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An introduction to carbon and it many sources

Table 2 Predicted global power generation. US Energy Information Administration. The message from these figures is that coal consumption is likely to rise substantially over the medium term.

  • Many old coal-fired plants commonly found in developing countries operate at an efficiency of only 30 per cent;
  • The key to higher efficiency in such engines is to increase the operating temperature and pressure;
  • Elsewhere capacity may grow, but never sufficiently to reduce coal use.

Coal is a cheap source of electricity and it is available in large quantities in many parts of the world. The Western world has built much of its prosperity on coal and now the developing world is intent on doing the same. If greenhouse gas emissions are to be reduced then this must, at least for the next 20—30 years, take place against a background of increased coal use for power generation.

Alternatives to coal combustion One of the reasons why coal use will not fall is that no viable alternative exists today. The only other comparable source in terms of size and technology is nuclear power; but nuclear power is unlikely to be able to replace even a small part of current coal capacity. A nuclear power station is both cost effective and produces much lower greenhouse gas emissions than any fossil fuel power plant. There are some major environmental objections to increased nuclear generation but perhaps the largest hindrance to a massive growth in nuclear capacity is the availability of uranium to fuel the plants.

While the nuclear fuel industry would almost certainly argue otherwise, it is not clear today that it can support anything more than a modest growth in global capacity IEA 2006 ; Breeze 2007a. So, while existing nuclear plants may be replaced in, for example, the USA and the UK, additional plants are unlikely to be built. Elsewhere capacity may grow, but never sufficiently to reduce coal use. Renewable technologies offer the other major alternative to fossil fuel combustion. Today, however, these technologies are simply not in a position to meet the growing demand for new capacity across the globe.

Growth in wind power capacity, perhaps the best option for the medium term, is already showing signs of being constrained by global manufacturing capacity. Solar power is almost certainly the earth's long-term solution to electricity supply but it will probably be another generation before it can begin to provide the sort of capacity the world needs.

Hydropower might an introduction to carbon and it many sources able to provide significantly more output, particularly in Africa where the infrastructure associated with increased hydro capacity can have other major benefits. Biomass, marine technologies, tidal power: The other important way of constraining growth in coal consumption is by introducing energy efficiency measures. There are simple measures that can lead to major savings but these will mostly take place in the developed world.

The growth in the use of coal will mostly take place in the developing world. Inevitably, therefore, coal use will increase. Facing up to coal The politics of coal have already been alluded to briefly but it is worth emphasizing its significance once again.

Coal is a cheap, widely available, high energy-density fuel. Since the industrial revolution, it has provided the energy that has driven industry in the West. Indeed, coal is arguably the fount of Western prosperity. Today it continues to provide both energy and energy security in many Western countries and particularly the USA where, as noted, it accounts for over 50 per cent of electricity generation.

The recognition of greenhouse warming and the identification of carbon dioxide emissions as a primary cause have led to a reappraisal of fossil fuel use.

Coping with carbon: a near-term strategy to limit carbon dioxide emissions from power stations

As a result international efforts are taking place, under the auspices of the United Nations, to reach a comprehensive agreement to control and eventually reduce greenhouse gas emissions.

Unfortunately, this comes at a time when the economies of two major developing nations, India and China, are growing rapidly. And, like the Western nations before them, they are growing on the back of coal. It is unrealistic to expect either of these nations, or any of the other developing nations that currently rely on coal, to sacrifice their prosperity for the sake of the planet.

Any international agreement will therefore have to take this into account. In practice, this means that while Europe and one hopes, eventually, the USA will aim for drastic cuts in its greenhouse gas emissions, coal use will continue to increase. If, therefore, overall emissions are to be limited, then technological solutions based around coal use will have to be implemented. Fortunately these already exist.

Applied pragmatically, they can do a lot to ameliorate the problems associated with coal combustion. In such a plant coal is burnt in air to generate heat that is used to raise steam in a boiler and the steam is used to drive a steam turbine generator, producing electricity.

The most highly developed of this type of plant, and the one that is of most interest here, is the pulverized coal-fired power plant.

  • Steam beyond this stage is superheated before being pumped to the steam turbine from which it is eventually condensed and returned as water to the boiler;
  • The released carbon dioxide must also be compressed prior to being pumped away for sequestration and this adds to the total energy burden, making an overall efficiency reduction with the two processes of approximately 25 per cent Breeze 2006.

This plant burns coal that has first been reduced in grinders to a fine powder, which can be injected pneumatically into the boiler combustion chamber where it is burnt under carefully controlled conditions in order to minimize the production of nitrogen oxides. Such plants probably account for 90 per cent of current coal-fired generating capacity. Under these conditions, nitrogen oxides are easily produced from the nitrogen in air, so the amount of air and hence oxygen is restricted in order to maintain reducing conditions in this hottest region.

Further air is added in a cooler part of the furnace to complete the combustion reaction.

The heat released during combustion is both radiant and convective. Radiant heat is captured by passing water through piping within the walls of the furnace while convective heat is captured higher up with bundles of water-containing tubes placed in the path of the exhaust gases. The product from the combustion of coal in air is, bar some traces of impurities, carbon dioxide. This may amount to 15 per cent by volume of the exhaust gases from the plant.

