Creating a Future for Coal30 July 2007 Mitch Beedie
With major projects happening right around the world, coal gasification is getting a lot of attention at present. But who will get it right, and what will it take? Mitch Beedie reports.
It almost sounds too good to be true – a more power-efficient, reliable and economical-for-large-scale way of making hydrogen from fossil fuel – coal. But many companies are touting zero-emission, coal-based electricity as the way of the future – and throwing the right amounts of money behind it to prove the point.
Already, BP has been part of a large failed project in Aberdeen, Scotland, in the UK which was focused around the carbon capture and sequestion (which sees carbon sequested as carbon dioxide in deep geological formations). It is still working on a similar project in California with GE.
Given the fact that the US has already placed funding of $1.2bn for a demonstration project for the world's first zero-emissions, coal-based electricity and hydrogen power plant, it is likely efforts here will come to fruition.
The prototype should be the world's first integrated coal gasification, CO2 sequestration and hydrogen production plant and the world's cleanest fossil fuel plant. The project is being led by the FutureGen Industrial Alliance, with results being shared among all participants and with the industry as a whole.
The US wants to continue using coal because it has such huge reserves (more than 250 years' worth). The country sees it as a domestic resource that will help its energy security. Gasification cuts the emissions and waste that accompanies traditional coal burning but does not address the huge amount of carbon dioxide that is produced. Carbon sequestration will compress and inject this into geological formations underground.
The aim is to produce electricity without advancing global climate change, while also producing hydrogen that can alternatively generate electricity in fuel cells to power the huge number of vehicles on US roads. Electricity generation is the world's largest source of man-made carbon dioxide emissions and transportation is the fastest growing.
The three major elements are gasified coal with combined cycle electricity generation, carbon sequestration to prevent CO2 emissions and additional hydrogen for fuel cells. The project aims to build a demonstration plant that sequesters 90% of carbon emissions by 2012 and 99% by 2020. To replicate these plants economically improvements to all the various processes will be required, especially to carbon dioxide capture / sequestration and fuel cell technology.
HOW COAL GASIFICATION WORKS
To understand coal gasification you must first understand coal. Coal is a complex mixture of carbon, hydrocarbons, metals, sulphur, hydrogen sulphide, ammonia, water and tar molecules. Its composition varies widely depending on where the coal was formed.
Traditional methods of coal burning release a cocktail of the various chemicals that have to be removed from the exhaust gases. Coal gasification aims instead to separate out the resulting high-value fuels and chemicals before burning.
Gasification makes coal react with oxygen and steam under high pressures and temperatures to form a synthesis gas (syngas). The composition depends on the origin of the coal, but can typically be around 50% hydrogen, 25% carbon monoxide and 10% carbon dioxide by volume. The syngas is cleaned of impurities and fed to shift reactors.
The shift reactors agitate the carbon monoxide with steam at relatively low temperatures in the presence of catalysts to yield more hydrogen and convert the CO to CO2. Hydrogen can then be separated using pressure swing adsorption (PSA). That captures the CO2 by passing it over an adsorbent material in a molecular sieve under high pressure, and desorbing it under low pressure.
Maximum integrated gasification combined cycle (IGCC) efficiencies are now below 50%, partially because of the complex processing needed to remove pollutants. But this could rise to nearly 60% over the next decade. IGCC plants are more like complex chemical plants than electricity generation stations.
SHORT AND LONG-TERM CO-PRODUCTION APPROACHES
There are two approaches for large-scale electricity / hydrogen co-production: the first (short term) aims for 90% carbon capture, while the second (long term) aims to have above 99%. Both approaches use the syngas and shift-reactor approach, with integrated CO2 collection followed by permanent sequestration.
The short-term approach aims to improve process efficiencies with, for example, warm gas cleanup and improved separation membranes. Power will come from combined-cycle gas / steam turbines, most likely using fuel produced in Fischer-Tropsch reactors. These are already commercially available and use catalysts to convert syngas to liquid fuels and chemicals.
Major turbine manufacturers such as GE and Siemens-Westinghouse have studied the modifications needed to existing combustion turbines, and at present they say these seem technically feasible.
