Ernst Riensche

Pre-reforming of natural gas in solid oxide fuel-cell systems

Internal steam reforming is an attractive option offering a significant cost reduction and higher system efficiencies of a solid oxide fuel cell (SOFC) power plant. Furthermore, faster load response may characterise systems with internal reforming. However, complete internal reforming can lead to several problems, which can be avoided with partial pre-reforming of natural gas. For SOFC systems the ratio between internal and pre-reforming has to be optimised on the basis of experimental performance data. A detailed study concerning the pre-reforming in a reformer of considerable size (10 kW) is carried out. The influence of operating temperature and mass flow variations on conversion of methane and ethane is analysed. During internal reforming of methane with substrate anodes, large temperature gradients were detected. Substrate samples with low catalytic activity show large response times with respect to parameter variations.

Optimization of a 200 kW SOFC cogeneration power plant: Part I: Variation of process parameters

In order to benefit from the high electrochemical efficiency of solid oxide fuel cells (SOFC), a detailed balance of plant (BOP) has to be developed. An energetic and economic analysis of a decentralized natural gas-fuelled SOFC-power plant in the range of 200 kW capacity is carried out. All calculations start from a basic plant concept with a simple flowsheet and a basic parameter set of SOFC operation and economic data. Changes in costs of electricity (COE) and plant efficiency are determined for the variation of cell operation parameters. This includes the influence of air temperature increase in the stack, degree of internal reforming, cell voltage and fuel utilization. The results indicate two classes of cell parameters. Cell voltage and fuel utilization show a cost optimum characteristic, whereas the other parameters have a uniform influence on efficiency and costs of electricity.

Clean combined-cycle SOFC power plant — cell modelling and process analysis

Abstract The design principle of a specially adapted solid-oxide fuel cell power plant for the production of electricity from hydrocarbons without the emission of greenhouse gases is described. To achieve CO 2 separation in the exhaust stream, it is necessary to burn the unused fuel without directly mixing it with air, which would introduce nitrogen. Therefore, the spent fuel is passed over a bank of oxygen ion conducting tubes very similar in configuration to the electrochemical tubes in the main stack of the fuel cell. In such an SOFC system, pure CO 2 is produced without the need for a special CO 2 separation process. After liquefaction, CO 2 can be re-injected into an underground reservoir. A plant simulation model consists of four main parts, that is, turbo-expansion of natural gas, fuel cell stack, periphery of the stack, and CO 2 recompression. A tubular SOFC concept is preferred. The spent fuel leaving the cell tube bundle is burned with pure oxygen instead of air. The oxygen is separated from the air in an additional small tube bundle of oxygen separation tubes. In this process, mixing of CO 2 and N 2 is avoided, so that liquefaction of CO 2 becomes feasible. As a design tool, a computer model for tubular cells with an air feed tube has been developed based on an existing planar model. Plant simulation indicates the main contributors to power production (tubular SOFC, exhaust air expander) and power consumption (air compressor, oxygen separation).

Optimization of a 200 kW SOFC cogeneration power plant. Part II: variation of the flowsheet

An energetic and economic analysis of a decentralized natural gas-fuelled solid oxide fuel cell (SOFC) power plant in the range of 200 kW capacity is carried out. All calculations start from a basic plant concept with a simple flowsheet and a basic parameter set of SOFC operation and economic data. Changes in costs of electricity and plant efficiencies are determined for variations of the plant concept. Flowsheets with gas recycling by blowers or jet boosters are described. Cathode gas recycling by jet boosters turns out to be more advantageous with respect to the costs of electricity than gas recycling by hot gas fans. The influence of pressure drop in the cathode gas circuit is analyzed. In case of anode gas recycling an internal steam circuit exists. This has the advantage that the external steam generator is eliminated and that the steam concentration in the exhaust gas is reduced. Therefore, a higher amount of excess heat can be used. Removal of useful heat at higher temperature levels diminishes the driving temperature differences and enlarges the heat exchange area of the recuperative heat exchangers located downstream.

- CO 2 sequestration from solid oxide fuel cells: Technical options and costs

Publisher Summary This chapter illustrates the vision of electricity generation from hydrocarbons by a tubular Solid Oxide Fuel Cell (SOFC) without carbon dioxide emission. It presents a conceptual design study involving the combination of a 20 MW SOFC with an electrochemical afterburner (ECAB) to reach the high CO2 concentration of about 90 mol-% in the exhaust gas that is required for efficient CO2 sequestration. A commercial simulation program (PRO/II) is used to quantify the influences of flow-sheet variations on plant efficiency and costs. Particularly the process variants involve different types of ECABs (oxygen pump, mixed oxide conductor, and second SOFC) and different possibilities of turbine integration into the cooling air system of the SOFC/ECAB generator. By comparison with a conventional reference system without CO2 capture CO2 abatement costs are calculated and economic break-even points on a scale of increasing CO2 tax values, expected for the future, are determined.