An Experimental Evaluation of Low Calorific Value Gaseous Fuels as a Heat Source for Stirling Engines

Abstract

This report, An Experimental Evaluation of Low Calorific Value Gaseous Fuels as a Heat Source for Stirling Engines, was written by Johan Westerberg as a Master's thesis at Chalmers University of Technology. It covers the topic of MILD combustion of liquid propane gas, G25 natural gas, synthesis gas and landfill gas, of which the latter two represents low calorific gaseous fuels. The investigated engine was a C9G Stirling engine from Cleanergy. The primary incentive to this study was to determine if steady and reliable operation of the C9G Stirling engine could be maintained while using low calorific gaseous fuels. Another incentive to this project was to achieve further understanding of the combustion device, which purpose is to supply the Stirling engine with heat. Thus, an understanding of the changed characteristics caused by varying the air to fuel ratio was targeted. Engine operation by use of landfill gas, made up by 20 percent methane in carbon dioxide and nitrogen, was successful at the lowest load setting allowed by the C9G engine, generating 1:5kW of shaft power. For liquid propane gas and G25 natural gas, up to 10:9kW of shaft power was generated. Combustion of synthesis gas was not attempted due to the hazard of carbon monoxide poisoning, caused by insufficient ventilation in the test cell. Apparent changes in energy ows and temperatures within the combustion device at varied air to fuel ratios suggest that incomplete combustion prior to the heat sink of the Stirling engine was an issue, especially at low to moderate air to fuel ratios. Further, a reversed ow direction through the ue gas recirculation cavities was observed when high ow rates of fuel, which primarily occurs for high load settings and low air to fuel ratios, was supplied to the engine. Consequently, a higher air to fuel ratio resulted in a higher system efficiency for all tested fuels and load settings. Thus, the system efficiency was concluded to be heavily impacted by the air to fuel ratio.