Pas de Deux with Swing & Stash: using IGCC for wind back-up1 February 2014
An improvement on the “Pas de Deux” concept presented in MPS June 2013 (pp 9-10) – which has two PSA (pressure swing adsorption) units operating in parallel – the coal-based IGCC concept described here, called Swing and Stash, offers an elegant and efficient means of providing a seamless and reliable back-up for wind. It uses feedstocks, equipment and technologies available now, while the byproducts are all marketable and the environmental impact from emissions minimal. John Griffiths
The "Swing and Stash" IGCC scheme offers a new approach to providing fast response back-up power generation to support renewables, and in particular, wind.
The idea is to employ coal gasification to produce a continuous fixed quantity of "syngas" (a mixture of hydrogen and carbon dioxide), which is priority used for power generation back-up for wind power.
The main objective is to provide a counterbalance to the irregularity of wind generation and thus turn an intermittent renewables supply into a steady supply source which can be readily absorbed into the grid system.
The amount of syngas available to produce the back-up power should be sufficient to cover the anticipated amplitude of variance. The surplus syngas is used to make hydrogen for use within the power plant and for sale.
This hydrogen for sale needs a market which is essentially a commercial sink, ie, capable of taking a fluctuating supply.
This can be achieved by using the hydrogen to make ammonia or methanol (which could be synthesised from the hydrogen plus some carbon dioxide from a CO2 capture system) and storing it in a large tank.
The hydrogen could also be injected into the natural gas grid, initially at small scale. In summer 2013, for example, E.On inaugurated a power-to-gas unit at Falkenhagen, which injected hydrogen produced by electrolysis into the German natural gas grid for the first time on an industrial scale. It is estimated that up to about 10% of hydrogen could be injected into European and US natural gas pipelines.
The proposed Swing and Stash scheme can substantially improve security of fuel supply and capability to respond to large and rapid changes in output requirement, while at the same time achieving low carbon emissions.
What it cannot do is to reduce capital costs as power production is reduced, ie, there will always be a capital cost penalty for redundant equipment not needed when wind power back-up is not required. However the proposed system reduces these redundancy capital costs to a bare minimum by operating continuously under steady conditions with the same total production rate of fuel/chemical feedstock.
Hydrogen could be produced and stored as an alternative to ammonia. But this is unlikely to be commercial because of the expense of storing very large quantities of hydrogen gas, which would entail large underground chambers taking considerable time and a great deal of expense to excavate.
The concept proposed avoids the need for pressurised hydrogen storage yet facilitates the instant availability of hydrogen as fuel for power production. It also avoids the thermally inefficient route of manufacturing an intermediate product that is then decomposed as required to yield extra power generation fuel.
Swing and Stash: Process description
The proposed scheme uses a solid feedstock, coal (or petroleum coke), as this may be stored in the open at point of use in very large quantities without excessive expense.
The coal is gasified in a water quench gasifier utilising high purity oxygen from an ASU. A water quench gasifier is preferred as the resulting high water content of the raw syngas is advantageous for the subsequent carbon monoxide shift step.
The syngas is shifted in a high temperature shift reactor over a sulphur tolerant (sour) shift catalyst. The shifted syngas stream is split and sent to two parallel pressure swing adsorption (PSA) units.
The first PSA unit (power PSA) produces a low carbon fuel which is sent to a commercial scale combined cycle unit. This PSA is designed for 'power mode' hydrogen production with a high H2 recovery (~94%), greater than 90% of the CO remaining in the H2 stream, in order, as far as possible, to preserve the fuel value of the stream to the gas turbine, while at the same time removing all the sulphur species and more than 90% of the CO2 to the PSA tail gas. The hydrogen rich stream is then blended with nitrogen from the ASU to limit the H2 content to 65% and sent to the gas turbine as low carbon fuel. This stream provides the 'baseload' fuel requirement to the gas turbine.
The second PSA unit (PSA 2) produces a pure (>99.99%) hydrogen stream which can be sent to an ammonia loop for chemical production (or in the future injected into the natural gas grid), with a portion diverted to the gas turbine as additional fuel when additional back-up power production is required. This second PSA is designed to produce high purity hydrogen (<10 ppm CO + CO2) and has a hydrogen recovery of only 88% - lower than of PSA 1. All the sulphur species plus CO and CO2 end up in the tail gas along with a portion of the H2. As a result, the tail gas from PSA 2 has a calorific value around 3 times higher than that from PSA 1.
The pure hydrogen stream is blended with nitrogen from the ASU, the quantity depending on application, eg whether it is to be used for NH3. synthesis (75% H2, 25% N2.) or sent to the GT.
In the ammonia synthesis case, the PSA units are sized to balance the amounts of H2 required for:
- baseload power production;
- baseload NH3. production;
- swing production between CO2-free back-up power and additional NH3 production.
A major feature of this concept is the recovery of the energy in the two PSA off-gases combined with their total combustion to carbon dioxide.
This is achieved by a combination of two technologies, one developed by Clean Energy Systems (CES) of California and the other by Jacobs Engineering - in a COTEC (COmbined TEChologies) unit.
