Many nations are moving away from fossil fuels in order to reduce their greenhouse gas emissions. The problem with doing so is that the replacement sources of energy ― solar and wind ― are intermittent. They cannot supply a reliable base load. Nor can the energy that they supply be ramped up and down to meet daily and hourly changes in demand.
This is a dilemma.
Nuclear power does provide a reliable base load, but that technology has its own problems. Therefore, it is likely that many governments will decide to build (or restart) coal-fired power plants, thus defeating the original goal of reducing greenhouse gas emissions. Therefore, there is considerable interest in finding ways of using coal without increasing said emissions.
Dirty Coal
The phrase ‘dirty coal’ seems to have become one word. Yet all fossil fuels emit carbon dioxide (CO₂) and other greenhouse gases. So why does coal have such a bad reputation?
There are two possible explanations. The first is that coal, being a solid, it creates behind ash and slag that has to be disposed of. This is an environmental challenge, but it is a manageable challenge. The second reason for coal’s bad reputation is that it contains a higher carbon to hydrogen ratio than other fuels, particularly natural gas — which is primarily methane (CH4 has four hydrogen atoms for each carbon atom). Therefore, for a given energy output, coal emits more carbon dioxide (CO₂) than other fuels.
The following are representative numbers for various types of fossil fuel. (The specific values vary considerably according to the type of fuel used.)
Coal: 205 lb CO₂/MMBtu
Oil: 170 lbs CO₂/MMBtu
Natural Gas: 117 lbs CO₂/MMBtu
Therefore, coal emits 75% more CO₂ per unit of heat delivered than natural gas, and 25% more than oil. This is why it is ‘dirty’.
Oxy-Combustion
One response to this problem could be to burn coal using an oxy-combustion process. The same amount of CO₂ is created, and it still has to be captured and sequestered. But, once the water vapor is condensed, the flue gas is almost pure CO₂. Hence it is much easier and more economical to dispose of the CO₂ using Carbon Capture and Sequestration (CC&S) technology.
The sketch illustrates how this process would work.
Air is fed into an Air Separation Unit ― the nitrogen produced is a commercial product. The oxygen is fed to a boiler.
The fuel for the boiler can be coal (or natural gas). The heat of combustion spins a turbine which delivers electrical power to the grid.
The flue gas from the boiler contains just CO₂ and water vapor. It is cooled and sent to a condenser which separates the water.
Part of the CO₂ stream is recycled to control the combustion properties of the flame at the boiler.
The exported high purity CO₂ stream can be sent directly to a sequestration site ― probably an abandoned gas well. Alternatively the CO₂ can be converted to solid carbonate.
This process does not reduce the amount of CO₂ produced by burning the coal, but it make the capture and sequestration of that gas much simpler.
Economics
Oxy-combustion is costly. There is a substantial capital cost. In particular, the Air Separation Unit can cost between 50-100% of the power plant itself. (The process can be retrofitted to existing coal power plants.) There are also the capital costs to do with disposing of CO₂, either as a gas or as solid carbonate. (These costs apply to any technology to do with greenhouse gas reduction so they need not be associated with the oxy-combustion process specifically.)
There are also increased operational costs as compared to a conventional power plant. For example, the Air Separation Unit may take up to 15-30% of the power plant’s output, and the extra equipment will increase maintenance requirements.
Some of these costs may be offset through sales of the produced nitrogen. Power plants currently discharge nitrogen to the atmosphere; it is not economic to consider its recovery. The oxy-combustion process adds a new revenue stream. Nitrogen has a wide price range, but $0.50/m3 can be used as a starting point. (If the nitrogen is liquified ― adding yet more capital and operational costs ― its price is considerably higher.)
Other ‘Clean Coal’ Technologies
Oxy-combustion is just one technology that may allow us to develop a ‘clean coal’ option. There are other options ― pre- and post-combustion CC&S come to mind. But, regardless of which option is chosen, the cost of electricity will rise.
Conclusions
As nations shift toward renewable energy sources, particularly wind and solar, they will realise that they also need to build supplemental, 24/7 power plants that can operate when the sun doesn’t shine or the wind ceases to blow.
Many of those power plants will burn coal.
Coal emits more CO₂ per unit of energy delivered than other sources.
Hence we lose many of the benefits of moving toward renewables in the first place.
‘Clean coal’ technologies can help overcome this dilemma.
However, and this is true of all renewable energy options, there is a substantial financial cost ― there are no free options.
