Influence of anodic gas mixture composition on solid oxide fuel cell performance: Part 1

Abstract

Solid oxide fuel cells can be fed not only by conventional fuels (methane or hydrogen), but also by nonconventional fuels such as syngas (H2, CO, N2, CO2, H2O). In terms of CO/H2 ratio, N2, CO2 and H2O concentrations, and mass flow, the anodic gas mixture composition can affect SOFC performance and create a carbon deposition phenomenon. In this paper a careful analysis of SOFCs performance for varying concentrations of the gas components of a generic stream of syngas and operating conditions (pressure, temperature), was performed in order to plan the actual test campaigns. This study showed that the CO/H2 ratio is the parameter to which must be given the most attention. This factor not only directly affects the performance of the fuel cell, but also affects its useful life, because carbon deposition is directly determined by the concentration of CO in the gas-mixture. The ratio has been found that provides the best performance without incurring in carbon deposition, which is in the range 20/80 ÷ 35/65. The paper reports a first stage of analysis (Part 1), whereas the second stage of analysis is reported in a subsequent paper (Part 2) where performances are assessed with regards to main operating parameters.

Performance Analysis of an Intermediate Temperature Solid Oxide Electrolyzer Test Bench under a CO2-H2O Feed Stream

“…The thermal power surplus produced by the stack, P th,s expressed in W is calculated by Equation 10: (10) where P th,lost,s is the thermal power generated by irreversibility, Ohm and contact overvoltage and polarization phenomena in the stack expressed in W; P th,rwgs,s is the thermal power absorbed by the RWGS chemical reaction in the stack expressed in W; The thermal power required to heat the feeding gases at the stack inlet, P th,heat,gas, s expressed in W is calculated by Equation (11): inlets expressed in K; c p,gas,in,ca (T) and c p,air,in,a (T) are the constant pressure specific heats of the gas mixtures and of the air at the cathode and anode inlets expressed in J•kg −1 •K −1 . If the SOE system is in thermal equilibrium, the operating condition (12) must be verified: P th,tot,sys = P th,heat,gas,s − P th,s − P f ur = 0 (12) where P f ur is the thermal power produced by furnace expressed in W. This condition expresses the equality of the thermal powers (P th,s + P f ur ) and P th,heat,gas,s . The net production efficiency of the SOE system is calculated through Equation 13: m CO are the mass flows of hydrogen and carbon monoxide produced expresses in kg•s −1 ; LHV H 2 and LHV CO are the lower heating values of hydrogen and carbon monoxide expressed in J kg −1 ; P th,tot,sys is the total additional thermal power (>0) required by the SOE system (SOE stack and furnace) expressed in W; P th,ph,H 2 O is the thermal power required to produce the steam at the stack inlet starting from water at a temperature equal to 25 • C at the SOE system inlet expressed in W; η el,re f /η th,re f is the ratio between the electrical and thermal reference efficiencies, which is used to convert the thermal power P th,tot,sys > 0 + P th,ph,H 2 O in equivalent electrical power.…”

“…Solid oxide fuel cells (SOFCs) are able to convert hydrogen and/or carbon monoxide and oxygen-rich air [11,12] or biofuels produced by biomass gasification and anaerobic digestion [13,14] in electric energy with high conversion efficiency. Some research studies were conducted on innovative components' materials to reduce the SOFCs operating temperature [15][16][17][18][19][20].…”