From waste problem to renewable energy resource : exploring horse manure as feedstock for anaerobic digestion

Abstract: A sustainable energy system requires, according to energy policies, reduced emissions of greenhouse gases, increased ratio of renewable sources of energy and more efficient use of energy. Horse manure could be regarded as waste, but also as a resource for renewable energy and plant nutrients. This thesis explores the potential of horse manure as a renewable energy source, and its possibilities to support and contribute to energy and environmental objectives. To do this, data was collected from literature, simulations, study visits and interviews.A number of horse keeping activities were identified in the assessment of horse manure as a feedstock for energy and as a plant resource: feeding, indoor housing, outdoor keeping, manure storage, fertilizing and transport, all with effect on amount and content of horse manure. Results indicated that choice and amount of bedding are important for both energy performance and plant nutrient content in the biofertilizer. Operational conditions such as long hydraulic retention time and high temperature had less impact for horse manure as a biogas feedstock. Anaerobic digestion resulted in the lowest global warming potential compared to incineration and composting, while large-scale incineration reduced primary energy demand, acidification potential and eutrophication potential. In a subsequent simulation, anaerobic digestion had lower potential environmental impact than unmanaged composting, regarding all chosen environmental impact categories in the study. Experiences from energy companies suggest that horse manure can be used in small quantities in co-incineration, with suitable incineration technology, but odor was mentioned as a problem. Farm-scale incineration required continuous maintenance and monitoring and mixing with pellets. As a feedstock for anaerobic digestion horse manure was regarded as suitable for plug-flow processes while stirred processes experienced more technical problems leading to increased cost for plants. With adaption of horse manure to the energy recovery technology to be used, and adaption at energy conversion plants to homogenous materials, this not yet fully utilized bioenergy resource has potential to contribute with renewable energy to the energy system, and thereby also reduce environmental impact from horse manure treatment