Converting PC to very-high-efficiency IGCC: the MaGIC™ of mild gasification1 November 2008
MaGIC™ offers a new approach to “hybrid” coal-fuelled IGCC, combining gasification with combustion. It uses pyrolysis and “mild” (partial) gasification of the char. Using warm gas clean-up, volatiles are preserved in the syngas, doubling its heating value compared with conventional air-blown syngas. MaGIC is much more compact than conventional gasifiers and it is the first IGCC concept that can claim lower capital costs than PC, with significantly lower generating costs. It is retrofittable to existing PC plants (using them as its HRSG), with the potential to dramatically lower CO2 emissions from the existing coal fleet.
MaGIC™ is an innovative variation on coal-fuelled IGCC (integrated gasification combined cycle) technology being developed by Wormser Energy Solutions, Inc (www.wormserenergy.com). It uses proven technologies, but in a new combination, to deliver efficiencies over 50% (LHV) – the highest for any IGCC. It significantly reduces CO2 emissions, and the estimated levelised cost of electricity from MaGIC is only 70-80% of that for pulverised coal (PC) or natural gas (NG) plants.
MaGIC’s main envisaged application is the retrofit of existing coal-fired power plants, effectively converting them into IGCCs. This dramatically lowers carbon emissions from the existing coal fleet and avoids the need for flue gas desulphurisation. As part of the retrofit, the existing boiler is decommissioned; the rest of the plant remains in use.
How it works
MaGIC (see Figure 1) can be described as a “hybrid” combined cycle plant. At its heart is a carboniser, where pyrolysis and gasification occur, producing not only syngas for use in a gas turbine but also char for combustion in a boiler. So it is a hybrid of gasification and combustion. The gasification is partial, or “mild”, so the hybrid is also known as mild gasification,
hence the acronym MaGIC (“mild air-blown gasification integrated combined cycle”).
Following burning of the syngas in a gas turbine, producing power, the waste heat is recovered via a heat recovery steam generator (HRSG).
The char remaining after gasification is burned in a pressurised circulating fluidised bed combustor (PCFBC), whose airflow originates from the gas turbine’s compressor. The PCFBC’s exhaust is returned into the gas turbine’s combustor. Steam generated in the PCFBC is used to superheat and reheat steam from the HRSG.
The carboniser (Figure 2) consists of in effect two reactors working in parallel: a pyrolyser; and a char gasifier. At the centreline of the carboniser is a draft tube, where hot gases entering from external burners entrain particles of hot char from the surrounding bed. The coal is injected into the bottom of the draft tube, and is immediately heated and pyrolysed by the circulating char.
A deflector at the top of the draft tube returns the char leaving the draft tube back into the surrounding bed. Steam and air are injected at the bed’s bottom to gasify and fluidise the char. The bed level is maintained at its control point by removing excess char through the cooler at the bottom of the carboniser.
Unlike conventional air-blown gasifiers, external burners preheat the char bed during startup and heat incoming flows (coal, air, and steam) to bed temperature during operation. The external burners are fueled by recycled syngas. The syngas is burned to completion, converting its carbon into CO2. The external burner design needs only about half of the airflow required by conventional air-blown gasifiers (which form CO, rather than CO2).
Mild gasification also needs less energy – and thus less air for combustion – than the full char gasification of conventional gasifiers.
Altogether, MaGIC’s carboniser reduces the air flow by two thirds, and the syngas volume by half, compared with conventional air-blown gasifiers. This translates directly into reduced equipment size and cost.
The pressure vessel containing the carboniser also incorporates the syngas cooler, needed to cool down the syngas to the temperature required by the syngas cleanup system. The cooler consists of a fluidied-bed with embedded coolant tubes. The bed’s turbulence prevents tar build-ups from occurring on the coolant tubes, while its high heat transfer rate minimises its size and cost.
Warm-gas clean-up system
A warm-gas clean-up system (Figure 3), using recently commercialised technology, removes contaminants from the syngas to avoid damaging the gas turbine. Conventionally, cold-gas clean-up has been used in IGCCs, although warm-gas clean-up is more desirable because it contributes to higher plant efficiency. The research on warm-gas clean-up needed for MaGIC has now been completed, and in recognition of that milestone, its developer (RTI) is now making the arrangements to license the technology. A public announcement to that effect is expected later this year.
In addition to efficiency improvements, the warm-gas clean-up used in MaGIC has a second function: it allows the syngas to remain sufficiently hot that vapours in the volatiles don’t condense. Without warm-gas clean-up, some of the syngas volume reductions cited above would be lost. Volatile matter is a medium-Btu fuel, of about 500 Btu/SCF, with four times the heating value of the syngas emerging from conventional air-blown gasifiers. Not all of MaGIC’s syngas is volatile matter; the other constituents consist principally of carbon monoxide, hydrogen, nitrogen, and steam. By preserving the volatiles, the heating value of syngas produced by MaGIC is about 300 Btu/SCF – more than twice the heating value of conventionally-produced syngas.
