(488c) Biogasification of Cellulosic Ethanol Stillage in an Anaerobic Fluidized Bed Bioreactor

Global concerns about climate change and other environmental impacts of fossil fuels have accelerated efforts to commercialize and introduce biofuels into the transportation fuel supply chain. The Energy Independence and Security Act (EISA) passed by US Congress and Renewable Fuel Standards (RFS) administered by USEPA, mandates that biofuels should account for 36 billion gallons of the fuel mix used by the transportation sector by year 2022. Ethanol from corn-starch having been capped at 15 billion gallons, major fraction of the remaining mandate would be required to be sourced from cellulosic feedstocks like agricultural wastes, forestry residues and dedicated energy crops. Ethanol from such feedstocks are referred to as ‘cellulosic ethanol’. To be classified as a cellulosic biofuel, in addition to the feedstock, the fuel must also generate 60% less lifecycle greenhouse gas emissions compared to the baseline emissions for gasoline or diesel in the year 2005.

Several initiatives are underway to demonstrate the production of cellulosic ethanol. One such operation is the Stan Mayfield Biorefinery in Perry, FL located within the premises of Georgia-Pacific’s Foley Cellulose Plant. This biorefinery was funded by the State of Florida and operated by the University of Florida until 2018. In this biorefinery cellulosic ethanol was produced from sugarcane bagasse using the following process steps. Bagasse was first soaked in dilute phosphoric acid and steam exploded, followed by enzymatic saccharification with cellulase enzymes and then fermentation using recombinant E. coli engineered to ferment both five and six carbon sugars to ethanol. Ethanol was recovered by distilling the whole fermentation broth. The waste stream from the distillation step is referred to as ‘stillage’. Large volumes of stillage are generated in an ethanol plant. Stillage generation will be 10 to 14 times the volume of ethanol of produced and is a highly polluting waste stream containing high content of organic carbon and other nutrients like nitrogen and phosphorus. Stillage from corn-starch ethanol plants is usually dewatered and dried to produce distillers dried grain with solubles (DDGS) which is a high-value animal feed. However, the nutrition value of cellulosic ethanol stillage is poor so the waste stream will have to be treated prior to disposal. Anaerobic digestion (AD) may be employed to treat stillage. AD is a biochemical process that mineralizes organic carbon compounds to biogas (a mixture of methane and carbon dioxide) through the syntrophic action of several groups of anaerobic microorganisms. Biogas can be used as a fuel for process heat or converted to electricity. In a cellulosic ethanol facility, on-site utilization of biogas can lead to further reductions in lifecycle greenhouse gas emissions and help to meet EISA mandates. AD is a mature technology and many installations are operating world-wide for treatment of municipal, agricultural and industrial wastes. The AD process can be implemented in many types of bioreactor designs. Choice of design is dependent on waste characteristics and can influence capital and operating costs as well as the footprint of the facility. There are only a couple of studies reported in the literature on the anaerobic treatment of cellulosic ethanol stillage.

In this research, stillage generated from the Stan Mayfield Biorefinery was treated in a semi-continuously fed laboratory-scale fluidized bed reactor. Among several designs that are available for anaerobic digestion, it has been shown that a fluidized bed reactor could be operated at high organic loading rates with very short hydraulic retention times, thus requiring a small footprint. Hence, a fluidized bed design was chosen for the study here. The fluidization medium used was activated carbon granules. The bioreactor was a 7 liter total volume (5.5 L working volume) glass vessel with a conical bottom. Heating was provided by a heating tape wound on the outside of the vessel. The reactor was operated at 55 ± 2 ^oC and temperature was measured by a thermocouple and controlled by a CR10 data logger. Long term experiments were conducted to investigate effect of dilution rate on reactor performance, and pH control and nutrient requirements for reactor operation. Reactor performance was measured in terms of biogas production rate, methane composition of biogas, pH, total volatile fatty acid concentration, and organic carbon reduction (as soluble chemical oxygen demand or sCOD). The methane yield on the basis of both volume and COD of feed was compared to the true yield obtained by operating the bioreactor in a batch mode.

Stillage characteristics were as follows: pH = 5.15; TS = 6.89 ± 0.33 %(w/w) wet weight; VS= 5.36 ± 0.33 %(w/w) wet weight; sCOD = 36 g/L ± 2.65 g/L; Ethanol = 0.75 g/L. The stillage also contained levulinic acid, hydroxymethyl furfurals and furfurals at concentrations of 1.6, 0.09 and 0.05 g/L, respectively. True methane potential of stillage was 342 ± 5 ml methane at STP/ g sCOD or 12.31 L methane at STP/ L of stillage. Currently experiments are underway to determine bioreactor performance at various dilution rates. The dilution rate was increased from 0.027 d^-1 to 0.0545 d^-1 and again to 0.082 d^-1. At each dilution rate the reactor was operated until steady state was reached (as determined by consecutive daily measurements of methane composition, biogas production rate, and total volatile fatty acid concentration). As dilution rate is increased the methane yield dropped from 12 L methane at STP/ L of stillage at 0.027 d^-1 to 11 L methane at STP/ L of stillage at 0.082 d^-1. Total volatile fatty increased from 4 -7 mM to 50 – 60 mM. Until now the reactor has been operated for over 100 days without the need for any nutrient addition or pH control. Based on the alkalinity, it is expected that the dilution rate could be further increased without acidifying the system.

A techno-economic analysis of the integrating a process for anaerobic digestion of stillage to a cellulosic ethanol plant was conducted using ASPEN Plus flowsheeting software. The cellulosic ethanol plant simulations were calibrated using data from Stan Mayfield Biorefinery. Laboratory scale anaerobic digestion data was scaled up to simulate AD in ASPEN Plus. An energy analysis showed that biogas produced by anaerobic digestion if used on-site can displace about 40% of the fossil energy consumed for generating steam for distillation.

The optimal dilution rate and the performance of the fluidized bed reactor under these optimal conditions will be presented. A technoeconomic analysis of integrating an anaerobic fluidized bed bioreactor to treat stillage will also be presented. The impact of introducing anaerobic digestion on the overall economics and energy inputs of a cellulosic ethanol facility will be presented.