(438k) Process Simulation of Integrated Liquid-State and Solid-State Anaerobic Digestion of Dairy Manure and Crop Residues

Dairy manure is the most common agricultural digester feedstock in the United States. The bioenergy from dairy manure is produced by producing biogas using a technique, liquid state anaerobic digestion (LAD). The major problem in LAD is disposal of large volume of effluent, which also produce odor problem if it is used to irrigate the farms. Crop residues can be used in solid state anaerobic digestion (SS-AD) to produce biogas. Advantages of SS-AD include smaller reactor capacity requirements, less energy used for heating, and no processing energy needed for stirring. Due to its lower water content, the digestate of SS-AD can be used as fertilizer and pelletized fuel, so it is much easier to handle than the effluent of LAD. Disadvantage of SS-AD systems are requirement of larger amounts of inoculum and longer retention time. By integrating LAD and SS-AD, effluent disposal problem of LAD and larger amounts of inoculum need of SS-AD is possible to resolve. The effluent from LAD is fed into SS-AD to wet the crop residues and add more inoculum for anaerobic digestion. The digestate comes from the SS-AD, which has no problem of odor issue can be used in the farm for fertilizing. In order to evaluate the cost and energy efficiency of this integrated AD system, process simulation model was developed using Super Pro design software. An accurate computer simulation process of the complete operation is developed to improve the operation of designed integrated anaerobic digestion system. The goal is to expand the dynamic anaerobic reactor model to improve its ability and simulate continuous liquid anaerobic reactor designs and continuously feed the SS-AD where crop residues and other solid waste are digested to increase the biogas production substantially. This process simulation model was developed by assuming 90% of the manure is collected from 500 head of cattle, manure is produced and collected constantly throughout the year, manure is managed as liquid and is free of bedding materials. For SS-AD, crop residues such as wheat straw and corn stover were considered for digestion. Monod kinetic model was used for anaerobic digestion reaction of organic matter to produce biogas in both reactors. The produced biogas was further cleared remove carbon dioxide and sulfur and converted into compressed natural gas or used for production of electricity. The capital and operating costs of producing clean biogas from the integrated LAD and SS-AD were presented.