Table of Contents:
: Chemicals from Coal: A Smart Choice
In the early 20th century, prior to the age of oil and gas, the coke oven industry, as it used to be called, provided ammonia, benzene, toluene, and phenol. DuPont produced a large amount of methanol at its Belle, W.V. coal mine. However, as low-cost natural gas became available around 1950, the unit was shut down. Likewise, during the 1930s and continuing through WWII, development and advances in Fischer- Tropsch technology enabled the production of straight chain hydrocarbons, waxes and fuel from coal. Production of synthesis gas also provided the route to many organic chemicals. From there to about 2000, the primary use of coal was limited to electric power generation.
The relative cost and convenience of crude oil derived fuels resulted in the displacement of coal as the premier energy source. Nonetheless, coal continues to be a staple in the energy diet of many countries, and the current prices of crude oil have triggered renewed interest in coal utilization for the production of chemicals. Table 1 shows coal utilization for the production of electricity for various countries. For some countries, like Poland and South Afr ica, coal accounts for more than 80% of electric power generation. As recently as 2018 in the United States, 28% of the electric power generated is from coal (LEA, 2011; LEA, 2018). As shown in Table 1, not much has changed between 2011 and 2018 regarding the utilization of coal for electricity generation.
From a carbon management perspective, the conversion of coal to chemicals is a far better option than its conversion to electricity or transportation fluids. For electricity or the transportation fluids the entire carbon content of the coal is ultimately converted to COv In the case of coal to chemicals, assuming that the energy required for the several processing steps in coal to chemicals is provided by coal, about 70% of the carbon remains in the final product, polymers, plastic, fibers, whatever the final product. Furthermore, many of these products are recyclable. Thus, it is a far better option with significantly less CO, per unit of coal utilization.
Table 1. Percent of total electric power from coal in 2011 and 2018.
Fossil Fuel Resources
Before we discuss chemicals from coal, it is important to look at resources, particularly world oil. gas and coal reserves. Figure 1 compares the geographic distribution of recoverable oil reserves in 1997 and as estimated in 2017 (BP, 2018). There has been significant consumption of etude oil between 1997 and 2017, however, the estimated recoverable etude oil reserves in 2017 are actually 45% greater than that of the 1997 estimate. In other words, we har e been discovering more oil reserves as well as increasing the estimate of recoverable oil. due to technological advancements, than we are consuming. During the same period, the share of Middle East reserves declined from 59% to 48%. As shown in Figure 2, a similar effect is also true for the natural gas reserves. However, in this case, the Middle East has slightly increased its share.
Figure 2 shows the geographic distribution of natural gas reserves (BP, 2018). Total gas reserves increased from 128 trillion cubic meter in 1997 to 193 trillion cubic meter in 2017.
Figure 3 shows the geographic distribution of coal reserves in 2017 (BP. 2018). Total coal reserves in 2017 are estimated at 1035 billion tons, a net decline of 6.5% from the 1997 estimate of 1106 billion tons. However, in terms of barrels of oil equivalent (BOE) energy, this is 25% greater than the estimated combined oil and gas reserves in 2017. A further point is that there is a geographic mismatch between areas of oil and gas reserves and areas of high demand, namely, North America, Japan, Western Europe and the emerging high demand areas of Asia. On the other hand, coal reserves are advantageously located in these high energy demand areas. As a result, coal can provide security of raw materials for the energy demand of these nations.
Over the last two decades, coal prices have ranged from 30 to 110 U.S. dollars per metric ton (BP,
2018). One ton of coal generates roughly 27 million BTUs (nnnBTU) of energy. In terms of energy value, coal at S50 per ton is equivalent to $1.90 per nmiBTU. This is approximately lA of the cost of natural gas in the United States during the fourth quarter of 2018. That is, in North America, the cost of coal in terms of its energy content is 'A of the cost of natural gas. However, natural gas is much more expensive than coal for the countries where natural gas is imported. For example, in India and China coal is priced at 70 to S100/MT, that is, about $2.70 to $3.80 per mmBTU, while natural gas is valued at S7 to $12 per nmiBTU.
Though coal has been and continues to be a major fossil fuel source, its utilization presents significantly greater environmental challenges than the use of oil or gas. Coal combustion produces higher levels of sulfur dioxide, nitrogen oxides and particulates. The presence of mercury, arsenic, lead and other heavy metals in coal is also of concern. These are critical factors that must be considered when looking forward to future uses of coal.
The key to the utilization of coal reserves will be to use clean coal-buming technology and develop efficient processes for coal to chemicals. When these problems are solved, coal can again play a major
Figure 1. Distribution of world oil reserves, % (in this nomenclature, one billion denotes 10s and one trillion 1012).
Figure 2. Estimated natural gas reserves, %.
Figure 3. World gas reserves 2017.
