Energy Systems and Technologies for the Coming Century ...

efficiency and turbine inlet temperature of the four plants (52.5% and 827°C,respectively) is found in methanation plant 2 and is reached by combining product gaspreheating and adiabatic methanation, thus avoiding an anode in/out heat exchanger. Ifan anode inlet temperature of 600°C (instead of 650°C) was allowed in methanationplant 1, the anode in/out heat exchanger could also be skipped here and a turbine inlettemperature of 843°C could be reached. The electrical efficiency would increase verylittle, though (~0.3 percentage points, without taking into account any SOFCperformance penalty due to the lower anode inlet temperature).Studying reference plant 2 and methanation plant 1, it can be seen that installingadditional product gas preheating results in a better plant performance than installing anadiabatic methanator. In methanation plant 1, the turbine inlet temperature reaches thehighest level of the two plants, but the lower mass flow of air limits the MGT net poweroutput. In reference plant 2, a relatively high turbine inlet temperature is reached withoutreducing the air flow resulting in a greater performance gain.From Table 2 it is also evident that the S/C ratio is rather constant in all plant scenarios.In this study, an adiabatic methanator cannot lower the risk of carbon formation in theSOFCs. If the theoretical maximum CH 4 level from full methanation was reached, theS/C ratio would only increase to 0.53.5 DiscussionMaking use of an adiabatic methanation step has a positive, though limited, impact onthe net electrical efficiency of the plant. Besides affecting the overall plant performance,including a methanation step reduces the size and duty of several components due toreduced SOFC cooling by excess air flow. These are the air compressor, recuperator,SOFC air preheater, burner, MGT expander and exhaust cooler. This is most pronouncedin methanation plant 1 with the lowest air mass flow. So from an investment point ofview, it is expected that adiabatic methanation is beneficial due to reduced componentsizes. Solely from a plant performance point of view, additional product gas preheating(reference plant 2) or combined methanation and product gas preheating (methanationplant 2) is better. In these cases, component sizes are not reduced or not reduced assignificantly as in methanation plant 1.The temperature levels in the methanation reactors in the two methanation plants cover arange from 209°C at the coldest inlet to 659°C at the hottest outlet. Catalysts with highand stable activity in this temperature range are commercially available [22]-[23].Furthermore, the turbine inlet temperatures reached in this study are all acceptable withregard to material constraints. Commercially available MGTs reach 950°C in turbineinlet temperature [24].6 ConclusionThe present study investigated the benefits of introducing an adiabatic methanation stepin a novel hybrid CHP plant combining two-stage biomass gasification, simple gasconditioning, an SOFC stack and a MGT. It was confirmed that by introducing anadiabatic methanation reactor prior to the SOFCs, the excess air flow for SOFC coolingwas reduced due to additional endothermic reforming reactions internally in the SOFCs,thus lowering the air compressor work. Regardless of the limited CH 4 levels achievedfrom adiabatic methanation (between 3.3 and 6.8 vol-%), gain in plant performanceswere shown due to higher MGT net power production. Installation of an adiabaticmethanator in reference plant 1 reduced the mass flow of cathode air by 27% andincreased the turbine inlet temperature by 17% resulting in an electrical efficiencyincrease from 48.6 to 50.4% (LHV) and reduced the size of several components. On theother hand, by introducing additional product gas preheating from the raw product gasinstead of an adiabatic methanation step, the plant performance was better (50.4 versusRisø International Energy Conference 2011 Proceedings Page 306