Warm-gas clean-up removes three pollutants in sequence. The halide scrubber is a fixed-bed system using a sodium-carbonate-containing mineral (nahcolite or trona) that removes the hydrogen chloride and fluoride. The desulphuriser uses interconnected transport reactors that circulate pellets of zinc-based sorbent. Finally, sulphur compounds are removed from the syngas in the first reactor, and the sorbent is regenerated in a stream of air in the second reactor. The sulphur in the regenerator’s off-gas is converted to sulphuric acid in an acid plant. The sulphur-free syngas is then ducted to the filter assembly, where the char is removed by metallic candle filters.
Char from the candle filter is cooled, pulverised, and injected into the pressurised circulating fluidised-bed combustor (PCFBC) – see Figure 4. Further desulphurisation is required in the PCFBC because mild gasification results in the sulphur originally contained in the coal’s pyrites remaining in the char. Thus, limestone is continuously fed into the PCFBC to desulphurise the char, and serves as the bed material for the fluidised bed. Air from the gas turbine’s compressor is supercharged and used to burn the char. The PCFBC’s temperature is controlled with the fluidised-bed heat exchangers in the return loop; their energy is used to superheat and reheat steam.
MaGIC can use coal of any rank, as well as high-ash and high-moisture coals. It can also co-fire biomass and/or refuse-derived fuel with coal. A drier is used with the high moisture coals.
As Figure 5 shows (LHV basis), mild-gasification IGCCs are potentially the most efficient of any power plant technology. With the latest generation of gas turbines, eg a high-efficiency, high-pressure-ratio machine such as GE’s LMS-100, LHV efficiencies can be as high as 57%.
At high efficiency levels the combustor must be pressurised so its exhaust can be returned to the gas turbine rather than used to generate steam. High temperature heat from MaGIC’s PCFBC is recovered and used to increase steam cycle temperatures; in conventional combined cycle plants, the relatively cool gas turbine exhaust limits the steam cycle’s efficiency. Finally, the levels of gasification (how much of the coal is gasified) is controlled to maximise efficiency. With MaGIC, the level is typically over 80%.
Carbon dioxide emissions
Due to its high efficiency, coal plants retrofitted with MaGIC can produce 57% more power than without increasing either the amount of coal used for fuel, or CO2 emissions. In contrast, capacity added by building additional natural-gas combined cycle plants (NGCC) increases total CO2 emissions by 30%, as well as increasing fuel costs.
Significantly, the emissions of MaGIC in all cases (see Figure 6) fall below 500 kg/MWh, the standard for new coal fired capacity that was recently proposed by the European parliament.
The amount of additional power from the MaGIC plant can vary with the power required. As can be seen in Figure 7, there is a tradeoff between CO2 emissions and added capacity; but the additional power can be as much as 157% of the original coal plant’s rating.
As with other IGCCs, MaGIC enables carbon capture to be done at lower cost and higher efficiency than other power generation technologies. Unlike other IGCCs, the capital cost savings of MaGIC, relative to new pulverised coal plants, are nearly enough to pay for the cost of upgrading a MaGIC plant to provide CCS. In part due to MaGIC’s efficiency gains, once fitted with CCS the CO2 emissions of retrofitted steam plants can be reduced by more than 90%.
MaGIC with CCS uses a shift reactor and absorption system to convert the pre-combustion syngas to a mixture of hydrogen, carbon dioxide and nitrogen. Absorbers separate the CO2 from the hydrogen/nitrogen mixture; the hydrogen/ nitrogen is used as fuel for the gas turbine while the CO2 is dried, pressurised, transported and sequestered. This process captures 70-90% of the CO2; the balance is removed by the pulverised coal plant’s stack gas scrubbers.
Prior to the syngas entering the CCS system, MaGIC uses a partial oxidiser and a syngas cooler to eliminate the tars and most of the methane from the syngas. The partial oxidiser is essentially a pressurised furnace; the syngas cooler is a pressurised tube-and-shell heat exchanger.
In the oxidiser, operating temperature – controlled by the incoming airflows – is set to reduce the tars and methane to acceptable levels. The cooler downstream of the oxidiser returns the syngas to the temperature required by the CCS shift reactor. The heat from the cooler is recycled into the gas turbine’s discharge air; therefore partial combustion has only a minor negative effect on plant efficiency.
