21. 3 Gasification Methods and Technologies 8

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May 4, 2010 Ch 21 Coal Gasfication_KBR_mods_Rev 4.0_April_2--2010


Coal Gasification

Ravi K. Agrawal, KBR


Table of Contents

21.1 Introduction 2

21.2 Theory 6

21.3 Gasification Methods and Technologies 8

21.3.1 Moving Bed Gasifier 10

21.3.2 Fluidized Bed Gasifiers 14

21.3.3 Entrained Flow Gasifiers 19

21.4 Gasification Island Design Issues and Cost Impacts 29

21.5 Applications of Coal Gasification 37

21.5.1 Power Generation 37

21.5.2 Liquid Fuels 42

21.5.3 Synthetic Natural Gas (SNG) 45

21.5.4 Hydrogen and Ammonia 47

21.6 Outlook 49

21.6.1 Global Energy Consumption Pattern 49

21.6.2 Identifying Growth Markets 51

21.6.3 Energy to Fuel the Growth Markets 53

21.6.4 Outlook for Coal-to-Liquids 54

21.6.5 Outlook for Coal-to-Synthetic Natural Gas (SNG) 60

21.6.6 Outlook for Coal-to-Electricity 61

21.6.7 Outlook for Coal-to-Chemicals 64

21.7 References 68

Chapter 21. Coal Gasification

21.1 Introduction

Fossil fuels supply almost all of the world’s energy and feedstock demand. Among the fossil fuels, coal is the oldest, most abundant, and widely available form of fossil fuel. Coal constitutes over 75 percent of the world’s fossil fuel. Since the beginning of the industrial revolution, coal has been the backbone of the world energy system. Before the discovery of oil and gas, coal was used to generate town gas for lighting and heating purposes. During World War II, coal was extensively used in Germany to produce oil substitutes. After World War II, oil replaced coal as the major source of energy, and the interest in coal gasification remained dormant until the recent rise in energy prices. Since 2000 several gasification plants have become operational, most of these plants are for the production of chemicals and only a handful of plants for power generation.

Coal is among the cheapest fuel available. Figure 21-1 compares the historical and projected price of major energy sources in the United States [1]. Projections of the Energy Information Administration (EIA) indicates that coal is likely to remain the cheapest fuel in the foreseeable future and will likely become cheaper as the prices of other preferred energy sources rise. Based on the energy content in terms of price per BTU, electricity commands the highest premium, followed by oil and gas. This price differential is a key driver that determines the interest in coal and its conversion into other energy substitutes.

Figure 21-1. United States Energy Prices and Projections (2008 dollars per million BTU) [1].

Since electricity trades at the highest premium, the cheapest source of energy, coal, has been extensively used to generate electricity. About 60 percent of global electric power is generated from coal, and about two thirds of coal produced is used for power generation. Therefore, generation of electricity and the consumption of coal are intricately linked. Unfortunately, coal has the largest carbon footprint when compared to conventional fuels derived from oil and natural gas. Historically, most coal based power plants are considered to be “dirty” as only a few had controls to lower the emissions of SO2, NOx and particulates, and even to date virtually none have controls on mercury and CO2 emissions. Tightening of environmental regulations in countries like United States, Europe and Japan has prompted installation of emission control devices. Table 21-1 summarizes the current U.S. new source performance standards (NSPS) on SO2, NOx, particulates and Hg. Also shown in Table 21-1 are emissions from a typical coal fired power generation unit and an Integrated Gasification Combined Cycle (IGCC) unit.

Table 21-1. Comparison of U.S. NSPS emission standards versus emissions from

an IGCC Unit and coal fired power plant [2,3].

The control of contaminants and pollutants from an IGCC unit is generally dictated by the more stringent downstream requirements, and not necessarily the environmental regulations, especially those of the gas turbine and/or the downstream catalyst requirements. As a result of these requirements, the emissions from IGCC units are significantly lower than the current NSPS requirements. For IGCC applications, additional power can be generated by diluting the syngas with hot water, steam or nitrogen from the Air Separation Unit (ASU) unit; while simultaneously reducing NOx. Therefore the design requirements for IGCC ensure that the emissions from the gasification plants are significantly lower than those from a traditional coal fired steam generation units.

Future regulations on greenhouse gas emissions will have a major impact on traditional coal fired power plants. This will lead to accelerated implementation of coal gasification based power plants, as gasification provides for an efficient method to generate power from coal while minimizing the environmental impact.

Another important application of coal gasification is for the production of chemicals and oil substitutes. Clearly, gasification is one technology that converts “dirty fuels” such as coal, petroleum coke, and other carbonaceous materials, into alternates for cleaner and high value energy sources such as oil and natural gas. This conversion is possible because all fossil fuels come from the same source and have very similar chemical composition. All fossil fuels are made up of carbon and hydrogen, with a small amount of oxygen. Coal is very heterogeneous and varies widely in chemical composition. Carbon is the dominant ingredient of coal and forms the basis for classifying coal. In broad terms, coal is classified into four ranks: anthracite, bituminous, sub-bituminous and lignite. Anthracite and bituminous coals are referred to as high rank coals and have the highest carbon content. Sub-bituminous and lignite are considered low rank coals as they contain significant amount of moisture. Bituminous and sub-bituminous are the most common coals used. Table 21-2 summarizes properties of selected fuels [4].

Table 21-2. Chemical and Fuel Properties of Selected Fuels [4].

Methane is the main ingredient of natural gas and its properties are shown in Table 21-2 to represent natural gas. Table 21-2 shows that coal has similar constituents as oil and natural gas, but also contain undesirable components such as oxygen and inorganic mineral matter (ash). Ash is an oxidized form of inorganic mineral matter that is inherently present in coal. The presence of undesirable components lowers the heating value of the fuel and/or increases material handling requirements. The fuel characteristics defined by the atomic H/C ratio indicates that the normalized hydrogen content of coal is about half that of oil, and only one fifth that of natural gas. This ratio indicates that coal is lower in hydrogen and higher in carbon, when compared to oil or natural gas. Therefore coal can be converted to oil or natural gas by either adding hydrogen or rejecting carbon. Gasifying coal with steam to generate hydrogen and rejecting excess carbon as carbon dioxide can make up the hydrogen deficiency in coal. The excess O/C ratio of coal is adjusted by rejecting the oxygen with hydrogen and carbon. Gasification also helps in removal of inorganic matter from carbon and hydrogen as ash. Syngas produced from gasification is rich in CO and H2, and can be used for the production of electricity, chemicals and liquid fuels. Typical chemicals that can be produced from syngas include: hydrogen, methanol, ammonia, acetic acid, and oxygenates. Liquid fuels such as: naphtha, diesel, ethanol, dimethyl ether, and methyl tertiary butyl ether can also be produced from syngas. Thus coal gasification can be effectively used to provide syngas as a feedstock for a variety of applications.
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