The production of this carbon dioxide cannot be avoided in a plant of this type but the environmental performance of the plant may be improved significantly by improving its overall energy-to-electricity conversion efficiency.

  • These are termed post-combustion capture, pre-combustion capture and oxy-fuel combustion;
  • The only other comparable source in terms of size and technology is nuclear power; but nuclear power is unlikely to be able to replace even a small part of current coal capacity;
  • The heat released during combustion is both radiant and convective.

A steam turbine cycle approximates to that of a Carnot cycle engine. The key to higher efficiency in such engines is to increase the operating temperature and pressure. The steam system of most conventional coal-fired plant includes a drum located part way through the system where water is converted to steam.

Steam beyond this stage is superheated before being pumped to the steam turbine from which it is eventually condensed and returned as water to the boiler. The resultant efficiency is approximately 38 per cent. Plants based on this type of steam an introduction to carbon and it many sources are known as subcritical plants. Some of the most modern plants operate with steam under conditions that are above the critical point of water.

In these plants, there is no need for a steam drum within the steam cycle since the phases of water can coexist. Two types of supercritical plant are in use today. The second type, the ultra-supercritical plant, uses yet more extreme conditions. These efficiency gains are extremely important when it comes to environmental performance. Many old coal-fired plants commonly found in developing countries operate at an efficiency of only 30 per cent.

Even in a technically advanced country such as the USA, the average efficiency of the coal-fired fleet is only 33 per cent Science Daily 2007. Increasing the efficiency of a coal-fired plant from 33 to 45 per cent would reduce the amount of carbon dioxide produced for each unit of electricity by 27 per cent. The extreme operating conditions in supercritical and ultra-supercritical coal-fired plants require technically advanced materials.

As these are developed further, even higher performance can be expected. The European Commission-funded AD700 programme aims to achieve an efficiency of 50—55 per cent for an advanced coal-fired plant by 2015.

This appears to put the USA in the shade: Converting the world's coal-burning fleet to high efficiency plant offers one effective short-term strategy for limiting carbon dioxide emissions albeit modestly. This is a measure that can begin to be implemented immediately. And while it will not reduce overall emissions it will slow their increase.

Emission reduction from coal plants will require another set of technologies: If higher efficiency in existing plants represents a short-term strategy to limit power plant carbon dioxide emissions then the zero-emissions plant is the medium-term solution. If we accept the arguments presented here that coal burning will continue well into the middle of this century, it is important that these technologies are brought into service as quickly as possible. In practice, that time scale is likely to be another decade at least.

To date there has been no coal plant scale demonstration of either capture or sequestration of carbon dioxide. Significant problems remain to be solved before either capture or storage can be implemented widely. Nevertheless, the basic technologies are in place and with sufficient investment there is no reason why they cannot be perfected over this time scale. Three different carbon-capture strategies are being developed in parallel today. These are termed post-combustion capture, pre-combustion capture and oxy-fuel combustion.

It is likely that each will have a part to play in the future of coal combustion and all three will be considered briefly here. The carbon dioxide, approximately 15 per cent by volume of the exhaust gases, is mixed primarily with nitrogen, so post-combustion capture involves the separation of these two gases. Separation can be achieved today most easily using a chemical absorption technique involving an aqueous solution of the solvent monoethanolamine MEA.

This is carried out in a plant similar to that used for sulphur dioxide scrubbing. The flue gases from the power plant are passed up a tall tower from the sides of which a solution of MEA is sprayed into its path. With currently available technology, this can result in the capture of 80—95 per cent of the carbon dioxide within the exhaust gases.

The scrubbed flue gases are released into the atmosphere. Meanwhile, the MEA—carbon dioxide-containing solution is collected at the bottom of the tower, pumped to a second reactor and heated to release the carbon dioxide and regenerate the solvent.

This is an energy intensive process that is likely to reduce the overall plant conversion efficiency by 15 per cent. The released carbon dioxide an introduction to carbon and it many sources also be compressed prior to being pumped away for sequestration and this adds to the total energy burden, making an overall efficiency reduction with the two processes of approximately 25 per cent Breeze 2006.

The major advantage of post-combustion technology is that it can, in principle, be retrofitted to existing power plants. The effectiveness of this will depend both on the existing efficiency of the plant and the availability of space for the capture plant. Modern supercritical and ultra-supercritical plants would make good candidates for future post-combustion capture owing to their already high efficiencies. Pre-combustion capture, the second of the capture strategies envisaged for coal combustion plants, takes a different approach.

In this case, the idea is to avoid entirely the need to capture carbon dioxide after combustion by removing all the carbon from the fuel before combustion takes place, using a process of coal gasification. This process involves reacting coal at high temperature with a limited proportion of either air or oxygen and steam. The gasification reaction, which takes place under reducing conditions, produces a mixture of carbon monoxide and hydrogen, a mixture called synthesis gas, syngas for short.

The syngas still contains carbon in the form of carbon monoxide; this is converted in a second process called a shift reaction in which it is reacted again at high temperature with steam. The result of the reaction is a mixture of carbon dioxide and hydrogen. The separation of carbon dioxide from hydrogen can be carried out relatively simply using current pressure swing absorption technology that selectively removes the carbon dioxide, leaving almost pure hydrogen.