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COAL GASIFICATION AND ITS APPLICATIONS
By DAVID A BELL BRIAN F TOWLER MAOHONG FAN
William Andrew
Copyright © 2011 Elsevier Inc.
All right reserved.
ISBN: 978-1-4377-7851-9
Chapter One
The Nature of Coal
Contents
The Geologic Origin of Coal 1 Coal Analysis and Classification 2 Coal Rank 4 Ash Thermal Properties 5 Coal as a Porous Material 9 Spontaneous Combustion 10 Reserves, Resources, and Production 11 References 15
THE GEOLOGIC ORIGIN OF COAL
Coal is fossilized peat. A peat bog is a marsh with lush vegetation. Plant matter dies and falls into the water, where partial decomposition occurs. Aerobic bacteria deplete the water of oxygen, and bacterial metabolic products inhibit further decomposition by anaerobic bacteria. Plant matter accumulates on the marsh bottom faster than it decomposes, and, over a period of many years, a layer of peat forms. The peat that became today's coal was laid down millions of years ago.
Buried peat is converted to coal when high pressure and elevated temperature is applied to the buried layer. This process is known as coalification. The physical and chemical structure of the coal changes over time. As shown in Figure 1.1, the youngest (least converted) coal is known as lignite, which can be further converted to sub-bituminous coal, bituminous coal, and finally anthracite. These coal types strongly influence the properties and use of coal, and will be discussed further.
Petrography is the visual inspection of a rock sample to determine the mineral types in the sample. When applied to coal, the different coal types are known as macerals. Table 1.1 lists coal macerals, and shows how they are derived from plant material.
COAL ANALYSIS AND CLASSIFICATION
Coal is used primarily as a fuel, so its most important property is its heat of combustion. Gross calorific value, also known as higher heating value (HHV), is determined by measuring the heat released when coal is burned in a constant-volume calorimeter, with an intitial oxygen pressure of 2 to 4 MPA, and when the combustion products are cooled to a final temperature between 20 and 35°C (ASTM D 5865-04). The tests mentioned in this book are primarily based on the American Society for Testing and Materials (ASTM) specifications. Coal is a variable, widely distributed and widely used material so a wide range of standard tests have been developed by a variety of individuals and organizations.
Coal is a porous medium, and these pores, especially in low rank coals, can contain substantial quantities of water even though the coal appears to be dry. The water is either adsorbed onto hydrophilic surface sites or held in pores by capillary forces. When this moist coal is burned or gasified, a substantial fraction of the combustion heat is required to vaporize water. Since the final temperature in the gross calorific value test is 20 to 35°C, most of the water is condensed, thereby recovering the heat of vaporization.
Water in the HHV test is primarily a non-combustible diluent. For example, a Wyoming Powder River Basin coal typically has an HHV of 19.8 MJ/kg (8500 Btu/lb) and a 28% moisture level. One can then calculate an HHV value for the coal if it is dried:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] Eqn. 1.1
If coal is burned or gasified near atmospheric pressure, then the heat of condensation for the water may not be recovered. For example, in a coal-fired power plant, the water contained in the coal may go up the stack as steam. In other situations, the heat of condensation is recovered, but the value of this heat is relatively low because of its temperature. In these cases, a better estimate of coal heat of combustion is the net calorific value, also known as Lower Heating Value (LHV), which assumes that vaporized water remains as steam and that the heat of condensation is not recovered. Water in the coal reduces its heating value by its heat of vaporization, 2.395 MJ/kg water (1055 Btu/lb water). Again, for a typical PRB coal:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] Eqn. 1.2
Proximate Analysis (ASTM D 3172-89) involves a series of tests that heat and burn coal. Moisture is measured (ASTM D 3173-03) by determining the weight loss after coal is dried at 104 to 110 °C. Volatiles are then measured (ASTM D 3175-02) by determining additional weight loss when coal is pyrolyzed at 950 °C. Ash is determined (ASTM D 3174-04) by the weight of inorganic materials remaining after coal is burned. Fixed carbon is the fraction of coal that is not moisture, volatiles, or ash. Fixed carbon, which is mostly carbon but can contain other elements represents the combustible portion of the coal char that remains after the volatiles have been removed.
Proximate analysis results are sometimes reported on a dry mineral matter-free basis. Mineral matter is calculated using the following equation:
Mm = 1:08A + 0:55S Eqn. 1.3
Where: Mm = percent mineral matter
A = percent ash
S = percent sulfur (ASTM D 3177 or D 4239)
The 1.08 factor presumes that minerals in the coal are hydrated. This water of hydration is lost when the coal is burned. The 0.55 factor assumes that sulfur is present as pyrites, which in many areas are converted to the corresponding oxides during combustion.
Ultimate analysis (ASTM D 3176) describes coal in terms of its elemental composition. For a dried coal, weight percentages of carbon, hydrogen, nitrogen, sulfur, and ash are measured. The remainder of the coal sample is assumed to be oxygen.
