Fuel Quality Analysis for Biogas Utilization in Heavy Duty Dual Fuel Engines

Paper in proceeding, 2012

Internal combustion engines using oil-derived fuels are dominating the heavy transportation sector today. However, the climate issue and security of supply drive the development towards new fuels and engine technologies. In the short term, Natural Gas (NG) is expected to have a dominant role, due to its high availability and a favourable H/C ratio. Thereafter, it is expect an introduction of biofuels of second and third generations. In this scenario the engine suppliers need to develop engines for various fuels of both fossil and renewable origin. One possibility is the Dual Fuel engine (DF), which uses a Diesel pilot to ignite a gas mixture and, it can be used for natural gas of various qualities as well as synthetic natural gas (SNG). To obtain significant share of second and third generation biofuels into the transportation sector a key process is gasification of the raw solid biomass to gas, as it can offer high production capacity and high efficiency. One interesting biofuel is SNG and at present there are a number of projects focusing on SNG production through gasification of biomass to be fed to the NG grid. However, this is a rather advanced and several stage process. The initial gas from the gasification before the gas is upgraded to CH4 (SNG) contains of a blend of various gas components such as H2, CO, CO2, CH4 and fractions of C2H2, C2H4, C3H6, and C3H8, as well as, longer hydrocarbons. The upgrading takes place in many process steps, where each step involves a cost and loss of energy. The question raised is if there are more efficient routs to introduce biomass derived gas than refine it all the way to SNG, from a well to wheel (WTW) perspective? The first step in such an analysis is to investigate how different gas mixtures could meet emission limits, together with the required performance of efficiency and load. This issue has been addressed in this work, where the operability in DF engines using gaseous fuels with a variation in fuel quality has been investigated. The operability has a key role in the optimization of the WTW efficiency, since it influences both the production process and the combustion in the engine. The definition of fuel quality for gaseous fuels to be used in gas engines is still not in place and proper legislation and standards are not available. Here, three parameters which are fundamental for a proper combustion in a DF engine: the Methane Number (MN), the Lower Flammability Limit (LFL) and the Lower Heating Value (LHV) have been studied. All parameters influence the combustion performance in the DF engine of the Port-injected type, which is more sensitive to the fuel quality than the Direct-injected type. The components available from biomass gasification were evaluated together with those from different NG compositions on the European market. Specific relations between the composition and fuel quality parameters have been derived, which can be used as starting point for future well to wheel analysis.