Coal-to-Hydrogen Process Description

The U.S. Department of Energy (DOE) has sponsored many design studies on the production of hydrogen (H₂) from coal, with or without the co-production of power.  Hydrogen production from coal is a proven technology; there are 10 coal gasification-based ammonia plants currently in commercial operation worldwide.¹  In these coal-to-ammonia plants, coal is first converted to H₂ before reacting with nitrogen to form ammonia.

In addition, production of H₂ from synthesis gas (syngas) generated by liquid or coke gasification is currently practiced in 10 commercial plants worldwide.¹ Refineries are one of today’s largest H₂ consumers, and all 10 plants are located at or near a petroleum refinery. Nine of the 10 plants use residual or waste oil as the gasification feedstock, with the remaining plant using petroleum coke.

Recent DOE studies presented the following four process design schemes as possible options for centralized-large-scale H₂ production from coal, and discussed their performance and efficiency:²

• Co-producing H₂ and power in today’s coal-based integrated gasification combined cycle (IGCC) plants
• Co-producing H₂ and power in coal-based IGCC with carbon capture
• H₂ production from coal without power export
• Co-producing H₂ and power in future IGCC based on advanced warm gas clean-up and advanced membrane (combined shift and H₂ separation) technologies

IGCC and H₂ Co-Production
Figure 1 shows a simplified block flow diagram (BFD) of an IGCC/H₂ co-production plant.  Syngas from a slagging gasifier is cooled by generating high pressure (HP) steam in the (high temperature gas cooling (HTGC ) system before being water quenched and scrubbed to remove fine particulates.  The scrubbed syngas then goes through sour water-gas-shift (WGS) to convert steam into H₂.  Depending on the desired amount of H₂ to be produced, supplemental steam injection into the sour WGS feed may be necessary.  The syngas from sour WGS is then cooled in low temperature gas cooling (LTGC) before mercury removal, and hydrogen sulfide (H₂S) is removed in a single-stage acid gas removal (AGR).  Only a small portion of the carbon dioxide (CO₂) that is co-absorbed with the H₂S is removed in the single-stage AGR.  Part of the sweet syngas from AGR is routed through the PSA unit to recover a 99.9% purity H₂ product.  The remaining sweet syngas from AGR is bypassed to the gas turbine (GT) combustor for power production.  After H₂ removal, the residual PSA feed is present as a low pressure (LP) purge gas containing all of the CO and CO₂, and 10 to 30% of the H₂ in the original PSA feed.  This LP purge has is subsequently compressed to the GT combustor for supplemental power production. 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 the SRU.

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Figure 1: IGCC/Hydrogen Co-Production Block Flow Diagram

IGCC and Hydrogen Co-Production with CO₂ Capture
Figure 2 shows a simplified BFD of an IGCC/H₂ co-producing plant with CO₂ capture. Key design differences from the reference case IGCC/H₂ plant with no CO₂ capture (Figure 1), in addition to the deployment of a new gas turbine capable of firing fuel with very high H₂ content, are:

• A full quench gasifier without HTGC to maximize the water content in syngas feed to the sour WGS system for maximum conversion of CO to H₂.
• The selective removal all of the CO₂ in the sour syngas with a two-stage AGR to produce a separate CO₂ product stream to be dried and compressed for sequestration.
• All of the sweet gas from the AGR (mainly H₂ with small amount of nitrogen [N₂], methane, and unconverted carbon monoxide [CO]) is routed through the PSA to recover 99.9% H₂ product.
• An amount of H₂ product is combined with the compressed PSA purge to meet the GT fuel requirement.
• Some of the waste N₂ from the air separation unit (ASU) is compressed for use as a diluent in the GT to offset mass loss from CO₂ removal.

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Figure 2: IGCC/Hydrogen Coproduction (With CO2 Sequestration) Block Flow Diagram

Hydrogen Production Only
In situations where the goal is to design a plant to produce H₂ from coal with no (or minimal) power export, the overall flow arrangement can be simplified.  Figure 3 shows a typical BFD for such a design. Onsite power generation is kept to a minimum to meet in-plant consumptions.  If necessary, some power can be imported to meet internal consumption.  Key differences from H₂ co-production in an IGCC plant with carbon capture (Figure 2) are:

• The replacement of the H₂ fired GT with a low or medium Btu fuel fired boiler plant.
• Changing the PSA purge compressor to a lower head blower.
• Eliminating the need to compress ASU waste N₂ for use as a diluent.

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Figure 3: Coal-Based Hydrogen Production (Without Power Export) Block Flow Diagram

IGCC/H₂ Co-Production with WGCU and Advanced WGS Membrane
Figure 4 shows a BFD of an IGCC/H₂ co-producing plant incorporating future technologies of warm gas clean-up (WGCU) and advanced metallic membranes capable of combined WGS and H₂ separation into a single operation, at elevated temperatures. With this process, syngas from the gasifier is cooled to 500-1000 °F by generating HP steam in the HTGC system before going to the advanced WGCU system, where particulates, sulfur compounds and other trace contaminants are removed at elevated temperature.  The cleaned syngas then goes through the advanced membrane system where WGS and H₂ separation occurs simultaneously and continuously.  The high pressure, hot, carbon-rich purge gas is burned with oxygen (oxy-combustion) in a combined cycle plant, to generate power and steam.  Water is condensed out of the oxy-combustion exhaust to generate a nearly-pure CO₂ product which is compressed and ready for sequestration.  The bulk of the H₂ product from the advanced membrane is exported to meet contractual demands.  The balance is compressed and burned with air in the gas turbine of the combined cycle system to generate additional power to meet internal and export demands.   The WGCU system directly converts the removed acid gas into sulfur to be exported as a byproduct.

Figure 4: IGCC/H₂ Co-Production with Warm Gas Cleanup and Advanced WGS Membrane

For performance and efficiency data on each of these plant schemes, go to the page on Coal-to-Hydrogen Efficiency & Performance, or refer to the reference reports 2 through 4 listed below.

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