Crucially, the MaGIC retrofit of coal plants paves the way to upgrade the existing fleet to provide CCS, once that technology has been demonstrated. Most of the cost of CCS with IGCCs is for the IGCC itself, with the cost of upgrading adding as little as 10% to the plant’s cost (see Carbon capture and storage: assessing the economics, McKinsey and Co). That means that the cost of upgrading to CCS is largely, if not entirely, covered by the savings of a MaGIC retrofit – compared with that of building a new PC plant – which is the next-lowest-cost source of new power. A key objection to CCS is thereby eliminated.
We believe that because of its low capital costs, MaGIC could have a major impact on the speed with which CCS is adopted, and with it, the speed with which the emissions from coal, the biggest single source of CO2, are reduced.
Stack gas emissions
The stringent requirements of the gas turbine result in lower air emissions than the current air pollution standards, as seen in Table 1.
Together, MaGIC’s carboniser and syngas cooler are less than 10% of the size of a conventional air-blown gasifier. Relative sizes of fluidised bed gasifiers are shown in Figure 8. This size reduction is key to MaGIC’s low capital costs.
Capital cost estimates for MaGIC are shown in Table 2.
Air-blown gasifiers cost less to build than the gasifier/cooler assemblies of oxygen-blown IGCC systems because they don’t need oxygen plants, and don’t use radiant syngas coolers, which can be about 27 storeys high. However, the relatively low temperatures of air-blown IGCC’s result in a slow gasification-rate for the coal. Slow gasification rates require a fairly large gasifier vessel in order to fully gasify the char; this consumes much of the cost savings.
Hybrid gasifiers overcome this problem by burning the char, rather than gasifying it. At the low temperatures involved, char burns much more quickly, and in less space, than if it had to be gasified. This reduces the overall size and cost of the equipment.
MaGIC further reduces the size and cost of equipment by use of the draft tube, where velocities are much higher than in a fluidised bed. The low syngas volumetric flowrate resulting from the use of the external burners also reduces equipment size and cost, as does the use of the existing coal plant. As a result, MaGIC is the first IGCC whose estimated capital cost is below that of a pulverised coal plant.
The cost of electricity
Figure 9 shows estimates for the levelised cost of electricity for MaGIC, subcritical coal and natural gas combined cycle.
MaGIC’s low capital cost and high efficiency combine to reduce its cost of electricity to 20-30% below that of a conventional pulverised coal plant.
MaGIC retrofits add a gas turbine and retain the existing steam system; the retrofit uses only 20% of the water required for a new pulverized coal plant.
Retrofitting, construction and payback – a summary
MaGIC increases the likelihood a plant will be commissioned, reduces the time required to bring the new power online, and offers outstanding payback figures. It also promises a plant efficiency of 50%+, produces electricity at a low LCOE (70% of the LCOE of a subcritical PC plant), therefore allowing the building of a CCS development fund.
It is estimated to be retrofittable at a very low cost, <$1100 per kW and avoids the need for FGD installation (average £75 million per 1 GW capacity). The investment cost is expected to be recovered in 5 to 6 years.
MaGIC retrofits can provide new power with less CO2 than a new NGCC plant, yet avoid the conflict between lowering CO2 and keeping costs low.
As compared with building a new power plant on a greenfield site, retrofitting an existing plant reduces permitting time and requires no difficult-to-permit new transmission lines.
Decommissioning the existing boiler greatly extends the original plant’s useful life, and minimises the downtime of tying in the MaGIC system. If necessary, the existing boiler and scrubbers can be scrapped to make room for the new equipment.
Because of MaGIC’s low cost, it could become the technology of choice in China and other developing countries. Also once MaGIC has been retrofitted, most of the cost of a CCS system will have been paid for.
If it becomes common practice to use the savings afforded by MaGIC to pay for CCS, the CO2 emissions from coal-fired power plants could drop from the current 35% of the world’s emissions, to just 3%.
Fast track development needed
US government studies have predicted that the CO2 emissions from coal plants in developed nations would be reduced by 80% by the 2030s with a technology matching MaGIC’s description.
MaGIC uses proven technologies, albeit in an innovative configuration, so no new research is needed. This surely qualifies the concept for fast track development.
Development of MaGIC will make a substantial contribution to the energy needs and CO2 reduction goals of developed economies. Unlike other technologies, however, MaGIC's low capital costs and LCOE make it an extremely attractive choice for emerging economies. Thus, fast track development will have a "multiplier effect" in lowering world CO2 emissions – once it's available, developed economies will choose it because it lowers CO2 and is CCS ready; emerging economies will chose it because of its low cost.
A utility-scale demonstration plant could be on-line in 2013.