compensate about 30% of the energy input in the conversion process.

A mass balance was also carried out on the overall orthophosphate-phosphorous released in the process. About 70% of the phosphate content of the stillage comes from the acid pretreatment step, with the remaining 30% released from the feedstock itself. Other than the phosphate that is used for biomass growth in the digester, there is no loss of phosphate throughout the process. Therefore, about 99% of the phosphate is recovered as struvite-containing sludge that can be used as a fertilizer.

Every ton of sugarcane produces 0.3 tons of bagasse. The amount of sugarcane required to meet the feedstock requirements of a 3.8 million L (1 million gal) per year ethanol plant would be 270 tons d-1. The recommended phosphate dosage for P-limited soils is about 36 kg of P per hectare. Based on this information, the phosphate precipitated as struvite-containing sludge can supply approximately 50% of the phosphate needed to cultivate the sugarcane required to produce an adequate supply of bagasse feedstock for bioethanol production.

This integrated treatment system allows successful recovery of resources while reducing the carbon and water footprints of the biofuel production process. Similar research is now being conducted on stillages obtained from the fermentation of other types of feedstock, including eucalyptus and wheat straw.

ASABE member Pratap Pullammanappallil, Associate Professor, Department of of Agricultural and Biological Engineering, University of Florida, Gainesville, USA,