Welcome to this series of newsletters on the theme Net Zero by 2050: Technology for a Changing Climate.
In response to the climate crisis, many business and industry leaders have committed their organizations to a ‘Net Zero’ program. By this they mean that their organization will not be emitting greenhouse gases by a specified date — often the year 2050. The purpose of these letters and posts is to help these leaders by providing realistic and practical information to do with net zero technologies. The emphasis is on the word ‘realistic’.
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At the conclusion of the recent COP26 conference (November 2021 in Glasgow, Scotland) many people were disappointed, even distressed, regarding the language to do with the use of coal as a fuel. The final statement of the conference used the phrase “phase down of coal” rather than “phase out of coal”.
The British president of the COP26 climate summit, Alok Sharma, said he wished he had been able to preserve the originally agreed language on phasing out coal power.
The dilemma can be stated as follows:
Coal is perceived as being a “dirty” fuel because it has a higher carbon/hydrogen ratio than natural gas and so produces more CO2 per unit of energy delivered than gas. (Being a solid, it also creates ash when burned, another reason for it being considered dirty.)
Coal is a crucial source of energy for many nations. A rapid phase out of its use would leak to severe economic setbacks.
A theme of the posts at this site is that there may be technical solutions, or at least partial solutions, to some of the dilemmas that we face. Sometimes, reframing an issue in a different way can help generate new ways of thinking and can generate helpful responses. So, rather than simply writing off coal as being “dirty”, we could define the objections to its use, and then see if there are any technical options that may allow for its continued use without the associated emissions. In this post we illustrate this point of view by showing how oxy-fuel technology can provide a useful way of burning coal without most of the associated CO2 emissions.
Figure 1 shows a schematic for the combustion of any hydrocarbon fuel. The fuel (coal, oil or gas) is a chemical containing carbon (C) and hydrogen (H) and small amounts of other materials. The air is a mixture of nitrogen (N2) and oxygen (O2) in a ratio of 4 to 1. When a fuel is burned in the presence of air its carbon content creates carbon dioxide (CO2) — a greenhouse gas; its hydrogen creates water (H2O) vapor.
The fuel/air mixture is burned in a boiler or some other type of combustion device. Steam or hot gas is generated, the steam spins a turbine and electricity is delivered to the customer. The exhaust/flue gas from the boiler consists of a large amount of nitrogen (which has taken a free ride through the process), some unburned oxygen, water vapor (created from the burning of the hydrogen in the fuel), carbon dioxide (CO2), along with a variety of other compounds in small quantities.
When the fuel being used is natural gas the CO2 concentration in the flue gas is about 5%. When the fuel is coal then that CO2 concentration is about 8%.
It is this small difference between 5% and 8% that makes coal “dirty”, but natural gas “clean”.
One solution to this dilemma is to implement Carbon Capture & Sequestration (CC&S) technology, i.e., to remove the CO2 from the flue gas, compress it and permanently sequester it underground. This process is illustrated in Figure 2.
Conventional Combustion with CC&S
CC&S systems are expensive to install and operate because the CO2 is at such a low concentration. Moreover, CC&S technology has not yet been fully commercialized.
We started this post by saying that insights as to potential solutions may be obtained by looking at problems from a different perspective. In this case we see that the core difficulty is not that the coal is “dirty” but that the concentration of CO2 in the flue gas is so low that CC&S technology is not economical. The reason for the low concentration of CO2 is that that large quantities of unreacted nitrogen are present. In other words:
The problem is not “dirty” coal — the problem is nitrogen.
If nitrogen is the core problem, then the challenge changes from finding a “clean” fuel to replace “dirty” coal. It becomes,
We need a means of burning coal without having nitrogen present in the combustion process.
If we can remove the nitrogen from the combustion process, then the flue gas will consist only of CO2, water vapor and small amounts of other compounds. The water vapor can be condensed and disposed of safely. The CO2 can be sequestered underground without any further treatment. Hence, there is no need for a Carbon Capture facility. (The need to dispose of the solid ash remains.)
The process for burning the fuel without nitrogen being present is known as oxy-combustion; it is illustrated in Figure 3.
Air is supplied to an Air Separation Unit (ASU). This is a technology that is very well understood and that is already widely used for many commercial processes. The nitrogen can be discarded into the atmosphere or sold as a commercial product. The oxygen is mixed with the hydrocarbon fuel (coal, oil or gas) and burned. The flue gas consists just of the combustion products carbon dioxide (CO2) and water vapor. The flue gas is passed through a condenser/cooler. The water effluent is treated and discarded; the CO2 is sequestered underground. (Some of the CO2 is recycled in order to maintain controlled combustion.) There is no need for the carbon capture part of CC&S.
This post to do with “dirty” coal leads to the following four conclusions.
The use of oxy-combustion and carbon sequestration will allow for the continued use of coal (or any other hydrocarbon fuel) without the creation of greenhouse gases. The economics favor the current process (Figure 1). However, if the addition of a CC&S system is required (Figure 2), then capital and operating costs increase substantially. In such a situation the economics of oxy-combustion (Figure 3) become competitive.
Nevertheless, responses to climate change generally cost money. There are no free solutions.
It is vital to structure a problem properly. If we say that coal is “dirty” then the only solution is to phase out its use, which is what many of the activists as COP26 wanted. However, if we say that we need to burn coal without emitting greenhouse gases, then other responses, such as oxy-combustion, may become viable. By defining the problem in a different way, different responses may present themselves.
Finally, it is not enough simply to say that, “There is a problem and ‘they’ (or ‘we’) need to do something about it”. Safety professionals sometimes use the term, “If you see it, you own it”. The same philosophy applies to climate change issues. We have a responsibility to come up with viable responses.