Co-Electrolysis of Simulated Coke Oven Gas with Carbon Dioxide Using a Solid Oxide Electrolysis Cell
Authors: Czachor Michal , Laycock Christian James , Carr Stephen , Maddy Jon , Lloyd Gareth , Guwy Alan 
Published: July 2021

A significant contributor to carbon and air-pollutant emissions is the steel industry, which is responsible for 2.8 Gt of CO2 annual emissions and accounts for a quarter of industrial CO2 emissions. A significant and valuable by-product of steelmaking is coke oven gas (COG), which is produced from coke-making via high-temperature dry distillation of coal in the absence of oxygen. COG typically consists of 55-60 vol% hydrogen (H2), 23-27 vol% methane (CH4), 5-8 vol% carbon monoxide (CO) and impurities such as H2S and tars [1]. Every ton of coke produced yields 300-360 m3 of COG and an estimated 650 Mt of COG is produced in the steelmaking industry worldwide each year, with up to 50 % re-utilised within steelmaking [2]. However, the rest is flared, contributing to carbon emissions and wasting valuable and useful gases.

In this paper, co-electrolysis of simulated COG (CH4/H2 30/70 vol%) with carbon dioxide using a commercially available solid oxide fuel cell (SOFC) was investigated at 750 °C. The electrochemical performance of an anode supported button cell was characterised using open circuit potential measurements, current-voltage curves and electrochemical impedance spectroscopy. The product gas composition was analysed using quadrupole mass spectrometry (QMS).

The QMS measurements show that CH4 was initially converted at the open circuit potential into synthesis gas (H2/CO) via dry reforming of CH4 and the reverse water gas shift reaction, yielding a gas mixture composed of 39.2 vol% H2, 38.3 vol% CO2, 22 vol% CO and low levels of unconverted CH4 (see figure). Increasing the voltage from the OCP to 1.4 V caused the CO2 to decrease and the CO to increase, yielding a gas mixture consisting of 42.6 vol% H2, 26.7 vol% CO2 and 29.5 vol% COincreased the synthesis gas content (H2 + CO) of the output gases by 44% relative to the OCP, with the levels of H2 produced increasing by 9% over this voltage range. In addition, the level of unconverted CH4 increased slightly, indicating that the CO2 conversion processes were competing with catalytic dry reforming of CH4. Possible reaction mechanisms are discussed. The I-V and EIS measurements established that cell electrical performance was increased by addition of CO2 through dry reforming and also improved diffusion of the fuel gases through the anode. The effects of COG fuel variability have also been investigated and show that increasing the CH4/H2 ratio from 10/90 vol% to 40/60 vol% of the COG increased the syngas production by 92%. The I-V curve and EIS measurements show that increasing the CH4/H2 ratio decreased the electrical performance of the cell due to prevalent carbon deposition. This work presents a method of efficiently utilising CO2 and suggests SOFC co-electrolysis of COG is an effective way to increase the usefulness and value of this waste gas stream.

References:

[1] Czachor M, Laycock C, Carr S, Maddy J, Lloyd G, Guwy A,. Co-electrolysis of simulated coke oven gas using solid oxide electrolysis technology. Energy Conversion and Management, 2020;225:113455. 10.1016/j.enconman.2020.113455

[2] Gao R, Zhang C, Kwak G, Lee Y, Chang S. Techno-economic evaluation of methanol production using by-product gases from iron and steel works. Energy Conversion and Management, 2020;213:112819. 10.1016/j.enconman.2020.112819

Figure. The effect of increasing the voltage on the composition of the anode output gases of an anode-supported cell under a 2:2 fuel/CO2 ratio. The fuel composition was 30/70 vol% CH4/H2. The cell temperature was 750 °C.

Figure 1