Category: Blog

A White Paper from the Chesapeake Climate Action Network:

April 2012

Contact: Mike Tidwell, director, 240-460-5838, mtidwell@chesapeakeclimate.org

The Climate Impact of Poultry Waste Disposal Practices and Manure-to-Energy Power Systems

The volume of manure produced by Maryland’s intensive poultry industry creates numerous environmental problems. When spread on fields in excess of what can be appropriately applied to crops as fertilizer, poultry waste can cause damage to surrounding air and water systems. CCAN’s research into this issue reveals that operations which aim to convert Manure to Energy (MTE) have the potential to be climate-appropriate only insofar as they utilize excess manure and employ the best technological means for capturing emissions. However, given the difficulty of measuring these key benchmarks and given the ongoing gaps in our scientific understanding of many key issues involved, significant additional research is needed before large-scale manure-to-energy systems can be responsibly developed in the Chesapeake region.

OVERVIEW

The intensive poultry production system on Maryland’s Eastern Shore raises nearly 300 million broiler chickens[1] each year, which produce approximately 550,000 tons of chicken “litter” (a manure and poultry bedding mixture).  Currently the most common use for poultry litter is as a fertilizer for crop lands.[2] [3] Poultry litter’s nutrient profile makes it an excellent fertilizer;[4] however the rate and method of application can significantly affect how much of the litter’s nutrients remain in the soil for crops and how much escapes into water and air systems. For example “no till” soil techniques have been promoted to reduce soil erosion,[5] yet several reports point to this technique as a major source of nutrient leakage into the Chesapeake Bay.[6] 

GREENHOUSE GASES (GHGs) AND LITTER

The primary mechanism through which poultry litter influences the climate is through release of the greenhouse gases (GHGs) methane (CH4) and nitrous oxide (N2O), both of which have many times the ability of carbon dioxide (CO2) to trap heat in the atmosphere. Over a 20-year time frame, CH4 is a staggering 72 times more powerful than CO2 at trapping heat, and N2O is 298 times more powerful than CO2 over a 100-year frame.[7]  Many sources, including the US EPA, consider decomposing poultry manure to be CO2 neutral, since the process recycles carbon which already existed in the atmosphere before being sequestered into the plant matter in poultry feed.[8]

Nitrogen and nitrous oxide: Poultry manure is nitrogen (N) rich;[9] however the amount of manure N which actually becomes converted to climate-damaging N2O through the nitrogen cycle depends on the way in which it decomposes in/on the soil. Many complex variables shape this process, including the litter’s source (content of poultry feed affects litter’s nutrient content) and when it is applied, crop rotation, tillage and drainage of soil, and climate (temperature and precipitation).[10] One study observes N2O emission rates of between 0.5 and 1.9 percent of the total N[11] applied; [12] however much further research is needed to better understand how manure decomposition on fields influences the climate.

CH4 (Methane):  Unlike swine and cattle manure, poultry manure tends to be stored in solid form, in which the anaerobic environment hospitable to CH4 emissions is comparatively rare. [13] [14] ^[15] One study estimates methane emissions for poultry are less than 0.2 lbs per animal per year. [16]

EXCESS MANURE

Poultry litter can be a feasible and economical substitute for synthetic fertilizers on many farms. If poultry litter which has an alternative, appropriate use as a fertilizer is diverted to power an MTE operation, the unmet nutrient needs of crops will likely be satisfied by synthetic fertilizers. The Haber-Bosch process used in creating synthetic fertilizers relies heavily on natural gas.[17] This implies that MTE is only climate-appropriate if it employs excess manure and does not introduce additional fossil fuels into the system through fertilizer production.

Most sources agree that excess poultry manure is produced on Maryland’s Eastern Shore, though estimates of exactly how much can vary.  A 2008 report from the Chesapeake Bay Program noted availability of excess manure depends on factors like animal population concentrations, existing soil nutrient levels, crop choice and overall acreage,[18] highlighting this as an area for further research.[19]  A 2010 report from Water Stewardship reported that in four counties in eastern Maryland excess poultry manure (in terms of crop phosphorous needs) is being applied annually at a rate of over 300 thousand tons. Other sources list the excess manure on the entire Delmarva Peninsula at 411 thousand tons per year. [20] An exact accounting of how much manure is being produced in the region, and how much of this should be used as fertilizer, is crucial to understanding the true influence MTE may exert on climate.

REDUCING GHGS BY REPLACING FOSSIL FUELS: MANURE TO ENERGY

Poultry production uses intense amounts of (fossil-fuel derived[21]) energy, particularly in providing for the heating and cooling needs of poultry houses. [22] If MTE can meet these needs using waste material it has the potential to generate a positive effect on climate. Some sources estimate that 85-90 percent of the 3-6,000 gallons of propane [23] being used annually by an average grower to heat poultry houses could be met by an onsite MTE operation.[24] As a relatively local fuel source, poultry waste may also eliminate some of the energy consumed in transporting fossil fuels.

