Barbosa, F., "Evaluation of Biogas Use in Transit Bus Fleets," SAE Technical Paper 2015-36-0227, 2015, doi:10.4271/2015-36-0227.
Current massive urbanization process concentrates high amount of population and impose an increased demand on transport systems. In this context, transit bus system plays an important role, as the most dynamic and less capital intensive transit option available. At the same time, it is strongly dependant on fossil fuels, predominantly diesel fuel, with its intrinsic polluting and greenhouse (climate change) effects. This has boosted research and investments for alternative and renewable fuels. One solution currently receiving widespread recognition is biogas use in transit bus fleets, as it allows the use of a renewable fuel, made from substrates derived basically from waste and sewage that otherwise would produce methane released to the atmosphere. Biogas contains basically methane, carbon dioxide, trace amounts of hydrogen sulfides and water, and to be used as engine fuel need to be upgraded, which means increasing the methane content up to 97% and removing water and other gases, when its composition becomes similar to fossil natural gas. From a technological perspective, biogas engines are predominantly spark ignited - SI and can be used in both lean burn (diesel derived) and stoichiometric (SI derived) combustion concept, as well the so called Diesel Dual Fuel - DDF technology, which uses a mix of natural gas and diesel fuel as a “liquid glow plug”, each one with its own strengths and weakness related to efficiency, thermal loads and cleanness (emission potential). The most suitable technology will depend basically on the emission targets to be achieved as well on the flexibility desired. Investment costs with biogas buses are generally higher, while operational costs use to be lower than those of baseline diesel buses, with the net value dependant on the size of the fleet relative to the installed infrastructure (bus garage, distribution, storage and filling system) as well as the cost of biogas production and engine maintenance. This paper is supposed to give an overview of biogas potential as a renewable fuel and its potential production chains, a technological heavy duty gas engine roadmap as well as an alternative fuel cost analysis.