ReviewLivestock waste-to-bioenergy generation opportunities
Introduction
With the massive consolidation of confined animal feeding operations (CAFOs) over the past decades, there is a need for new, state-of-the-art waste management systems that make animal operations economically viable and environmentally benign. In addition to the potential environmental threat traditional waste management systems pose (McNab et al., 2007, Stone et al., 1998, Szogi et al., 2006), there are rising energy prices and concerns over petroleum supplies. Thus, there is expanded interest in on-site biofuel production. Bringing biofuel production to the farm-scale provides an opportunity for the agricultural sector to reduce their reliance on imported fossil fuels while improving the soil, water, and air quality (Muller et al., 2007). Currently, animal production annually provides 35 million dry tons of sustainable biomass/manure feedstock that comprises 18% of the total available sustainable biomass from the US agricultural lands (Perlack et al., 2005, Ro et al., 2007). The use of animal manure and other organic-based waste products as bioenergy feedstocks for waste-to-bioenergy conversion processes would allow farmers to take advantage of new markets for traditional waste products. In effect, livestock waste-to-bioenergy treatments have the potential to convert the treatment of livestock waste from a liability or cost component into a profit center that can: (1) generate annual revenues; (2) moderate the impacts of commodity prices; and (3) diversify farm income.
Two basic platforms exist for converting organic biomass – the biochemical (biological) and thermochemical platforms (Fig. 1). Within these platforms are treatment processes that can be designed to solve odor problems, reduce volume, recover inherent nutrients, decrease pollution potential, as well as recover energy from the manure. As discussed by McKendry, 2002a, McKendry, 2002b, when selecting a conversion process, economics and both the available feedstock’s quantity and characteristics are important factors. In most instances, the desired energy form of the final end-product is the overriding factor. The end-products from each conversion process can be placed into three main groups: heat and power generation; transportation fuels; and chemical intermediates (Cantrell et al., 2007, McKendry, 2002a, McKendry, 2002b).
Biochemical conversion processes are defined by the US Department of Energy as the use of living organisms or their products to convert organic material to fuels (USDOE, 2002). These conversion processes can be realized by both anaerobic and photosynthetic microorganisms to produce gaseous and liquid fuels. Many times, the solid/slurry-phase residual by-product from these processes is nutrient-rich and can serve as an alternative fertilizer. The thermochemical platform is a physical conversion of biomass using high temperatures to break the bonds of organic matter and reform these intermediates into synthesis gas, hydrocarbon fuels, and/or a charcoal residual (Bridgwater, 2003, Cantrell et al., 2007, McKendry, 2002a, McKendry, 2002b). While the biological-based conversion processes require an extended amount of reaction time (days, weeks or even months), thermochemical conversion processes (TCC) can quickly (seconds or minutes) yield multiple complex end-products (Bridgwater, 2006). Consequently, the short residence time requirement of TCC drastically reduces the footprint requirements. Thermochemical conversion processes include combustion, pyrolysis, gasification, and liquefaction. Combustion converts manure’s energy into heat; however, this method does not provide a way to store the energy until it is needed. Additionally, the ash product from combustion has yet to find a suitable recycle use. As such, pyrolysis and gasification have received the most attention because they have more versatility.
In this paper, we reviewed currently available biological and thermochemical conversion technologies that can be applied to produce bioenergy while treating livestock wastes. We also suggested a biological–thermal hybrid system concept that appears to treat livestock wastes while at the same time reduce greenhouse gas emissions and produce bioenergy.
Section snippets
Biological conversion
This section intends to examine several biochemical conversion processes that are either established or emerging technologies. Waste-to-bioenergy technologies involving biological treatment of livestock waste have been dominated by anaerobic digestion with full-scale production of combustible biogas. Less known and reported at laboratory-scale has been the use of photobiologic microorganisms like algae and fermentative processes for production of bio-hydrogen. Even less known is the biological
Thermochemical conversion (TCC)
Thermochemical conversion (TCC) is a high-temperature chemical reforming process that breaks apart the bonds of organic matter and reforms these intermediates into char, synthesis gas and highly oxygenated bio-oil. In addition to TCC being a mass consumer of a manure’s organic portion that extracts all available energy, TCC processing has a number of other benefits and advantages: (1) small footprint; (2) efficient nutrient recovery; (3) no fugitive gas emissions; (4) short processing time on
Bio-thermochemical opportunities
Carbon dioxide is a major component in the product gases from anaerobic digestion and thermochemical conversion processes. Since an increased atmospheric concentration of CO2 is considered one of the main causes of global warming (Schneider, 1989), it is important to recover CO2 to limit short-term release. By naturally fixing atmospheric CO2 via photosynthesis ten times more efficiently than terrestrial plants (Usui and Ikenouchi, 1997), algae can rapidly generate both algal biomass and
Conclusions
In this paper, we reviewed multiple livestock waste-to-bioenergy processes to help combat rising energy prices and reduce the environmental threats from traditional livestock waste management practices. The biochemical process of anaerobic digestion is an established technology capable of biogas production; however, other biological processes like bio-hydrogen and bio-methanol production are still in early research stages and show promise to become a sustainable, renewable energy resource.
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