COAL RANK
In the coalification process, the coal rank increases from lignite to anthracite, as shown in Figure 1.1. Coal rank is useful in the market, because it is a quick and convenient way to describe coal without a detailed analysis sheet. A more detailed description of coal rank is shown in Tables 1.2 and 1.3.
Bituminous and sub-bitumous coals are the primary commercial coals. A relatively small amount of anthracite is available. In the USA, anthracites are produced only in northeastern Pennsylvania. Lignites are abundant. But the economics of hauling a low-grade fuel long distances are unfavorable; so most lignite is consumed close to where it is mined.
Peat is also mined and generally used close to where it is mined. Peat may be either considered old biomass or very young coal. In nations that regulate greenhouse gas emissions, the difference between the two is more than mere semantics. Carbon dioxide emissions from biomass combustion are not considered a contributor to global warming, because these emissions are offset by carbon dioxide uptake by growing biomass. On the other hand, the same emissions from fossil fuels, are restricted. Emissions from peat combustion are a regulatory gray area.
Some coal, particularly bituminous coal, has the tendency to cake. With increasing temperature, coal particles simultaneously pyrolize and partially melt, causing the coal particles to stick to one another. Some gasification reactors, especially moving bed and fluidized bed gasifiers, are limited to processing coal that does not cake.
ASH THERMAL PROPERTIES
The melting temperatures of coal ash impose temperature limits for coal gasification. Fluidized bed gasifiers and dry-bottom moving bed gasifiers, such as the Lurgi gasifier, require free-flowing ash. The maximum operating temperature for these gasifiers is the initial deformation temperature. When the temperature rises above the initial deformation temperature the ash becomes sticky. Fluidized bed gasifiers often run near the initial deformation temperature to maximize carbon conversion.
Entrained flow gasifiers and slagging moving bed gasifiers such as the BGL gasifier require a fluid slag, so they must operate at a sufficiently high temperature to completely melt the ash. Operation at significantly higher temperatures increases oxygen consumption.
Ash is a complex mixture of minerals, which will cause the coal ash to melt over a temperature range rather than at a fixed temperature. Temperatures in this range are specified by ASTM D-1857-04. A coal ash cone, 19 mm high and with an equilateral triangle base 6.4 mm on each side, is placed in an oven. Temperatures are reported for reducing or oxidizing gas environments. The initial deformation temperature (IDT) occurs when rounding of the cone tip first occurs. The softening temperature (ST) occurs when the cone has fused to produce a lump which has a height equal to its base. The hemispherical temperature (HT) occurs when the lump height is half the length of its base. The fluid temperature occurs when the fused mass has spread out in a nearly flat layer with a maximum height of 1.6 mm.
A number of researchers have attempted to correlate ash thermal properties with ash composition. The most extensive effort was by Seggiani and Pannocchia, who correlated the behavior of 433 ash samples, based on nine elemental concentrations.
Note that mineral elemental compositions are reported as if the mineral sample were a blend of simple metal oxides. For example, the fraction of aluminum in a sample is typically reported as the equivalent weight percent of Al2O3. Seggiani and Pannocchia's correlations are based on mole percents, rather than weight percents, on a normalized, SO3-free basis.
The correlation for initial deformation temperature is given as:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] Eqn. 1.4
The correlation for softening temperature is given as:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] Eqn. 1.4
The correlation for softening temperature is given as:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] Eqn. 1.5
The correlation for hemispherical temperature is given as:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] Eqn. 1.6
The correlation for fluid temperature is given as:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] Eqn. 1.7
The temperature of critical viscosity, Tcv, is not part of the ASTM D1857 test but it is important for slagging gasifiers because it marks the transition of slag from a difficult-to-handle Bingham plastic, below Tcv, to a more easily handled Newtonian fluid, above Tcv. The correlation for temperature of critical viscosity is given as:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] Eqn. 1.8
Seggiani and Pannocchia report standard deviations for their correlations to be 70 to 88°C. Table 1.4 compares experimental results for four American coals from Baxter to the temperatures predicted by these correlations. The predicted results are very close to the experimental results for the lignite and the sub-bituminous coals. The exception is the predicted temperatures are substantially higher than the experimental values for the bituminous coals.
Inorganic additives have been added to coal gasifiers to modify ash thermal properties. For example; alkaline materials such as sodium, potassium and calcium compounds tend to lower ash melting temperatures. These can be added to an entrained flow gasifier to lower slag viscosity. Care must be taken with refractrory-lined gasifiers, because these compounds may attack the refractory. The opposite approach was taken by van Dyk and Waanders. They sought to increase the ash fusion temperature (ISO 540 and 1195E) to allow higher temperature operation in a Lurgi gasifier. Tests with Al2O3, TiO2, and SiO2 showed that Al2O3 was most effective. Addition of 6 weight % Al2O3 boosted the ash fusion temperature of a mixture of South African coals from 1,340 °C to greater than 1,600 °C.
(Continues...)
Excerpted from COAL GASIFICATION AND ITS APPLICATIONS by DAVID A BELL BRIAN F TOWLER MAOHONG FAN Copyright © 2011 by Elsevier Inc.. Excerpted by permission of William Andrew. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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