Table 2. Coal price, $/Mt.
role in the manufacture of chemicals. China, with its large coal reserves, made a national policy and stalled several projects based on coal.
Coal tar and coal oven-based chemicals, such as anthracene to carbon black and naphthalene to plithalic anhydride, have traditionally been produced, along with acetylene and its derivatives. China has exhibited continued interest and efforts for coal to chemicals, calcium carbide, VCM, PYC. ammonia and urea. More than 60% of Chinese vinyl chloride monomer (VCM) capacity is based on coal derived acetylene. All Chinese coal mine operators have long-term planning for coal to chemicals strategy.
Coal gasification and synthesis gas production
In conventional coal combustion, complete oxidation of the carbon and hydrogen content of the coal to CO, and H,0 is a primary goal. The heat of combustion is used to generate steam and power. On the other hand, coal gasification is a partial combustion, in which the amount of oxygen fed to the reactor is controlled in order to yield a fuel gas mixture of hydrogen and carbon monoxide CH, + 0.50, = CO + 2H,. The gasification product gas produced is called raw synthesis gas, and has considerable BTU value. The synthesis gas, after clean-up, can be more efficiently and cleanly burned in a downstream process. Alternatively, the cleaned-up synthesis gas can be used to manufacture a number of different chemicals.
One of the more significant developments in coal utilization is the cogeneration of “clean fuels”, where the heat of the reaction and energy content of the waste streams are converted into electricity, with the other gasification product being liquid fuels or chemicals from fossil fuels. This is more energy- efficient than producing electricity alone. It also reduces the emissions of greenhouse gases and other pollutants. Thanks to these improvements, coal gasification is receiving greater attention. The integrated gasification combined-cycle (IGCC) system is already playing an important role in power generation.
Carapellucci et al. (2001) have investigated the performance of an IGCC power plant combining electric power generation with methanol synthesis. Figure 4 shows a schematic flow diagram of such a system. In this scheme, synthesis gas from coal gasification is used for both methanol production and power generation. In a stand-alone methanol plant, a large recycle is required in order to maximize utilization of the synthesis gas. In an integrated operation, as shown in the flow diagram, the recycle can be reduced. Instead, a purge gas stream can be fully utilized in the power generation section. This simplifies the methanol synthesis and also achieves greater energy efficiency. A similar integration can be made for DME production or for liquid fuels production with a Fischer-Tropscli unit.
The chemical reactions of gasification which uses coal are as follows:
Figure 4. Integrated gasification combined cycle (IGCC).
There are several known technologies for coal gasification, the “Texaco” gasifier (now owned by GE), Shell Global gasifier, British Gas/Lurgi gasifier, KRW gasifier and IGT U-Gas gasifier. The Texaco, Shell and Lurgi gasifiers account for the majority of gasification units worldwide. For coal to chemicals, the gasifiers are designed and operated for maximum synthesis gas production as it is a key intermediate for coal to chemicals. Lee (1997) has given a detailed review of synthesis gas technology and its uses. Synthesis gas from coal gasifiers requires significant clean-up to remove particulates, carbon dioxide and sulfur oxides. There are several well-established processes for the synthesis gas clean-up (HP, 2012), such as the UOP Benfield™ Process and the UOP Selexol™ Process. The UOP Polybed™ PSA and/or Polysep™ membrane processes may also be used for the production of hydrogen or adjusting the CO-H, ratio for downstream process applications. After clean up, the synthesis gas can either be used for power generation or for the production of chemical products, such as ammonia, methanol, DME, liquid fuels, etc.
Conversion of coal to liquids (CTL) was widely practiced in smaller capacity units in Germany during World War II. In this process, coal is converted to synthesis gas, followed by Fischer-Tropsch synthesis to liquid hydrocarbons. According to Jager (1997), several units were built in South Africa during the 1960s and 1970s, and are currently operating with a capacity of about 150,000 barrels (One barrel is equal to 42 U.S. gallons) per day.
A similar technology to CTL is the conversion of natural gas to liquid fuels (“gas to liquids,” or GTL). This also involves the production of fuel from synthesis gas, though, in this case, the source of the synthesis gas is partial combustion of natural gas. Typically, the gas comes from “stranded gas” locations, where it cannot be easily utilized or transported by conventional methods. Estimates of known natural gas reserves are increasing as the rate of new discovery of unconventional gas reserves increases. Fuels and Lubes Weekly in 1993 reported that Shell began operation of a 12500 BPD gas-to-liquids plant at Bintulu in Malaysia (Fuel and Lubes, 2012). A Sasol-Chevron project of 35,000 barrels per day which began operation at the end of 2006 in Qatar was mentioned by the Catalyst Group (2004) and Shell began operation of a large GTL plant of 140,000 barrels per day in Qatar in 2011, which was reported by Independent Chemicals Information Sendee (ICIS, 2012).