Читать книгу Coal-Fired Power Generation Handbook - James Speight G., James G. Speight - Страница 31

Following from the statements in Chapter 1, coal is the biggest single source of energy for electricity production and plays an essential role in the energy mix of many countries, particularly for power generation, but there is an urgent need to use coal efficiently and reduce the environmental footprint of the coal. Thus, the measurement and reporting of efficiency performance and carbon dioxide emissions is a prerequisite to the more sustainable use of coal in power plants.

The direct and indirect utilization of coals for production of energy (and chemicals) is the foundation upon which interest in classifying coal resource is built. However, because of the complex, heterogeneous nature, and the variety of coals used throughout the world, classification is a difficult task. Identification of the most advantageous raw material, whether by quality, cost, availability or a combination of several such factors has always been one of the driving forces behind the development of classification systems. In fact, many of the systems currently in use in the coal industry were derived specifically from a need to identify quality coals for coke making, and in that respect only classify a relatively narrow range of coals. Other systems that have been developed to address the scientific need to understand the origin, constitution and fundamental properties follow the approach that any sound classification will identify all coals for all potential industrial uses (Speight, 2013).

Coal is a combustible dark-brown-to-black organic sedimentary rock that occurs in coal beds or coal seams (Chapter 1) and is composed primarily of carbon with variable amounts of hydrogen, nitrogen, oxygen, and sulfur as well as mineral matter and gases as part of the coal matrix. The types of coal, in increasing order of alteration, are lignite (brown coal), subbituminous, bituminous, and anthracite (Chapter 2).

Coal is the most abundant fossil fuel in the United States, having been used for several centuries, and occurs in several regions (Figure 2.1) (Speight, 2013). Knowledge of the size, distribution, and quality of the coal resources is important for governmental planning; industrial planning and growth; the solution of current and future problems related to air, water, and land degradation; and for meeting the short- to long-term energy needs of the country. Knowledge of resources is also important in planning for the exportation and importation of fuel.

Coal begins as layers of plant matter that has accumulated at the bottom of a body of water after which, through anaerobic metamorphic processes, changes in the chemical and physical properties of the plant remains occurred to create a peat-like solid material. It is believed that with further passing of time, lignite is formed from the peat-like product which is metamorphosed (due to thermal and pressure effects) to lignite. With the further passing of time, lignite increases in maturity to subbituminous coal thence to bituminous coal and finally to anthracite.

There are many compositional differences between the coals mined from the different coal deposits worldwide. The different types of coal are typically classified by rank which depends upon the degree of transformation from the original source (i.e., decayed plants) and is therefore a measure of the age of the coal (Chapter 1) (ASTM D2011). As the process of progressive transformation took place, the properties of the coal changed markedly, leading to the differentiation of coal based on rank (which is often cited incorrectly as carbon content but there are other factors involved in determining coal rank). Nevertheless, changes on properties can cause changes in efficiency of power plant operations.

Figure 2.1 Coal reserves and distribution in the United States (DOE/EIA, 1995).

Coal remains in adequate supply and at current rates of recovery and consumption, the world global coal reserves have been variously estimated to have a reserves/production ratio of at least 155 years. However, as with all estimates of resource longevity, coal longevity is subject to the assumed rate of consumption remaining at the current rate of consumption and, moreover, to technological developments that dictate the rate at which the coal can be mined. Moreover, coal is a fossil fuel and an unclean energy source that will only add to global warming. In fact, the next time electricity is advertised as a clean energy source, consider the means by which the majority of electricity is produced – almost 50% of the electricity generated in the United States is from coal (Speight, 2013, 2020).

However, there are considerations which can impact significantly on efficiency including: (i) moisture content, which influences latent and sensible heat losses, (ii) ash production from the mineral matter, which impacts on heat transfer and auxiliary plant load, (iii) sulfur content, which influences design limits on boiler flue gas discharge temperature, (iv) use of flue gas cleaning technologies, such as selective catalytic reduction (SCR), fabric filtration, flue gas desulfurization (FGD) and carbon dioxide capture, which increase on-site power demand, and (v) use of low-NOx combustion systems, which require excess combustion air and increases unburned carbon.

Thus, a plant designed for high-moisture, high-ash coal, fitted with flue gas desulfurization units and bag filters, and operating with a closed-circuit cooling system, could not be expected to achieve the same efficiency as one without flue gas desulfurization units using high-rank, low-ash, and low-moisture bituminous coal at a coastal site with cold seawater cooling. In most cases, there is little that can be done to mitigate these effects; it is sufficient to recognize that their impact is not necessarily a result of ineffective design or operation, but merely a function of real plant design constraints.

The efficiency of converting coal into electricity is of prime importance since more efficient power plants use less fuel and emit less climate-damaging carbon dioxide. However, with many different methods used to express efficiency and performance, it is often difficult to compare one coal-fired plant with another, even before accounting for any fixed constraints such as coal quality and cooling-water temperature. Guidelines are required that allow the efficiency and emissions of any plant to be reported on a common basis and compared against best practice. Such comparisons start with the classification of coal and, amongst other parameters, allow less efficient plants to be identified and steps taken to improve these plants. Having such information available will allow better monitoring of plant performance and, if necessary, regulate the means by which coal is used for power generation, leading to a more sustainable use of coal.