Deciding whether or not to build new generating capacity is a complex business, as is the choice of technology, against a backdrop of changing energy markets and uncertain environmental legislation. But globally, the arguments in favour of using coal for power generation are getting stronger. Proven coal reserves could last more than 200 years based on current consumption levels and coal does not suffer from the same price volatility, supply uncertainties and geopolitical complications that some fuels do.

Nuon, an independent international energy company based in the Netherlands, which focuses on north west Europe and aspires to be a provider of clean, reliable and affordable energy, is working on the development of a new clean coal plant called Nuon Magnum. This can be seen as the next step in the transition towards a sustainable energy future. While coal will be the principal fuel, Nuon Magnum is envisaged as a multi-fuel power plant, also able to use gas and biomass. It would be based on gasification technology in which biomass can be co-fired with the coal on a large scale and which provides possibilities for CO2 capture.

The new plant, with an installed capacity of 1200 MWe, is expected to start commercial operation in 2011. The final investment decision is to be taken this year. ABB Lummus is acting as project management consultant, helping Nuon get to this decision point. The gasification technology to be used is one of the cleanest and most flexible ways of generating electricity from solid or liquid fuels and will build on Nuon’s already extensive experience of coal gasification at the Willem-Alexander plant at Buggenum.

Why multi-fuel?

Coal gasification offers great advances in comparison with other large-scale power generation technologies and the Nuon Magnum plant promises a combination of high environmental standards with good economics. The benefits of the multi-fuel gasification concept can be summarised as follows:

• Sustainability. One of the most important benefits of coal gasification is the environmental performance. Measurements at Buggenum (see below) demonstrate very low emissions of dust, SO2, NOx and heavy metals. In addition it offers the most cost-effective route to CO2 sequestration using pre-combustion.

Nuon Magnum will also have the ability to fire substantial amounts of bio-fuels and other secondary fuels, which will be necessary to comply with future regulatory restrictions and to achieve a more sustainable energy balance.

• Flexibility. With energy markets prone to volatility, it is important that a plant is able to switch fuels. The multi-fuel concept of the Nuon Magnum plant provides this capability within one unit, maintaining high efficiency and low marginal costs.

A multi-fuel plant, operating on coal for baseload and gas for peak load, will be well placed in the Netherlands as well as in the merit order of north west European plants as a whole. Other technologies have limited potential in this respect.

In the long run the overall demand pattern could shift to a more baseload-like shape (eg if there was significant solar panel penetration). This requires units that can shift from load following operation today to baseload operation in the future, while maintaining economic viability. With an increase in wind generation the flexibility to balance this additional wind capacity requires spinning reserve to maintain grid balance. Combined heat and power production requires must-run units operating economically throughout their full lifetime span. This requires fuel flexibility and sufficient heat sources (additional modules) to keep the unit in operation at lowest marginal costs while maintaining/guaranteeing a reliable heat supply.

• Non-power business opportunities. Gasification also offers opportunities outside the confines of the conventional electricity generation business, for example the production of hydrogen or methanol if power prices (spark spreads) are too low.

An increase in cross-fuel volatility can be expected (eg, with increased gas-on-gas competition, LNG penetration, growth in biomass use), while gas reserves are limited and gas use is increasing its penetration into sectors such as automotive and chemicals, with an assumed higher substitution price. In contrast coal is eminently abundant and no significant penetration into new market sectors is expected in the near future.

• Advantages over the alternatives. Alternative large scale power generation technologies are associated with problems that are more difficult to mitigate. There are problems relating to the social acceptance of nuclear energy (with particular concerns about waste disposal). Natural-gas-fuelled combined cycle units are generally assumed to set the price in the long run and thus theoretically do not create margin. Gas fired CHP units are shifted to the right in the merit order and cannot be operated continuously while maintaining profitability. Ultra supercritical coal-fired plants do not offer the same potential advantages in terms of environmental performance and also require large amounts of cooling water.

Nuon Magnum design: some basics

As already noted Nuon owns and operates the Willem Alexander plant at Buggenum. This was the world’s first IGCC plant based on the Shell gasification process with a Siemens V94.2 gas turbine. The gasifier was designed for Drayton coal and is currently co-fired with biomass.

Based on operating experience with the coal gasifier, Nuon has taken the opportunity to develop a second generation IGCC plant, providing the design basis for the Nuon Magnum multi-fuel power plant.

The key characteristics envisaged for the new multi-fuel power plant are as follows:

• The power plant will consist of three to five combined cycle units and three gasification units, with a total net power output of 1200 MWe (as already noted).

• About 60% of the fuel input will be supplied by the gasification units and the remaining 40% by natural gas.

• Coal can be substituted by alternatives, such as secondary fuels and biomass up to 50% in weight.

• The gasification technology will incorporate a number of advances over that used at Buggenum, with operational and engineering experience acquired at Buggenum used in formulating the design basis for the Nuon Magnum plant.