In the long-term approach, the main fuel product will be high-purity hydrogen, with power produced using a combined cycle hydrogen turbine or solid-state fuel cell. Improved ceramic membranes will be required to recover the hydrogen with ion transport membranes for oxygen separation. Advanced water-gas shift reactors using sulphur-tolerant catalysts will have to produce more hydrogen from synthesis gas at a lower cost.
By 2010 the roadmap calls for a 275MW co-production plant suitable for commercial deployment. By 2020, a demonstration reactor also generating 275MW should implement the long-term approach for 99% CO2 capture.
Carbon sequestration aims to store carbon dioxide at great depths. Proponents say that potential storage capacity is enough to hold all carbon dioxide emissions for several centuries. They point, too, to a good match between large-scale carbon dioxide sources and storage formations.
One of the most promising ideas being investigated is to use depleted and declining oil fields, where sequestration boosts oil recovery and increases recoverable reserves. Such 'value-added' sequestration in enhanced oil recovery (EOR) operations dramatically alters carbon sequestration economics.
Similarly, the CO2 could displace methane in deep unmineable coal seams. Other possibilities include sequestration in saline reservoirs and basalt and other significant geological formations.
Capture from coal plant flue gas is difficult because low pressures and dilute concentrations mean the volumes of gas that are treated are high. The flue gas holds impurities that make the adsorbing process less efficient, and a lot of energy is required to compress the CO2 to pipeline pressures. Capture from IGCC plants has higher efficiencies and reduced costs because the CO2 is in higher concentrations and at higher pressures.
GAS CAPTURE AND RECOVERY
The short-term FutureGen approach captures the carbon dioxide by absorbing it from the shifted syngas using a physical solvent like Selexol. This absorbs acid gases like CO2 at fairly high pressures, with the gas being released as the pressure is decreased.
The process can even recover other gases (such as hydrogen sulphide) in separate streams. (This has not yet been practiced commercially at such large scale, however, and process efficiencies will anyway need to rise).
Chemical solvents, like amines, can capture the carbon emissions. An aqueous solution reacts quickly with CO2-laden gas streams and can be heated to release the CO2 for recovery. This is the approach used by Statoil's Sleipner T platform in the North Sea for removing CO2 from natural gas (the gas has CO2 concentrations above those allowed in European pipelines).
The CO2 is compressed and piped to an adjacent platform and injected into a saline aquifer 1,000m below the seabed. The platform sequesters around a million tonnes of CO2 a year.
The Carbon Sequestration Leadership Forum in the US has so far recognised about 20 projects to identify and investigate storage sites, including the CASTOR project, which aims to store up to a third of Europe's industrial emissions. There are also projects in China and India.
At present, sequestration is estimated to cost between $100 to $300 per ton of carbon emissions avoided. The programme aims to reduce that to $10 or less by 2015.
FROM HYDROGEN TO POWER
Hydrogen has the highest energy content per unit weight of any known fuel. Around half the world's hydrogen is now produced by steam reforming of natural gas, with the rest produced from coal, biomass or by electrolysing water. Hydrogen is also produced as a by-product of ammonia production for fertilisers.
One of the main aims hydrogen producers have at heart right now is the production of a 'hydrogen economy'. A major task of the long-term FutureGen approach will be to develop high-efficiency fuel cells which will help with this.
Fuel cells have been generating electricity for spacecraft since the 1960s but they cannot yet compete for everyday applications such as vehicles, homes or businesses. The aim is to make competitive fuel cells for conventional vehicles by 2010.
FUEL CELL TECHNOLOGY
Fuel cells are similar to batteries but consume the reactants to produce electricity. The principle is similar to electrolysis but in reverse.
Fuel cells generally have the fuel (hydrogen) passing over the anode, an ion-conducting membrane as electrolyte, and an oxidant (oxygen) passing over the cathode. Ions travel through the electrolyte with the electrical current given a return path through the load, with additional products being water and heat. Stacks of cells can be connected in series / parallel to increase the voltages and currents.
Fuel cells chemically combine the molecules of a fuel and oxidiser with the aid of catalysts (the electrode material) without burning or producing pollutants. The oxygen can come from air, separated out using ion transport membranes (ITMs).
