Coal-to-Chemicals (Primary)

Chemicals can be produced from coal derived syngas.  While the chemical industry is still predominately based on feedstocks derived from natural gas and petroleum, there has been increasing interest in using coal, as conventional oil and gas production are being strained by the rate of global economic growth. Production of chemicals from coal is of great interest in countries like South Africa, China, India and the United States where there are abundant coal resources.  Eastman Chemical Company has been successfully operating a coal-to-chemicals plant at its Kingsport, Tennessee, site since 1983. Similarly, Sasol has built and operated coal-to-chemicals facilities in South Africa.

Figure 1 shows the many potential products that can be produced from coal gasification derived syngas  Primary chemicals that are produced directly from the syngas include methanol (MeOH), hydrogen (H₂) and carbon monoxide (CO), from which a whole spectrum of derivative chemicals can be manufactured. The Eastman Chemical plant uses two primary products of CO and MeOH as intermediate chemicals to make acetic acid, methyl acetate and the final product of acetic anhydride, for the production of cellulose acetate – the basis for many of their photographic film, fibers and other plastic products.

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Figure 1: Coal to Chemical Routes

The following sections address typical processes for producing primary chemicals such as MeOH and CO. The process of gasification of coal to produce hydrogen for ammonia or other chemical products is addressed in the Coal to Hydrogen Production page. The production of higher level or derivative chemicals from coal is discussed on the Coal-to Derivative Chemicals page.

Coal-to-Methanol
Figure 2 shows a simplified block flow diagram (BFD) of a coal-to-methanol plant.  Syngas from the gasifier is cooled by generating high pressure (HP) steam in the high temperature (HT) gas cooling system before being water quenched and scrubbed to remove fine particulates. The scrubbed syngas then goes through a sour water gas shift (WGS) to adjust the H₂-to-CO ratio to approximately two.  Depending on the amount of CO needing to be shifted, supplemental steam injection to the sour WGS feed may be necessary.  The syngas from sour WGS is then cooled in low temperature (LT) gas cooling before mercury removal, and follow with hydrogen sulfide (H₂S) and carbon dioxide (CO₂) removal in an acid gas removal (AGR) unit.  Sweet syngas from AGR is sent to the MeOH synthesis block where it is compressed before going through the MeOH reactor to produce a crude MeOH product.  The crude MeOH is then purified to meet product specifications via distillation.  Purge from the MeOH reaction system is routed through a pressure swing absorption (PSA) unit to recover the H₂ for recycling back to the MeOH reactor. Net low pressure purge gas from the PSA is burned in low-Btu boilers to produce power and steam to meet in-plant power demand.  Acid gas from the AGR is sent to the sulfur recovery unit (SRU) to recover sulfur as a byproduct. Sulfuric acid production can be used as an alternative to SRU. Since CO₂ is removed and vented ahead of MeOH synthesis, carbon sequestration can be implemented by the addition of a CO₂ drying and compression system.

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Figure 2: Simplified Block Flow Diagram for Coal to MeOH

For co-producing MeOH and power in integrated gasification combined cycle (IGCC) applications, the liquid-phase MeOH synthesis process, LPMEOH™ from Air Products and Chemicals, is a very effective technology to convert part of the H₂ and CO in the IGCC power plant syngas into MeOH. Figure 3 shows a simplified BFD depicting the use of LPMEOH™ with IGCC for once-through production of MeOH.  Part or all of the treated syngas from gasification is routed through the once-through LPMEOH™ reactor to make MeOH. The MeOH reactor gaseous effluent is cooled to remove entrained oil, and to condense the crude MeOH product before the high pressure offgas is sent to the gas turbine for power generation.  Hydrogen recovery by a PSA unit is not needed.  The amount of MeOH conversion through the LPMEOH™ reactor can be increased by increasing reaction pressure, internal recycle, CO₂ removal, and/or by steam addition.

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Figure 3: Simplified Block Flow Diagram for IGCC/MeOH Co-Production

Coal-to-Carbon Monoxide
Carbon monoxide and H₂ are the building blocks of chemical production from syngas. High purity CO from syngas can be produced by several commercial processes: cryogenic purification, pressure swing adsorption, membrane separation, and salt solution absorption. Cryogenic purification is normally the preferred process except when the syngas feed contains large amount of nitrogen (N₂), due to the close proximity between CO and N₂ boiling points (approximately 8°F). Under this situation, copper salt-solution absorption can be used instead. The Eastman coal-to-chemicals plant at Kingsport uses cryogenic purification for CO/H₂ separation.

Figure 4 shows a simplified cryogenic partial condensation CO/H₂ purification scheme. Syngas feed is first pre-treated with molecular sieves to remove CO₂ and water before being chilled to approximately -300°F in the cold box by heat exchange against exit gases. Refrigeration is supplied by the cold product streams and by flashing the final CO liquid product stream exiting the stripping tower. Separation of CO/H₂ and purge gas is accomplished by a series of condensation/depressurization steps, of which, depending on the operating pressure cycle, the overall heat exchange/recovery and refrigeration streams may vary and can become very complex. Figure 4 shows a simple configuration of a condensation process. It includes a molecular sieve adsorber station, a cold box containing the plate fin heat exchangers to precool the feed syngas against the product streams.

Figure 4: A Simplified Cryogenic CO/H₂ Purification Scheme

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