METHODS

Poultry manure can be transformed into useful energy byproducts through the application of heat, or through ba
cterial digestion. Combustion, gasification and pyrolysis employ various ratios of heat and oxygen to alter waste material, while bio-digestion (anaerobic digestion) does so through the actions of microbes. Importantly, poultry litter’s chemical and physical composition is generally less consistent than fossil fuels or biofuels like wood. This irregularity, as well as variation in MTE operation sizes, emissions-capture technologies, and byproduct quality, makes it difficult to state definitively which MTE technique is most climate appropriate. [25]

Combustion processes burn litter material using great amounts of both oxygen and heat (3600 F)[26] to produce heat, CO2, H2O, and ash. This ash is generally a valuable fertilizer, high in phosphorous, potassium and other micronutrients.[27] The ash embodies the nutrients in a form that is greatly concentrated in both mass and volume compared to raw litter, making it more feasible to transport.[28]

Combustion technologies can be used to produce space heat for poultry growers, or on a larger scale the fuel can be burned in a boiler to produce steam heat and run a turbine to generate electricity. Since combustion occurs in a high oxygen environment, NOx emissions may be a greater concern with this technique than with others if the appropriate capture technologies discussed in the next section are not used.[29]  Combustion may also be the most economically feasible at the small scale.

Gasification is similar to combustion but employs lower temperatures (1100 to 1800F) and lower (or insufficient) amounts of oxygen to burn poultry litter. Gasification produces CO, H2, ash, and a biogas with a heating value equivalent to 10-20 percent that of natural gas (100-200 Btu/cubic foot).  The controlled burn and lower amounts of oxygen involved in the process may lower its risk of NOx production.[30]

Pyrolysis is the process through which organic matter is converted into charcoal (a concentrated form of fixed carbon). It employs almost no oxygen and very low temperatures (between 390 and 1100 F) to create a fixed carbon byproduct called bio-char.[31] The char acts to sequester carbon permanently, though its value as a traditional fertilizer appears low. [32]

Pyrolysis can produce a medium-value gas up to half the heating value of natural gas (350 to 550 btu/cubic foot).[33]  Other useful energy products include bio-oils which could be refined further to create a bio-diesel. Some studies claim that poultry manure is particularly well suited to pyrolysis because of its relatively low moisture content;[34] however pyrolysis may demand more expensive technology than combustion or gasification. Little to no oxygen involvement in the burn reduces the likelihood of producing NOx gases; furthermore using biochar as carbon-sequestering soil amendment may improve the climate profile of this technique. [35]

Bio-digestion/Anaerobic Digestion uses bacteria to convert manure into methane gas (60-70 percent), carbon dioxide (30-40 percent) and waste water. The methane has 60-80 percent of the energy value of natural gas (600-800 Btu/cubic foot) and can be used for heating, electricity generation, lighting or cooking.[36] [37]

The emissions profile for anaerobic digestion appears to be relatively low; [38] however, unlike the thermal processes which transform manure nutrients into more usable forms, digestion produces a large quantity of nutrient-rich waste water that must be refined further for use in agriculture.[39] Digestion also appears better suited to swine or cattle manures which have higher moisture contents,[40] and its technical requirements may make it less economical for small operations.[41] [42]

CAPTURING AND REDUCING EMISSIONS

The thermal or biological techniques used to transform poultry manure into energy are not without emissions. In order for manure to be considered a potentially renewable and climate-appropriate fuel, producers must make use of the best available technologies for capturing or reducing these emissions.

Two of the most effective techniques for reducing NOx gases are Selective Non-Catalytic Reduction (SNCR) and Selective Catalytic Reduction (SCR). SNCR involves the injection of ammonia or urea into the incineration furnace during combustion to reduce NOx emissions. One report anticipated the efficiency of this capture method to be 25-50 percent; [43]however some concern exists that a modest amount of ammonia may escape during this process (“ammonia slip”[44]).  SCR involves an additional control device to manage the injection of ammonia into flue gases and may have a capture efficiency of up to 90 percent. SCR, though more expensive, appears to be the most efficient capture method where technologically feasible.[45]

Other capture technologies to address air-quality concerns include a fabric filter or “bag house” which can control emissions of small particles[46] and “scrubber” techniques to capture 80 to 99 percent of sulfur dioxide and hydrogen chloride.[47] [48]

FURTHER RESEARCH

CCAN’s preliminary literature review is not intended to be an exhaustive, scientific accounting of the issues surrounding poultry manure disposal and climate change. Nevertheless, areas where further data is necessary did present themselves during the course of our research.

• We found few studies giving estimates of the precise amounts of N2O -a powerful GHG- which can result from poultry litter spread on soils, and those which did tended to be contradictory and conditional. Several researchers at area universities confirmed that N2O emissions from decomposing poultry litter under a range of circumstances (including different storage processes) are not well understood. Further research is needed to better understand the emissions profile of poultry manure, specifically concerning greenhouse gases.
• The amount of excess poultry manure which actually exists in any area is sensitive to a range of variables. This issue would also benefit from further study to ensure MTE functions as a climate-appropriate fuel option.
• Our investigation pointed to a scarcity of reliable, comprehensive and comparative reviews of the technological options for MTE, which have the express goal of measuring the GHGs each process produces.  A majority of the literature available evaluates a specific technique (often by its proponents) and does not focus on emission
  s from a global warming standpoint. CCAN feels that an independent and impartial review of the emissions profile of MTE techniques under comparable conditions is necessary before a reasoned endorsement can be reached for any single MTE technique.

We at CCAN feel MTE holds significant potential to be a tool in Maryland’s fight for a renewable and climate-safe energy base. In order to confidently move forward we advise industry leaders to accept the need for caution, utilize the best possible technologies, and continue to pursue a deeper understanding of poultry waste’s climate implications.  

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