Product and Economic Analysis of Direct Liquefaction of Swine Manure

Direct hydrothermal liquefaction of biomass is a technology that has shown promising results in treating waste and producing oil. A batch hydrothermal liquefaction system was used to treat swine manure, and it successfully converted up to 70% of swine manure volatile solids into oil and reduced manure chemical oxygen demand by up to 75% (He et al., Trans ASAE 43(6):1827–1833, 2000). A continuous-flow reactor was developed and resulted in similar conversion rates to the batch process, indicating the potential for scale-up (Ocfemia, 2005). This study investigates the hydrothermal process in relation to a livestock system to determine the impact on oil yields and fertilizer values that might be realized in a farm-scale application. Oil products from the hydrothermal process are maximized using manure containing around 20% solids, but typical swine confinement facilities contain wetter manure slurries. A preliminary investigation of liquid–solid separation methods was conducted to determine the resultant oil yields and the effects of the hydrothermal process on fertilizer values in the wastewater as compared to the unprocessed manure. Energy and economic analyses of the liquid–solid separation and hydrothermal liquefaction processes were also conducted. The hydrothermal process results in an oil product as well as a fertilizer product that retains the majority of its nitrogen value with a reduced level of phosphorus (as compared to the unprocessed swine manure). The economic analyses indicate feasibility for several different liquid–solid separation methods, dependent on the equipment and maintenance costs assumed for each method. Conveyor-belt manure collection systems in conjunction with hydrothermal liquefaction are especially promising.