Cogeneration of power and substitute of natural gas using an internal combustion engine

In a previous paper an option to obtain valuable electric power and valuable fuel starting from renewable variable electric power, biomass and/or wastes was analysed. Referring to the generic schematic, several different combinations could be carried out. In particular, the case of a biomass gasifier using electrolytic oxygen as an oxidant was examined. The syngas obtained could be used in different kinds of power unit: gas turbines, internal combustion engines, fuel cell stacks. And again electrolytic oxygen is available as an oxidant in such a way that the exhaust gas contains only steam and carbon dioxide, a part from very small amounts of impurities. Steam and carbon dioxide are easily separable so that water could be recycled to the electrolyser and carbon dioxide could be fed to a Sabatier process together electrolytic hydrogen to obtain a mixture of methane and hydrogen with characteristics very similar to natural gas. Such a mixture can be distributed and utilised just as natural gas. When the power unit is a gas turbine or an internal combustion engine, oxycombustion results in too high temperature. Temperature could be controlled by recycling water and/or carbon dioxide. A simulation of the entire system has been carried out using AspenONE® v8.4. The efficiencies estimated were in the range 0.507-0.586, with the lowest for the system based on internal combustion engine. In this paper we reconsider just the less favourable case trying to improve the reliability of the simulation using a specific software for internal combustion engine (ICE) simulation, the AVL BOOST one-dimension code, and using again AspenONE® v8.4 for the other components. The AVL BOOST is a dedicated and well proven engine simulation software so it is possible to obtain reliable values of ICE mechanical efficiency and exhaust gas temperature. The ICE has been tested at 1500 rpm (typical rotational speed value for stationary engines) and for different external carbon dioxide recycles, while the syngas composition and flow and the oxygen flow has been kept constant. The reference model adopted is a natural gas fuelled ICE. To lower engine exhaust gases temperature at about 750-775 °C, a low engine exhaust gas recirculation (EGR) has been adopted together with a 70% external carbon dioxide recycle. The ratio between oxygen and carbon dioxide (0.28) is not so different from the ratio between oxygen and nitrogen in the air. The efficiency of ICE is 0.27 and the exhaust gas temperature is 757 °C. The ICE efficiency (roughly 27%) is significantly lower than that of the same engine fuelled with natural gas (35%), even if the substitution of nitrogen with carbon dioxide and the presence of hydrogen into the fuel could lead to suppose another behavior. Actually it does not happen probably owing to the high percentage of carbon monoxide (characterized by a low laminar flame velocity) and water vapor (which absorbs heat) into the mixture fed to ICE. This suggests that a water gas shift unit before the ICE could improve the efficiency by reducing the carbon monoxide concentration in favor of hydrogen concentration. Another issue is the conversion of heat recovered at the exhaust of the ICE, which can be conveniently utilized, for example, to feed an organic Rankine cycle (ORC), but the high temperature could be suitable also for other energetic applications.