Producing Energy Pellets from Dried Animal Waste and Biomass

Chandra Theegala  |  9/20/2017 7:26:16 PM

Chandra Theegala

Forestry and poultry, the top two income-producing agricultural commodities in Louisiana generate significant quantities of waste that can be used for producing energy pellets or other value-added products such as soil amendments.

The forestry industry produces enormous quantities of sawdust that is not suitable for energy production because of its low bulk density and high moisture content. If end users are not identified within reasonable hauling distances, the full economic potential of the sawdust is not realized.

The magnitude of waste from the poultry industry is the primary reason for concern. Because of the costs associated with hauling the material over long distances, poultry litter is often overapplied to nearby land, leading to excess nutrients and pathogens in runoff water.

Apart from these two primary wastes, significant quantities of crop residues are underutilized. Louisiana also has the potential to generate enormous quantities of dedicated energy crops such as switchgrass, energycane and kenaf.

Therefore, generating energy from this waste and biomass is both economically logical and environmentally sustainable. Because most of these wastes have relatively high moisture and low bulk density, it is important to dry the waste and make it into pellets. If the waste is generated from animal operations, pathogen elimination or pellet sterilization may be warranted.

Energy content quantification experiments in the Department of Biological and Agricultural Engineering indicate the energy content is 6,879 Btu per pound of pine sawdust, 5,116 Btu per pound of separated dairy manure and 5,030 Btu per pound of poultry litter. Results also indicate that pine has the highest percentage of volatile solids (99.9 percent) and least amounts of ash. However, both animal waste samples had volatile solids contents around 83 percent, thereby indicating that pure animal waste pellets may not be ideal for burning and may require some degree of blending with biomass with low ash content. However, animal waste pellets are ideal as soil amendments because of their high nutrient content.

Freshly collected poultry litter and biomass have a moisture content of up to 50 percent or higher. Therefore, for low-value applications like producing bioenergy, using electrical energy or fossil fuel for removing moisture is prohibitively expensive and impractical. Oftentimes, the energy required for drying the material far exceeds the energy it contains. On the other hand, solar energy, which is abundant and free, can be used for drying these materials. LSU AgCenter research on custom-designed solar stills (Figure 1) has demonstrated that biomass with moisture content as high as 70-80 percent can be dried to less than 4 percent moisture in less than three to four summer days.

Research has demonstrated that the extremely high temperatures (above 200 degrees F) in the solar still may not be enough for bacterial sterilization or pathogen elimination for soil amendment or fertilizer applications. However, high moisture content in the still together with solar heat effectively lowered the fecal coliform, E. coli and total coliform counts.

Solar still-dried biomass can be pulverized, if necessary, and fed into a pellet mill, which extrudes the pellets from a rotating die on which frictional rollers are attached (Figure 2). Results indicate that biomass with a 10-15 percent moisture content is ideal for pelletization because the steam generated by heat from the frictional rollers improves pellet integrity. The produced pellets can be used in biomass gasifiers or pellet stoves for producing heat and electricity. Gasification research at LSU has shown that biomass with high ash content is not ideal for gasification as a sole feedstock. However, blending high-ash wastes with biomass with low ash content (such as pine with less than 1 percent ash content) can improve the combustibility.

Chandra Theegala is a professor in the Department of Biological and Agricultural Engineering.

(This article was published in the spring 2015 issue of Louisiana Agriculture Magazine.)

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Figure 1. Incident solar radiation heats the solar still to very high temperatures (about 210 degrees F during midday). In the regular non-convection mode (with no air blower), water evaporated from the wet waste condenses on the cooler glass surface and runs to the bottom of a special channel to be drained out. In the convection drying mode (with air blower), water is carried away with incoming fresh air. This concept was validated in the Department of Biological and Agricultural Engineering using separated diary manure in smaller, custom-built solar stills.

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Figure 2. Pellets

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