GSP: Gasify, Separate, Purify27 January 2017
GSP is a simplified scheme for integrated gasification combined cycle (IGCC) with carbon dioxide capture. A key simplifying feature is use of pressure swing adsorption instead of the complex systems of chemical washes normally employed in IGCC plants for acid gas removal. Co-production of hydrogen for transportation fuel is also a possibility. By John Griffiths
This article builds on work done by a group of experienced chemical and mechanical engineers committed to developing a commercially viable way of capturing CO2 from a coal fed power station. The product CO2 is of sufficient quality to be fed to a long-term underground storage system either as part of a means to control the rate of release of anthropogenic CO2 to atmosphere, or for enhanced oil recovery purposes.
The key equipment for the CO2 extraction is well proven and consists of a dry molecular sieve which is regularly regenerated by depressurisation followed by repressuring with fresh syngas. The process is known as pressure swing adsorption or PSA. The key to the GSP power generation concept is placing the processes in the correct order and paying careful attention to the details of their interconnection.
Some basic features of the proposed power plant design can be summarised as follows:
anticipated installed capacity, 700 MW net;
overall thermal efficiency, 35%;
carbon dioxide capture rate, 50%, but can be increased to 90%;
full water quench gasifier for blockage- free operation;
employs equipment and technologies already proven in operating plants;
dry, commercially available, system, ie PSA, as already noted;
no imports of solvent needed for sulphur removal as plant produces its own scrubbing agent, namely hydrogen peroxide;
modular delivery and construction.
Coal is an abundant resource – according to BP statistics, world total proved coal reserves in 2015 were sufficient to meet more than 200 years of global production – and may be shipped anywhere. But it is becoming increasingly clear that its combustion is not considered a long term option for power generation in many regions of the world.
Gasification has the potential to provide a much better way of using coal for power generation, inherently able to provide a route to low emissions and commercially viable carbon capture, which is part of the process rather than an afterthought, as in the case of post combustion capture.
Coal can be converted into a homogeneous gas mixture, syngas, through gasification of a dry or water-slurry feedstock of coal or petroleum coke, providing a clean and low carbon gas turbine fuel, an alternative to natural gas. However, there are few pressurised gasification plants in operation, and even fewer are coal feedstock based. The IGCC (integrated gasification combined cycle) scheme proposed here – Gasify, Separate, Purify (GSP) – aims at significant simplification relative to “conventional” IGCC technologies, particularly in the approach adopted to syngas cleaning and CO2 removal.
The nub of the proposed process is to gasify, ie partially oxidise, the coal fuel, and then immediately subject the bulk of the syngas generated to a catalytic water-CO shift reaction, which converts water and carbon monoxide to hydrogen and carbon dioxide. The objective is to produce: a syngas with the maximum proportion of hydrogen that can be used as gas turbine fuel; non-leachable slag; and an inert carbon dioxide/nitrogen stream, suitable for EOR (enhanced oil recovery) or injection into underground pressurised storage.
The gasification (or “partial oxidation”) reactions are carried out in a pressurised vessel designed to withstand the necessary combinations of heat and pressure. The oxygen feed rate is significantly less than that required to completely combust the coal feed to oxides, slag and ash.
Most important, as already noted, the GSP scheme replaces the complex systems of chemical washes currently proposed to remove “acid gases” (CO2 and S compounds) from gasification syngas in conventional IGCC plants with a concept based on pressure swing adsorption, essentially a mechanical process which is well proven and established in the chemical industry.
Use of these mechanical PSA units can considerably simplify and reduce the cost of IGCC power station design, construction, operation and maintenance.
The pressurised gasifier is contained in a single pressure shell in which the top section has a water cooled shell wall or refractory lined wall. The coal feedstock is first ground to a fine powder and fed either dry or as a water slurry into the top of the gasifier via a vertically mounted downward firing water cooled burner, together with a fixed ratio mixture of steam and oxygen. The gasifier internal dimensions and the reactants’ exit temperatures are chosen and designed to ensure that all the coal’s volatile components are gasified and the associated ash is liquid and free-flowing at the controlled firing/exit gasification temperature.