The steam/CO2 exhaust from the CES oxyburner is discharged into a Jacobs-designed Desaturator to separate the steam and carbon dioxide, which has a battery limit discharge pressure of 10 bars.
Upstream of the CES burner, any sulphur fed into the plant with the coal will exist in the reducing atmosphere of the syngas mainly in the form of H2. The gases even if wet may be contained using relatively inexpensive construction materials.
After combustion with oxygen, the sulphur will be in the form of acidic SO2 and careful choice of materials will be required. Aqueous condensates within the COTEC unit will require rigorous monitoring to ensure that acid formation remains under control.
If required, the desaturated CO2 gas stream may be desulphurised in a simple hydrogen peroxide wash system to provide a sulphur free CO2 stream, at pressure, for compression and export.
The 10 bar pressure significantly reduces the size, number of stages and power requirement of the CO2 compressor when compared to conventional CO2 recovery schemes.
The power recovered in the expander and generated from the LP steam produced are in excess of the compression requirements for the two PSA tail gas streams thus making the COTEC unit a net producer of electricity.
A worked example - ammonia synthesis case
In this example of a potential application, the gasification train is sized to support a single F-class gas turbine operating in combined cycle along with a 2000 tpd ammonia loop.
The plant produces enough crude hydrogen from PSA 1 to supply the gas turbine at 50% of its full load.
PSA 2 produces high purity hydrogen that is divided into two streams, one dedicated to supply an ammonia loop at 25% of full production (500 tpd) and the remainder as a 'swing stream' that is either fed to the ammonia loop to enable it to run up to its full production or to the gas turbine to allow it to ramp up to 100% output. This stream can also be split between the two users depending on the immediate demand for back-up power.
The scheme achieves a carbon capture of 74% for the base load electricity production.
For the hydrogen used for the swing power and ammonia production there is 100% carbon capture.
In this example the power export and ammonia production will be variable as follows:
55 MW base-load power to grid
2000 tpd NH3
1750 tpd CO2 to EOR
55 MW base-load power to grid
227 MW premium back-up power to grid
522 tpd NH3
1750 tpd CO2 to EOR
Average production (assuming 75% utilisation of NH3 loop and gas turbine)
55 MW base-load power to grid
77 MW premium back-up power to grid
1500 tpd NH3
1750 tpd CO2 to EOR.
Benefits of the scheme
The "Swing and Stash" scheme maximises the commercial and environmental advantages of variable renewable energy, especially wind power.
The intermittency of renewables is normally countered by a back-up power supply.
Such a back-up is necessary if a renewable source is to become an acceptable contributor to the grid system, ie, a system which is required to reliably meet fluctuating demand, with minimal disturbance due to meteorological, diurnal, weekly and seasonal variations.
Energy storage, in particular hydro pumped storage, is an ideal form of back-up but not widely available.
An alternative, commonly used, is the simple cycle gas turbine generator using natural gas fuel. Such a system suffers from the high cost of redundant capital (75-80% underused), and, certainly in Europe, high fuel costs.
By switching to a solid fuel, the short term fluctuations of the gas market are avoided and a substantial (several months) store can be established and maintained at little containment cost.
The equipment and technology required for the typical flowschemes described here have been proven at commercial scale at the required operating temperatures and pressures and could be supplied by several international OEMs and process designers.
Redundant equipment which would have to be temporarily shut down in order to make room for wind power on the grid is reduced to the minimum.
As already noted, any sulphur brought into the plant with the coal will be finally captured and contained within the product discharge carbon dioxide stream and may be removed by scrubbing with dilute hydrogen peroxide to produce sulphuric acid. This can then be removed and concentrated or precipitated as gypsum in a similar manner to a conventional power station flue gas desulphuriser.
Refined and concentrated hydrogen peroxide is very expensive but it may be manufactured within the plant using some pure hydrogen from the PSAs (as envisaged in the Pas de Deux scheme).
Carbon dioxide capture is an integral feature of the proposed scheme and could if necessary be increased to 100% (with some loss in power output). A preliminary optimum of a minimum of 92% carbon capture is considered in the worked example above.
Byproduct carbon dioxide in excess of that required for, eg, EOR, food and drink applications or chemical synthesis, and not transported to long term storage, can be heated and expanded to generate power before discharge to atmosphere.
If some such discharge to atmosphere is permissible, then it could also be used beneficially to displace nitrogen as the gas turbine flame temperature moderator.
Swing and stash & capacity markets
The potential advantages of Swing and Stash can be illustrated in the context of the UK Energy Bill, just enacted. This envisages the use of open cycle gas turbines as a source of back-up electricity for the proposed capacity market. But Swing and Stash overcomes the many shortcomings which will be experienced using such a scheme:
- Immediate and seamless transition to back up power controlled by varying the quantity of surplus hydrogen sent to the natural gas pipeline (or ammonia synthesis).
- No necessity to start up and shut down any gas turbine or other generator - the back-up is always available.
- Back-up power is based on the use of lower cost coal fuel and not expensive, on demand, spot purchased natural gas.
- The back-up power has had the bulk of its carbon dioxide extracted.
John Griffiths, Gasification Matters, UK, and chairman of the 2014 European Gasification Conference, Rotterdam, 10-13 March 2014