(290h) Optimal Production of Power in a Combined Cycle from Manure Based Biogas

Most countries have powerful agricultural systems to provide for food. As a result, large volumes of residues are generated. In particular, cattle and pigs farms generate vast amounts of residues (manures). Apart from the difficulty of dealing with such quantities of waste, the composition is also dangerous. Anaerobic Digestion (AD) provides the technology not only to dispose it but also to generate further value. The production of biogas through AD offers significant advantages over other forms of bioenergy production. It has been deemed as one of the most energy-efficient and environmentally beneficial technologies for bioenergy production [1]. Furthermore, biogas generation can drastically reduce greenhouse gases compared to fossil fuels by utilization of locally available resources. Compared to other fossil fuels, methane produces fewer atmospheric pollutants and generates less carbon dioxide per unit energy; as methane is comparatively a clean fuel, the trend is towards its increased use for appliances, vehicles, industrial applications and power generation [2]. Finally, the digestate represents an improved soil conditioner which can substitute mineral fertilizer [3]. However, most of the studies on power production from biogas limit to the use of Stirling engines or basic CHP plant analysis [4-6]

In this work, the production of power using a combined cycle gas turbine/steam turbine for wast heat recovery, which operates with biogas as fuel, is evaluated. The process begins with the production of biogas from pig and /or cattle slurry manure(s) using AD. Afterwards, the gas is cleaned up to remove humidity, hydrogen sulphide, carbon dioxide and ammonia. The cleaned gas (biomethane) is then used in a Brayton cycle (gas turbine) to produce energy. The flue gas that exits the Brayton cycle is typically at high temperature and it is further utilized to produce steam that generates power in a regenerative Rankine cycle (steam turbine). Two alternative steam production schemes are evaluated: either splitting the flue gas to have high temperature gas for the reheating step of the steam or sequential heating up. The model is formulated as a Mixer Integer Nonlinear Programming (MINLP) solved in GAMS^® for the optimal production of power and topology selection.

 For a typical production capacity of manure in farms in Spain, 2.6 MW are produced. The hot flue gas is used in sequence for producing superheated steam and next to reheat up the stream after the first expansion in the steam turbine. While typical efficiencies of biogas combustion engines are from 25-40%, the system presented in this work reaches 65% efficiency from the biogas to electricity. The investment for the plant turns out to be 26M� and the production cost of the electricity is 0.35�/kWh before including the credit from the conditioned digestate, that could be sold as fertilizer. The electricity cost goes down to 0.15�/kWh considering a reasonable credit from the digestate, whose composition depends on the feedstock processed in the facility.

References

[1] Fehrenbach H, Giegerich J, Reinhardt G, Schmitz J, Sayer U, Gretz M, et al. Criteria for a Sustainable Use of Bioenergy on a Global Scale. Germany: Federal Environment Agency; 2008; 245 (prepared by the Institute for Energy and Environmental Research (IFEU), Heidelberg).

[2] Chynoweth DP, Owens JM, Legrand R. Renewable methane from anaerobic digestion of biomass. Renew Energy 2001;22(1):1â??8.

[3] Weiland P. Biogas production: current state and perspectives. Appl Microbiol Biotechnol 2010;85(4):849â??6

 [4] Kang, J. Y., Kang, D.W., Kim, T S., Hur, K.B. (2014) Economic evaluation of biogas and natural gas co-firing in gas turbine combined heat and power systems Appl.Thermal Eng. 2014; 70: 723-731

[5] Japaraju, P., Rintala, J. 17 - Generation of heat and power from biogas for stationary applications: boilers, gas engines and turbines, combined heat and power (CHP) plants and fuel cells 404-427, In The Biogas Hand book. 2013.

[6] Kang JY, Kang DW, Kim TS, Hur KB (2014) Comparative economic analysis of gas turbine-based power generation and combined heat and power systems using biogas fuel Energy 2014; 67:  309-318