Coal

Coal is the most common source for electricity production all around the world. Coal provides 25% of global primary energy needs and generates 40% of the world's electricity [17].The consumption of coal is still increasing, especially in China, and it is expected that it will play a major role as an energy source the next decades. The remaining resources of coal around the world is proved to be significant larger than for oil and gas. The ratio of reserves to current production (R/P ratio) is 147, which means that with today's coal consumption the world's reserves will last 147 years. Corresponding numbers for oil and gas are respectively 40 and 63 [5]. These numbers varies from different sources. Gasification estimates with an R/P ratio for coal as 216 [6]. Anyhow this demonstrates that coal will be an important part of the energy market in the upcoming years.

Coal fired power plants are one of the largest source of CO2 world wide. A reduction of coal consumption in the nearest future seems unrealistic. CO2 capture from coal fired power plants will probably become one of the main sources to reduce CO2 emissions.

One of the actual technologies of CO2 capture from coal fired power plants is to gasify the coal and thereafter extract the CO2 before the power cycle. This gasifying process will be studied closer in the following chapters.

2.2.1.1 Composition

The composition of coal is complex. The resources are distributed all over the world and the formation process of coal differs and depends on location. Because of the variations in properties it is hard to make one definition of coal. The varying compositions cause different properties which affect the performance for instance in a power plant. The main components in coals are carbon, hydrogen, oxygen, nitrogen and sulfur. The amount of the different substances decides the heating value of the fuel. Table 2.3 shows the composition and lower heating value of several types of coal from different regions in the world.

Table 2.3. Variation of coal composition [6].

Coal Composition LHV

Country, region Class C H O N S MJ/kg Germany, Ruhr Anthracite 91.8 3.6 2.5 1.4 0.7 36.2 Australia, typical Bituminous 81.3 5.8 10.0 2.0 0.9 33.8 India, typical Bituminous 75.5 6.4 15.2 1.5 1.4 32.1 China, Datung Bituminous 84.4 4.4 9.5 0.9 0.8 33.4 South Africa, typical Bituminous 83.8 4.8 8.4 2.0 1.0 34.0 Poland, typical Bituminous 82.8 5.1 10.1 1.4 0.6 36.1 USA, Illinois Bituminous 78.4 5.4 9.9 1.4 4.9 33.7 USA, Montana Sub-bitiminous 76.4 5.6 14.9 1.7 1.4 31.8 USA, North Dakota Lignite/browncoal 71.0 4.3 23.2 1.1 0.4 26.7 Germany, Rhein Lignite/browncoal 67.5 5.0 26.5 0.5 0.5 26.2

The composition data of coal can be presented in different ways. There are four classifications for coal description [12]. Those are as-received (ar), moisture-free (mf), free (af) and ash-and-moisture-free (maf) basis. Table 2.3 shows the composition on a maf basis. That means that the moist and ash is not included in the composition and heating value calculations.

2.2.1.2 Proximate analysis

This analysis method helps to determine the composition on a macro level. That means to decide the fractions of moisture, volatile matter, ash and fixed carbon. Moisture is found both on the surface and inside the coal particles. The amount of moisture can vary from a few percent to 60-70% dependent on the coal type. The moisture is determined by drying the coal for 1 hour under standard coal drying conditions at 104-110˚C. [6] Volatile matter is a

mixture of many components. The most common are hydrocarbon gases, lighter oil, tar, hydrogen and water. To determine the volatile matter the coal is heated for 7 minutes to about 950˚C. The rest, which now can be completely combusted, is the fixed carbon. The amount of fixed carbon can be determined by measure the mass loss. When coal is classified it is ranked after the fraction of fixed carbon. Table 2.4 shows the numerical values for the most common coal classifications. Anthracite has the highest fixed carbon value and therefore the highest heating value. Anthracite is thus classified as a high ranked coal while Lignite is in the category of low ranked coal.

Table 2.4. Classification of coal [6].

Anthracite <8 >92 36-37

Bituminous 8-22 78-92 32-36

Sub-bituminous 22-27 73-78 28-32 Lignite/browncoal 27-35 65-73 26-28

2.2.1.3 Ultimate analysis

The ultimate analysis gives the percentages of the elements in coal, mainly carbon, hydrogen, oxygen, nitrogen and sulfur. Table 2.3 shows a typical ultimate analysis of different coals on mass basis. The browncoal from Rhein, Germany, contains for example 67.5% carbon on mass basis.

There are also traces of a lot of other minerals in coal. Actually a substantially part of the periodic table can be shown to be present in coals [6]. For the case of simplicity these components will be neglected in the calculation in this report. Despite their relatively low fractions, they will play a role connected to gasification and power production. A typical issue is damage on machinery which requires removal of some of the substances.

2.2.1.4 Conversion to gas

The process of converting coal to a useable gas is complex and passes different stages. The main stages are drying, pyrolysis and gasification. In a normal combustion process the

combustible matter goes through these three stages before the final combustion which releases heat. In this case, the issue is to get the intermediate matter between gasification and final combustion and use that gas in a gas turbine. This is called synthetic gas or just syngas and it is mainly a mixture of carbon monoxide (CO) and hydrogen (H2).

Figure 2.15. Transforming coal to syngas.

Before the final gasification stage the coal goes through the drying and the pyrolysis

processes. There will be given a short explanation of these stages before the final gasification is discussed in detail.

In theory, the stages appear chronological corresponding to Figure 2.15. One stage does not start until the previous stage is completely finished. In practice there is not a total separation of the stages and they are overlapped by each other. Figure 2.15 shows an overview of the different stages in a coal conversion process. In the modeling work presented later the whole conversion process will occur in one unit and the operation is described only as gasification.

In fact there will also be implemented a shift reactor to maximize hydrogen production as well.

Drying and pyrolysis

The drying and pyrolysis are not separate units, but are included as steps in the gasification in the gasifier. Combustion of coal requires dry coal. A gasifier can use additional steam supply and/or the humidity in the coal can operate as a reactant in the gasification reaction [10].

Drying is the easiest part of the process. The principle is simply to heat up the coal particles.

Even though drying is not technological advanced the speed of the heating has influence on the next steps. With a slow temperature rise the subsequent steps are more or less separated. A fast rise in temperature leads to pyrolysis and gasification in one simultaneous process. This is shown in Figure 2.16.

Figure 2.16: Influence of heating rate [17].

Pyrolysis of coal is also known as devolatilization or carbonization. It refers to the

decomposition of organic matter by heat in the absence of air [18].The hydrogen rich volatile matter is removed and a carbon-rich solid part is left behind. From pyrolysis one can go further and produce liquid or gasses. Anyway pyrolysis is considered as the first conversion step in the process. The pyrolysis products depend on the raw material. For coal the

composition is complicated and varying. Coal contains an inhomogeneous mixture of different organic and inorganic materials which varies from type to type. The complex composition of coal makes the pyrolysis hard to describe. There will always be an approximation instead of an exact answer in determining the pyrolysis process [18].

The knowledge concerning the pyrolysis process is not fully developed and further details will not be discussed in this report. The gasification process will be study in details in the next part.