• From the five potential licensors for the entrained flow gasification technology to be used in the new plant, Shell has been selected. Both Nuon and Shell learned many lessons from the Buggenum experience and more or less share a common understanding of what features are required in the gasification system to be used in the new Nuon Magnum plant.

• The air separation unit (ASU) will be located “outside the battery limits” of the multi-fuel power plant itself.

• A modular concept will be used to increase availability, while economies of scale will reduce specific investment and operational costs.

• Future extension of the coal gasification capacity will be possible, with a limited investment.

• Carbon dioxide capture and sequestration can be incorporated (ie, the power plant will be truly capture ready).

• The gas turbine technology will combine maximum robustness with high efficiency.

• The Magnum design will combine the fast ramp-up and ramp-down capabilities of gas turbine technology (for peaking) with the virtues of a baseload generation on coal, biomass and other secondary fuels.

“The devil we know”

In Nuon’s selection process for the choice of gasification system, a comparison was made between fluidised bed, moving bed and entrained flow gasification technologies.

Among the major reasons for selecting the entrained flow technology were the fact that it is already used in proven large-scale applications, the quality of slag/fly ash it produces and the fuel flexibility it offers.

Subsequently, a choice had to be made between a slurry feed or dry feed entrained flow gasification process. The slurry feed was seriously considered but was rejected mostly because of its lower coal conversion efficiency, the requirement for a spare gasifier, limited lifetime of coal burners and high volume of process water treatment requirements.

Once dry entrained flow technology had been selected a final choice had to be made between the Shell Coal Gasification Process (SCGP) and the Future Energy technology (now owned by Siemens). The SCGP technology was finally selected, mainly because, for Nuon, this was the best reference and enabled the lessons derived from experience gained at Buggenum to be fully exploited.

The number of coal-fuelled IGCCs in operation worldwide is still limited to just four and no installations meet the availability levels required by the energy industry. Therefore maximum attention was, and must be, given to plant operability in the Magnum project.

By selecting the SCGP technology, Nuon is able to ensure that the Magnum power plant design reflects its own experience, and means we are working with “the devil we know”.

Nuon Magnum will not be a demonstration plant and therefore it was decided to replicate Buggenum in the case of components that have worked well in that plant. For this reason the size and capacity of the Magnum gasifiers will be exactly the same as those of Buggenum, in other words 3 x 2000 t/d of coal instead of moving to 2 x 3000 t/d, for instance. The gasifier pressure was kept the same as that of Buggenum, 27 bar, despite the fact that the present candidate gas turbines all require a higher inlet pressure than the gas turbine used at Buggenum and thus (proven) booster compressors will be necessary.

The following design features based on major lessons learned from Buggenum will be implemented in the Magnum gasification section:

• 30% over-design in coal milling & drying capacity at 15% moisture (at Buggenum 12% was specified).

• Increased pulverised coal sluicing capacity plus modified design.

• Heat skirt in bottom of gasifier to be water cooled instead of ceramic-lined.

• Back up for slag dewatering chain system.

• Over-design in quench gas system (700-800ºC design quench temperature) to prevent sticky fly ash from plugging the syngas cooler.

• Proper de-carbonisation of slag bath water to prevent plugging of pipelines by carbonate scaling.

• Duplex stainless steel for slag bath water cooling system to prevent erosion/corrosion.

• Improved fly ash rapper design.

• Simplified syngas cooler (only IP steam system, no HP system and no economiser).

• Improved fly ash cyclone design.

• Improved hot gas ceramic filter.

• Carry over of Sulfinol (H2S absorbent) to be minimised by proper column design and elimination of thermal degradation of Sulfinol by proper re-boiler design.

• No ASU integration, and use of modern standard ASU design.

• ASU operated at fixed pressure instead of sliding pressure, as a result of sliding outlet pressure of the gas turbine compressor.

• Proper mole-sieve design (vertical position instead of horizontal).

• ASU based on internal compression concept (100% liquid O2 draw off, so less build up of contaminants in particular solid CO2).

• Proper training of operators and maintenance engineers (on the Buggenum plant) by experienced Buggenum staff, with use of simulator.

New environmental requirements

One driver leading to some departures from “the devil we know” is increased stringency in environmental permmitting requirements, which are having a major impact on the design of the Magnum gas treatment plant design. This will be superior to that used at Buggenum, with 99.8% desulphurisation efficiency vs 98.5%. The Magnum gas treatment system will also be equipped with a flash gas recycle system from the Sulfinol regeneration system back to the inlet of the HCN/COS conversion reactor.

In the water treatment section Magnum permit requirements are not comparable with Buggenum where a zero discharge permit for process water is applicable. The reason for this is that Buggenum is located on a river whereas Nuon Magnum is in a coastal area, with discharge of the treated process water to the sea (harbour). Therefore the use an evaporator can be avoided and less energy is thus consumed in this part of the plant.

