Gas Turbines + Carbon Capture

Two basic methods are possible:

• Capturing the CO₂ once it has been produced (post-combustion)
• Converting the methane to hydrogen before burning it, and capturing the CO₂ from that chemical conversion process

When trying to capture the hot CO₂ as soon as it is produced, the flue gas from the burnt fossil fuel will be mixed with nitrogen from the air, water vapour and other chemicals like oxides of sulphur and nitrogen and soot. The biggest problem is that the concentration of the CO₂ in the flue gas is fairly low – possibly as little as 4% in a gas-fired plant, or 15% from a coal plant.

The CO₂ concentration from cement and steel production is more like 20% — 30%, so capture or removal of the CO₂ in the emissions from those chemical processes, is much easier.

Currently, the easiest commercial way to remove the CO₂ from the flue gases is to pass it through a solution of either mono-ethanolamine or di-ethanolamine. This can capture 85% — 95% of the CO₂. However, it is expensive and the chemicals used degrade fairly quickly. Other promising methods are becoming commercially available. These include using some material to which the CO₂ would “stick” as it passed, or using some form of membrane which would filter out the CO₂. One group claim to have achieved a 90% removal of the CO₂. More details in  the discussion of DACCS.

Removal of the carbon, before combustion in fossil fuel-burning power plants, can be done particularly when starting with methane. This is still experimental and not yet deployed commercially. Here, the first step is to partially oxidise the fuel with pure oxygen to produce a mixture of carbon monoxide and hydrogen (CO and H₂), which can be burned to generate electricity in a combined-cycle power plant. This is called Integrated Gasification Combined Cycle (IGCC) power generation.

Once any solid particles have been filtered out of the gasified CO and H₂ mixture, the gas is taken through a two-stage water-gas shift reactor which converts the CO to CO₂, leaving a mixture of CO₂, H₂ and water. Given that the CO₂ is fairly concentrated at this stage, the CO₂ can be removed by passing the gas through simpler glycol-based solvents. This leaves almost pure H₂ which is burned in the combined-cycle generator to produce electricity.

The energy requirements of this fairly complex process significantly reduces the energy produced from the methane. Even without any processing, burning a quantity of methane produces 5 times more energy than burning all the hydrogen you could obtain from that same methane. The processing to extract the hydrogen and capture the CO₂ reduces the resulting energy even more. As a result it will not be economically attractive to power companies unless there are serious economic penalties imposed for producing the CO₂. Again some form of membrane to remove the CO₂ might make the process cheaper.

The Oxy-fuel process uses pure oxygen for the combustion process, rather than air, so that the flue gas is mainly water vapour and CO₂. If the flue gas is condensed to liquefy the water, the result is almost pure CO₂.

The problems with this process are that energy is required to produce the pure oxygen, and the combustion in pure oxygen happens at a much higher temperature, so the generating plant has to be designed differently.

In 2017 a UK government report seemed to concentrate more on the economic costs of these methods rather than the environmental benefits.

A version of the IGCC process effectively to extract hydrogen from water, using coal as the starting fuel, also exists. Some “clean coal” power stations using this cycle have been built in the USA. However, the initial ones did not capture the CO₂ at all. Instead, they were concentrating on removing other air-borne pollutants produced when burning coal, such as sulphur and nitrous oxides, mercury and heavy metals. Many of those “clean coal” plants also generated significant water pollution – from the mercury and heavy metals: in climate terms they were not “clean” at all. Ones which would capture the CO₂ have since been planned – more details in Wikipedia.