Investigation of Subsystems for Combination into a SOFC-Based CCHP System

Abstract

This paper presents the development of the subsystems for stationary biogas powered solid oxide fuel cell (SOFC)-based combined cooling, heat and power (CCHP). For certain applications, such as buildings, a heat-driven operation mode leads to low operating hours per year for conventional combined heat and power (CHP) systems due to the low heat demand during the summer season. The objectives of this study are the evaluation of an adsorber, a steam reformer, a SOFC, and an absorption chiller (AC). Biogas, however, contains impurities in the form of hydrogen sulfide (H2S), hydrogen chloride (HCl), and siloxanes in different concentrations, which have a negative effect on the performance and durability of the SOFC and, in the case of H2S, also on the catalyst of the steam reformer. This paper describes different experimental sections: (i) the biogas treatment with its main focus on H2S separation and steam reforming, (ii) the setup and start-up of a 10 cell SOFC stack, and (iii) test runs with an AC using a mixture of NH3 (ammonia)/H2O (water). The components required for the engineering process of the subsystem's structure are described in detail and possible options for system design are explained. The evaluation is the basis to reveal the improvement potentials, which have to be considered in future product developments. This paper aims at comparing experimental data of the test rigs to develop an understanding of the requirements for a stable and continuous operation of a SOFC-based CCHP operated by biogas.

Advances, challenges, and perspectives of biogas cleaning, upgrading, and utilisation

“…It is estimated that annual worldwide production of siloxanes is over 1 Mt [51,[167][168][169]. Table 8 summarises the use of siloxanes in various applications [170].…”

“…For example, high-temperature fuel cells cannot tolerate silica values above 69 ppb, to prevent deposits on the anode. The siloxanes in biogas convert to silica (SiO2) which deposits on turbine blades, heat exchangers, cylinder heads, etc., leading to abrasion of moving parts, or deposits on the inner surface of exchangers, reducing heat transfer efficiency [167,168,[171][172][173].…”

“…Solid oxide fuel cells (SOFCs) benefit from fuel flexibility, and so, can be directly (internal reformimg) and indirectly (external reforming) fed with biogas. SOFCs can achieve up to 50% electrical efficiencies, and up to 85%-90% efficiency in CHP and CCHP modes [168]. The electrical efficiency of fuel cells is higher than micro turbines and mHAT turbines making them superior to other methods.…”

“…This exhaust gas is used to preheat the air and supply the heat required for the absorption chiller. Heat from the exhaust gas may also be utilised to further increase the efficiency of the system [168].…”

“…Comparison of various technologies for siloxane removal[161][162][163][164][165][166][167][168][169][170][171][172][173][174][175][176] .…”