Implementation of engine exhaust gas recirculation in a fixed bed gasification reactor

This study aims to increase the energy efficiency of a micro combined heat and power (micro-CHP) system able to produce electrical energy and thermal energy from wood biomass through the gasification process and an internal combustion engine. Since nowadays the engine exhaust gas is released in the atmosphere, without being exploited, the goal is to recover this gas. A fraction of gas is taken from the engine exhaust pipe and is reinserted in the gasification reactor head to provide thermal energy and a series of new reagents: A sort of exhaust gas recirculation (EGR) is created. Some tests were carried out at different fractions of EGR, 10% and 20%, on a Power Pallet 30 system (PP30) built by All Power Labs, to obtain information about syngas quality and gasification efficiency. The gas high heating value (HHV) is indirectly determined by gas chromatography that gives the syngas composition and the concentration of single compounds. The results show an increment of the nitrogen content and a reduction of hydrogen in the syngas when the EGR percentage rising. The concentration of the other gases does not change. This variation causes an HHV drop, compared to the case without EGR, which is reflected in the PP30 efficiency. Tests have revealed an increment of efficiency from 70% to 71% only in the case with EGR sets at 20%, thanks to the higher temperature at which the exhaust gas was reintroduced in the gasifier head, compared to the reactor temperature itself, that leads to an increase of syngas flow rate. Therefore, an higher gasifier efficiency has been obtained although the drop of heating value, thanks to the larger production of syngas. This higher volume of produced syngas not only can compensate the HHV drop but also bring benefit to the PP30, compared to the case without EGR. Consequently, the temperature is more important than the flow rate value or the composition of the exhaust gas reintroduced in the head of the gasifier. Taking into account the low cost of the recirculation circuit components, the EGR is a good improvement for the fixed bed gasification reactors