Energy Production from Agricultural Waste

Although it is clear from above discussion that ethanol production through fermentative methods from crops and other renewable biomass sources has received much attention recently, crop-based feed-stocks are subject to seasonal fluctuations in supply, ultimately limiting ethanol generation (Kasper et al. 2001). The energy cost in harvesting these feedstocks (e.g., corn stubble) as well as their lost value as soil amendments can make ethanol production costly for farmers (Pimental, 1992). Animal manures avoid many of these problems because they are a truly renewable feedstock. Production of ethanol from animal waste through the process of gasification is another new technology that has been trialed (Kaspers et al. 2001).

Gasification of biomass has received much attention as a means of converting waste materials to a variety of energy forms such as electricity, combustible gases, synfuels, various fuel alcohols, etc. Gasification is a two-step, endothermic process in which solid fuel is thermo-chemically converted into a low or medium Btu gas. In the first step, pyrolysis of the biomass takes place; in the second step either direct or indirect oxygen-deprived combustion takes place during the gasification process. This process converts raw biomass into a combustible gas, retaining 60-70% of the feedstock's original energy content. A recent cost and performance analysis of biomass (i.e. wood) gasification systems for combined power generation indicated that such a steam system (Battelle Columbus Laboratory) had the lowest capital cost and product electricity cost (Craig and Mann 1997).

Huge amounts of swine waste are produced annually in many parts of the world. For instance, the quantity of swine manure produced in the USA is estimated to be 5 billion kg dry matter per year, sufficient to contribute substantially to ethanol supplies. Assuming a conversion efficiency of 40%, this is a theoretical ethanol yield of 500 million gallons per year (Kaspers et al. 2001).

Poletory Diagram Schedule
Figure 2. Flow diagram of poultry litter fuelled power plant (Source: Staff report, Modern Power System, 2000).

In the UK, poultry litter has been used for large-scale off-site electricity generation and on-farm space heating of broiler houses using two separate stages-gasification and combustion (Dagnell 1992). Figure 2 shows a flow diagram of poultry litter fuelled power plant. The size of poultry litter production in many countries worldwide indicates a sustained and increasing trend. For example, in the UK, the poultry farming industry produces nearly 1.5 million tons of litter annually per year (Fibrowatt). If land-application of litter is not a viable option due to the potential contamination of aquatic bodies through the run-off of nutrients associated with litter, an alternative mean of disposal is the production of energy, as has been demonstrated in the USA. Many promising projects are under way, both in the USA and Europe, researching the environmental effects and economic benefits of poultry litter biomass combustible. One such example of power generation, the Fibrowatt has built three power plants in the UK, consuming 800000 t of litter annually to generate approximately 64 MW of electricity (Fibrowatt; Morisson 1997). In one of the three plants, nearly 400000 t of poultry litter per year is used to produce enough electricity to power about 93,000 homes in Thetford, UK. The facility at Thetford has a maximum feed rate of 55 tonnes per hour. Feedstock is delivered from surrounding operations by covered trucks from nearby farms. The poultry litter is fed to boilers using spiral screw augers blown into the combustion chamber and burnt at 850oC. Water in the boiler is heated to 450oC and steam from the process turns the turbine connected to an electrical generator. This process is continued, steam is cooled, and water is recycled for boiler use, while ash from combustion is conditioned through the precise addition of nutrients to create a fertiliser product (Fibrophos), marketed as a concentrated organic fertiliser (ECW, 2006). Davalos et al. (2002) recently evaluated the usefulness of poultry litter as a feasible fuel and found that the calorific values or the massic energy combustion of dry-poultry litter was 14 447 kJ/kg. The authors also reported that if the water content is < 9%, it can burn without extra fuel.

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  • Jessica
    How much poultry litter waste is produced annually?
    1 year ago

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