Combined cycle aspects

For Nuon Magnum three to five identical gas turbines will be installed in combined cycle configuration. The tendering process is currently underway and the gas turbine type has not yet been selected, so not a huge amount of detail can be given at this stage. But, as already mentioned, the gas turbine design priorities are robustness and efficiency. The environmental performance is also very important of course but is largely realised by removing all kinds of impurities from the syngas before burning it in the gas turbine.

The gas turbines will be of a proven design and have a high degree of fuel flexibility while meeting the environmental requirements relating to air pollutants. The fuel flexibility will enable operation on syngas only, on natural gas only or on a combination of syngas and natural gas co-fired in the same gas turbine. The necessity of an air bleed to the air separation unit is avoided to maximise ease of operation and reliability.

The gas turbine design will be based on a standard natural gas fired machine, with modifications for firing syngas and co-firing.

The LHV of syngas is only 25% of the LHV of natural gas and therefore the syngas flow will be much higher when firing natural gas. Besides this, in the case of all IGCC plants running to date, water or nitrogen is applied as a diluent to reduce NOx emissions when firing syngas. Without modifications this would lead to inadmissibly high flue gas flow rates and pressure ratios and also to a too high output. These problems can be avoided by modifications such as reduced IGV angle, adaptation of the compressor blading, extra compressor stages and/or increasing the throttle area of the first stage turbine nozzle. Most such modifications have been successfully demonstrated in other IGCC applications.

The gas turbine burners will be capable of firing syngas and natural gas in diffusion mode while using diluents for NOx reduction. Dry low NOx (DLN) burners for syngas applications are under development but have not yet been demonstrated at full scale.

One possibility is that selective catalytic reduction (SCR) technology could be applied to meet the NOx requirements and to minimise the use of diluents for NOx suppression, resulting in improved efficiencies. Space for an SCR unit will be reserved in the heat recovery steam generators.

The HRSGs, downstream of each gas turbine and supplying steam steam turbine generators, will be of the multi-pressure-level type. The steam will be condensed in once through seawater-cooled condensers.

Steam produced by the gasifiers will be expanded in a separate steam turbine to avoid integration and so maximise flexibility and reliability. This steam turbine will also be connected to a once-through seawater-cooled condenser.

A RAM (Reliability Availability Maintainability) specification is being applied to ensure high availability and reliability, with low maintenance costs. This will also focus on the goal of achieving the best possible overall plant efficiency, which is a function of design efficiency coupled with reliability. Standardisation will of course be applied to the maximum extent possible in the interests of achieving good operability and ease of maintenance.

On being CO2 capture ready

In the Netherlands plans for a number of new coal-fired power stations have been announced recently and most of the energy companies making the announcements say their plants will be carbon capture-ready. The concept of capture-ready is eye-catching for policymakers, but what does it mean and how can it be substantiated? This is not yet clear. Let’s start with the definition:

A plant can be considered to be carbon capture-ready if, at some point in the future, it can quickly be retrofittedfor carbon capture and sequestration and still be economic to operate.

The concept of capture-ready is not specific to a certain plant design: rather it is a spectrum of investments and design decisions that an energy company might undertake during the design and construction of a plant.

IGCC technology offers advantages over pulverised coal plants for CO2 capture as the CO2 can be separated at higher partial pressures, reducing the amount of capital required and the energy penalty for the capture.

The table below gives a brief overview of the issues surrounding the retrofit of the Magnum IGCC plant with pre-combustion capture-ready technologies and the design options that should be deployed to minimise the impacts of the measures.

The best choice

Overall, Nuon believes that the planned Magnum IGCC constitutes the best choice of power generation technology for decades to come, given the uncertainties of the market and its low emissions. In addition its versatility will enable it to better address the as yet unknown challenges that the new power plants are likely to have to overcome in the future. It also lends itself to carbon capture and sequestration. Energy companies have a vital role to play in the ongoing process of creating a more sustainable and environmentally friendly energy balance and the Nuon Magnum plant promises to be key step towards achieving this end.

The author is head of Technology Development and Control, Nuon Technical and Project Development, Sparklerweg 20, Amsterdam, The Netherlands

Visualisation of a possible layout for the Nuon Magnum IGCC plant at Eeemshaven Location of Eemshaven The Willem-Alexander plant has achieved low emissions, and this is one of the attractions of IGCC Eemshaven provides a spacious site with a modern sea port, good infrastructure and high-voltage grid access. There is a plan to deepen the channel leading to Eemshaven, allowing deep-draft vessels carrying fuel to enter the port. The harbour will also neeSchematic of the multi-fuel concept to be employed at Nuon’s Magnum IGCC plant The Willem-Alexander IGCC plant at Buggenum