FutureGen will require improved ITMs. It will also need significant improvements in fuel cells themselves, not least in the interconnections that carry electricity and can also separate the fuel from the oxidant supplies.
SOLID OXIDE FUEL CELLS
For power plants, a major contender is the solid oxide fuel cell (SOFC) which is based on ceramics. The SOFC produces heat from exothermic chemical reactions and from ohmic losses.
High-temperature operation (sometimes above 1,000°C) increases efficiencies, producing heat needed for reforming the hydrogen. With large-scale generation the reforming can be done at the anode, with excess heat used to drive the turbines. This can increase overall efficiencies to about 70%.
These high temperatures do, however, mean materials for high-temperature SOFCs are expensive. The DoE's fuel cell coal-based systems programme has selected two research teams, led by General Electric and Siemens Westinghouse, to develop the 100MW+ fuel cells needed for central power stations. The aim is to reduce costs to $400/kW, excluding coal gasification unit and carbon dioxide separation.
The General Electric team is developing an integrated gasification fuel cell system that merges the company's SECA-based (Solid State Energy Conversion Alliance) coal gasification, gas turbine and solid oxide fuel cell technologies. A fuel cell / turbine hybrid is the main power generation unit. The Siemens Westinghouse team is developing large-scale fuel cell systems based on their in-house gas turbine and SECA-modified tubular solid oxide fuel cells.
FUEL CELLS IN VEHICLES
Intermediate-temperature SOFCs are aimed at domestic, leisure and transport applications – generating from around 1kW to 50kW. These work at below 700°C, allowing lower-cost steels and metals to be used for construction and interconnection, although the electrolyte conductivity decreases at lower temperatures. The waste heat from intermediate temperature SOFCs can be used for hot-water generation and space heating.
A major fuel cell application will be in vehicle use. Vehicles are the major reason the US needs to import over half its oil and that is expected to grow to nearly 70% by 2025. Hydrogen fuel cells promise pollution-free electricity to power cars, emitting only water. Fuel cells now have ten times the cost of internal combustion engines, but in the US the FreedomCAR initiative aims to reduce that.
For vehicles, proton-exchange membrane (PEM) fuel cells look promising. They can start up quickly and run at around only 65°C. PEM cells are lightweight and compact and have efficiencies of up to 60%. Hydrogen gives up electrons at the anode, with the PEM blocking these electrons but allowing the protons to flow through to the cathode. The electrons again flow through an external circuit to give the power, recombining with the hydrogen ions and oxygen atoms at the cathode to give water, which is exhausted.
HOW LONG UNTIL THE FUTURE OF FUTUREGEN?
Fundamental advances will be required to meet each of FutureGen's long-term aims of 99% CO2 capture. Gasification must also become more efficient, with huge fuel cells feeding power into national grids.
Carbon sequestration must prove itself technically and economically in the various regions it will be used. For example, cars will required to store hydrogen safely and convert it into fuel cells that are small and lightweight enough for their operation.
Two of the major issues surrounding FutureGen are carbon sequestration and the 'hydrogen economy'. Although large-scale carbon sequestration is feasible, there have already been setbacks. BP pulling out of the £500m Aberdeen plant, for example, was a result of UK government delays in supporting the project.
On top of this, new technologies are required for storing and delivering the flammable hydrogen gas. Hydrogen can't be delivered by standard natural gas pipes because it makes some high-strength steel pipes, compressors and valves brittle. A 'hydrogen economy' will therefore require substantial investments in a dedicated hydrogen delivery infrastructure.
With such major issues to solve, many feel that the financial and technological resources going to FutureGen might be better aimed at tried and proven technologies – particularly energy efficiency and small-scale electricity generation from renewable sources. Prevention is always better than cure, and not producing the carbon in the first place cuts out expensive sequestration. Much of the research is going to improving fuel cells but these don't actually generate energy, they only convert it.
Reducing unnecessary car use would cut the huge need for petrol, which could be reduced further by improved fuel efficiency and (for example) sustainable ethanol production. Even if FutureGen is successful by 2020, the resulting technology will still need to be rolled out across the industry. Gasification, however, is already a mature technology, and some critics see FutureGen itself as a stalling tactic to avoid having to invest in IGCC plants immediately. At best, they see the FutureGen project as 'too little, too late'.