Hydrogen Separation and Production
Synthesis gas (syngas) derived from most high pressure gasification processes already contains a significant amount of hydrogen (H2), which can be readily separated into a pure H2 product meeting industry product quality standards. There are several conventional H2 separation processes that can be employed, but the predominant process is pressure swing adsorption (PSA), which is well proven. PSA has the ability to produce high purity (99.9%) hydrogen at near feed pressure. New technologies are being developed to increase the efficiency and reduce the costs associated with H2 production from coal gasification.
Hydrogen production or co-production from syngas generated by liquid or solid gasification is currently done in ten plants worldwide.1 Since refineries are one of today’s largest H2 consumers, it is not surprising that all ten plants are located in or near petroleum refineries. Nine of the ten plants are residuals or waste oil fed, with the remaining plant being pet coke fed. Many studies and demonstrations on producing H2 from coal have been completed, but there is currently no commercial coal gasification plant producing significant quantities of H2. With the recent interest in an H2 economy and with the possibility of converting to hydrogen-based power generation, H2 production technologies are gaining attention.
Co-Producing H2 with Today’s IGCC Technology
Figure 1 is a simplified block flow diagram (BFD) for co-producing H2 in a current technology IGCC power plant without carbon sequestration. Syngas from the slagging gasifier is cooled by generating high pressure (HP) steam in the high temperature gas cooling (HTGC) system before it is water quenched and scrubbed to remove fine particulates. The scrubbed syngas then goes through a sour shift process to further generate H2 by reacting carbon monoxide (CO) with steam. Depending on the desired amount of H2 to be produced, supplemental steam injection into the sour shift feed may be necessary. The syngas from sour shift is then cooled in low temperature gas cooling (LTGC) before mercury removal, and followed by hydrogen sulfide (H2S) removal 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 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.
Co-Producing H2 in IGCC with Carbon Sequestration
Figure 2 is a BFD for co-producing H2 in a future IGCC plant with carbon sequestration. Key differences from H2 co-production in today’s IGCC (Figure 1), in addition to developing new gas turbines capable of firing fuel with very high H2 content, are:
- Use of a full quench gasifier without a HTGC system to maximize moisture in syngas feed to the sour shift system for maximum conversion of CO to H2.
- Selective removal of all of the CO2 from the sour syngas with a two-stage AGR to produce a separate CO2 product stream to be dried and compressed for sequestration.
- Routing all of the sweet gas from AGR (mainly H2 with small amount of nitrogen (N2), methane, and unconverted CO) through the PSA to recover 99.9% H2 product.
- Bypassing the required amount of H2 product to be combined with the compressed PSA purge to meet the GT fuel requirement.
- Compression of some of the waste N2 from the air separation unit (ASU) for use as diluent in the GT to offset mass loss from CO2 removal.
Hydrogen Production from Coal without Power Export
In cases where the goal is to produce H2 from coal without power export, the overall flow arrangement can be further simplified. Figure 3 is a BFD for producing H2 from coal with minimal power generation. Onsite power generation is kept to a level to meet internal combustion, and if necessary, some power can also be imported. Key differences from H2 co-production in IGCC with carbon sequestration (Figure 2) are:
- Eliminating the need for H2 fired GT by switching to 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 diluent.
Advanced Technologies for Production of H2 from Coal
DOE continues to support research and development projects which promote large scale H2 production from coal. These projects include the development and scale up of advanced separation materials and devices for producing ultra-pure H2, and the development of advanced technology that consolidates multiple processing steps into a single step to reduce overall project costs.2