Characterization of Biogas from Anaerobically Digested Dairy Waste for Energy Use
Bothi, Kimberly L.
As the third largest dairy producer in the United States, New York is faced with the critical issue of agricultural waste management. The environmental impacts and high long-term costs of poor waste disposal have pushed the industry to realize the potential of turning this problem into an economic and sustainable initiative. The anaerobic digestion of dairy manure-derived agricultural waste produces biogas, a valuable energy resource. Anaerobic digestion offers an effective way to manage manure by addressing the principal problem of odor control while offering an opportunity to create energy from conversion of biogas with a system of combined heat and power (CHP). Anaerobic digestion is a microbial process that produces biogas, a gas consisting of primarily methane (CH4) and carbon dioxide (CO2). The use of biogas as an energy source has numerous applications. However, all of the possible applications require knowledge about the composition and quantity of constituents in the biogas stream. This thesis presents the findings of a study conducted over several months at five New York farms to evaluate the characteristics of dairy manure-derived biogas. Relatively long term measurements of a biogas stream at Dairy Development International (DDI) provided information about the composition and quantity of the constituents of biogas over time (day, week, months). At DDI, methane averaged 60.3% (+/-1%) of the total gas composition with an average BTU per standard cubic foot of 612 (+/-12 BTU/SCF). Carbon dioxide averaged 38.2% (+/-1%) during this period with nitrogen at 1.52% (+/-1.1%). Hydrogen sulfide, a particularly hazardous component of biogas affecting the ultimate end use of biogas in energy generation technologies measured an average of 1984 ppm (+/-570 ppm) at DDI where measurements were taken about every 3 hours over numerous 24 hour periods from July to November 2003. Biogas samples at the other four dairies illustrated rather wide variations in hydrogen sulfide concentrations from about 600 ppm to over 7000 ppm. It is suggested that the lower H2S concentrations may be due to additions of food waste to the anaerobic digester at the dairy with the low H2S concentration. The high H2S concentrations measured at another dairy are believed to be related to the significantly higher concentrations of sulfur in the farm water. For dairies not adding food wastes and not having high sulfur content in the farm water, the H2S concentrations ranged between approximately 1000 and 3600 ppm. Water, waste and feed samples were also collected from the five dairies to determine whether the digester inputs had an effect on the components of the biogas as well as to explore the range in biogas quality at various dairies in the region. Based on the preliminary results shown in this study, it is suspected that higher sulfur contents present in feed water may have an impact on the hydrogen sulfide content in biogas generated through the anaerobic digestion of dairy manure. These results agree with often well-quoted generalized concentrations of approximately 60% CH4, 40% CO2 and 600 BTU/SCF for dairy-derived biogas. The data also show that the H2S concentration can vary significantly depending on the type of additives in the diet and the quality of the farm water, anywhere from 600 ppm to 7000 ppm.
anaerobic digestion; dairy manure-derived biogas; biogas characterization; methane; hydrogen sulfide
dissertation or thesis