To realize a carbon-neutral society, efforts are underway worldwide to develop CO2 resource conversion technologies that convert CO2 into valuable gas resources, as well as technologies to realize carbon footprint displays shown in terms of CO2 equivalents of greenhouse gas emissions throughout the entire life cycles of products and services, from raw material procurement to disposal and recycling.
CO2 conversion technologies include power-to-chemicals (P2C) technologies, which use electricity from renewable energy sources to convert CO2 into chemicals, and methanation technologies for synthesizing methane, the main component of natural gas, from CO2 and H2. These technologies convert CO2 into resources by decomposing it through electrochemical reactions or by reacting it with other gases. To achieve highly efficient conversion of CO2 into resources, it is important to control optimal gas reaction conditions while performing real-time monitoring of the composition and concentration of the reacting gases. To achieve reliable carbon footprint, it is also necessary to measure and accurately visualize the concentrations of each greenhouse gas.
In actual environments, however, gas reaction processes generate byproduct gases and water vapor in addition to CO2 and generated resource gases, resulting in a mixture of several types of gases. Efficiently converting CO2 into a resource and accurately determining greenhouse gas concentrations requires accurate real-time measurements of the components and concentrations in the gas mixture (Figure 1).
An analytical instrument called a gas chromatograph is currently used to measure gas concentrations, but the time required for those measurements makes real-time monitoring difficult. Also, the large size of those devices requires the installation of complex systems in facilities that will perform P2C or methanation. Gas sensors are currently being developed worldwide for increased speed and reduced size. Of the three main methods for gas sensing—metal oxide semiconductors (*6), catalytic combustion (*7), and thermal conductivity—the thermal conductivity type is most effective from the standpoint of tolerance. The gases generated by CO2 conversion technologies often contain gases with a strong poisoning effect (*8) such as CO, and metal oxide semiconductor and catalytic combustion methods have the issue of these poisoning causing changes in the gas reaction material (*9). By contrast, thermal conductivity-based methods measure gas concentrations based on the fact that different gases have different heat transfer properties (thermal conductivity). These methods are robust against gases with a strong poisoning effect because they do not employ a gas reaction material. However, when three or more gases are present, it has not been possible to determine which gas is transferring heat, meaning that concentrations could not be calculated.