The downward firing burner flame is kept at a temperature of typically 1300 – 1400°C by adjustment of the oxygen rate and the products of the gasification process are dunked in water at the bottom of the gasifier.
The reaction gases are rapidly cooled and the liquid melt solidifies as a particulate slag, which is discharged through a submersed water lock.
The syngas leaving the quench gasifier is super saturated with water. After any necessary indirect preheating to above water saturation temperature the syngas is ideal for catalytic shifting, which increases hydrogen and CO2 content. The shift catalyst used needs to be sulphur tolerant (aka “sour shift” catalyst). To achieve the required conversion rate in the shift reaction (some 93%), two stages of shift may be required.
The shift reaction (CO + H2O ↔ CO2 + H2) has been employed on an industrial scale since the 1920s to convert coke and other forms of high carbonaceous material to hydrogen fuel and feedstock via gasification. “Shifting” of the reaction to the right is encouraged by maintaining a high steam partial pressure throughout.
The shifted syngas, after heat recovery, is then passed through a PSA unit (the ‘Power PSA’) where all the sulphur compounds and the bulk of the carbon dioxide are adsorbed in a bed of molecular sieve material. The purified syngas needs no further treatment before use as gas turbine fuel.
The adsorbed carbon dioxide and sulphur compounds make up the single stream of tail- gas which is compressed for final discharge through the CES stoichiometric oxyburner from which any excess heat is extracted for recycle to the burner. However, this PSA tail- gas contains a proportion of non-combusted fuel (hydrogen and CO), and this energy must be recovered efficiently in order to attain a competitiveoverallefficiencyforthepower station. It has a very low calorific value and cannot self-support combustion even using pure oxygen, so a portion of unshifted syngas is added to augment the calorific value.
This is achieved through a controlled combined bypass of the shift and power PSA (see flow scheme).
A small side-stream of this unshifted syngas to be used for CV enhancement is taken off as feedstock for a second, smaller, PSA (the ‘Hydrogen PSA’). This produces pure hydrogen to feed an on-site package hydrogen peroxide synthesis unit. The hydrogen peroxide is employed for sulphur removal.
Tail-gas from this second PSA is compressed and recycled. Any gas surplus to stoichiometric requirement is fed as additional fuel to the CES burner.
The hot exhaust gases from the CES burner are fed to a power generator fitted with tail-end steam raising.
Using multiple adsorbent vessels allows continuous production of fuel gas. It also permits gas leaving a vessel being depressurised to be used to partially pressurise other vessels. This results in significant energy savings, and is common industrial practice.
The key plant systems employ proven technologies provided by reputable international suppliers who can demonstrate existing installations operating at similar conditions and throughputs envisaged for the GSP flow scheme presented here. These key systems include: coal preparation; air separation unit; gasifier; CO shift; PSA; combined cycle; offgas compressor; expander; desaturator; hydrogen peroxide sulphur wash; and pressurised oxyburner.
It is therefore expected that the plant can achieve competitive capital, operating and maintenance costs, with relatively short lead times for design, procurement and implementation, and competitive bidding possible for 90% of plant systems.
The focus of the GSP scheme presented above is a single large gas turbine and steam turbine in combined cycle. This is the simplest and smallest power capacity arrangement using the GSP design.
But another design objective of the GSP conceptual design team was to provide a co- production option, generating electric power plus production of pure hydrogen.
A possible scheme is shown in the diagram below left, which could be envisaged and prepared for during development of a GSP power station.
This would enable relatively low cost but clean coal fuel to be used for future transportation as well as power generation.
Acknowledgements: GE/Shell/Siemens, gasifiers & combined cycle technology; UOP, pressure swing adsorption systems; MAN Diesel & Turbo, compressors & expanders; Clean Energy Systems, pressurised oxy-burners; Solvay, hydrogen peroxide synthesis and scrubbing; Jacobs, technical co-ordination
Further reading: Related articles previously published in Modern Power Systems: May 2006; February 2009; February 2010; June 2013; and January 2014