Alstom is working with the US Department of Energy to improve the performance of Rankine Cycles. The study is examining the potential of supercritical systems as well as new technologies such as CMB combustors.

Mark Palkes, Alstom, USA

It is no secret that demand for electricity worldwide is growing rapidly and will likely continue for decades. So too, it seems, are the policy debates examining how best to supply new generation, given potential environmental impacts, security issues and economic variables. In the US, with continued higher cost and scarcity of oil and natural gas, limited renewable capacity and no growth of nuclear power generation, attention has focused on coal as a major energy resource for the nation’s future.

In search of higher efficiency and lower emissions, much of the new interest in coal has been directed toward second generation technologies such as coal gasification combined cycle and fuel cell systems that utilize hydrogen derived from coal gasification processes or natural gas fuel. There has also been research exploring other advanced power plant systems that generate chemical products in addition to generating power.

Less consideration has been given to potential improvements in conventional coal fired steam power plants, known as Rankine Cycles, that are also capable of high efficiency and lower emissions. These plants utilize pulverized coal or fluidized bed combustion systems.

In view of the possible near-term benefits, a US Department of Energy/Alstom Power Inc. (Alstom) consortium recently funded an assessment of Rankine Cycle power plants equipped with three different combustion systems: pulverized coal (PC), Circulating Fluidized Bed (CFB), and Circulating Moving Bed (CMB). The purpose of this study was to establish, through engineering analysis, the most cost-effective performance potential available through improvement in the Rankine Cycle steam conditions and combustion systems.

If you have not heard of CMB combustion systems, you are not alone. Still in the R&D development cycle, it is a novel method of solid fuel combustion and heat transfer, in which a moving bed heat exchanger heats the energy cycle working fluid to the high temperature levels required for advanced power generation systems. The CMB combustor is being developed in a separate DOE-sponsored programme in which Alstom participates.

Figure 1. Plant thermal efficiencies (HHV basis) � all cases
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Boosting efficiency

The main part of the study was the analysis of ten new steam power plants employing three different coal combustion technologies – PC, CFB, and CMB – integrated with five different steam cycles. Simply put, operating each of the three plant types at higher steam temperatures and pressures could boost typical current plant efficiencies from about 35 per cent to nearly 45 per cent. And that could have dramatic effects on reducing the amount of fuel burned, emissions produced, and cost of electricity $/kW.

This part of the study, referred to as ‘greenfield’, explored the technical feasibility, thermal performance, environmental performance, and economic viability of ten power plants that could be deployed currently, in the near, intermediate, and long-term time frame. For the five steam cycles, main steam temperatures vary from 538°C to 700°C and pressures from 165.5 bar to 349.9 bar. Reheat steam temperatures vary from 538°C to 720°C. The number of feedwater heaters varies from 7 to 9 and the associated feedwater temperature varies from 260°C to 330°C. The main part of the study therefore determines the steam cycle parameters and combustion technology that would yield the lowest cost of electricity (COE) for the next generation of coal fired steam power plants. With respect to environmental performance, the greenfield plants are designed to meet the following emissions performance based on CURC (Coal Utilization Research Council) 2010 targets:

  • >98 per cent sulphur removal
  • >90 per cent Hg removal

Mercury control was not considered in the investment costs or economic analysis for this study to simplify the analysis. However, new mercury control technology is currently being developed and it is believed that the investment and operating cost would be relatively small and approximately the same for PC, CFB, and CMB boiler technologies.

Figure 2. Plant investment costs ($/kW, EPC basis) – all cases
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The second part of the study explored one means of upgrading the efficiency and output of an older existing coal fired steam power plant by repowering. There are currently more than 1400 coal fired units in operation in the US, generating about 54 per cent of the electricity consumed. Many of these are modern units, clean and efficient now. But there are many older units in good condition still in service that could benefit from this repowering technology. The study evaluated the technical feasibility, thermal performance, and economic viability of this repowering concept.

For the ten greenfield cases, the calculated thermal efficiencies (HHV basis) range from 37.02 per cent to 43.55 per cent. With respect to thermal efficiency, for the same steam conditions, there is little difference when comparing among the combustion systems (PC, CFB, and CMB) as would be expected. In general the thermal efficiency of the PC fired systems are about the same as for the CFB based systems. The CFB and PC systems thermal efficiency are about 0.1 percentage points higher than the CMB systems which is due to only partial sulfation in the CMB combustor. The effect of the increasing steam cycle parameters (i.e., both temperature and pressure) is also clearly illustrated.

The specific investment cost results for the greenfield cases were estimated to range from about $1018 to $1168/kW net. For the same steam conditions, the CMB combustion system plants require the lowest investment costs as compared to PC or CFB plants. This cost advantage increases as steam cycle conditions are raised. The CFB systems are about $70/kW lower in cost than the PC type combustion systems. This difference is primarily attributable to the differences in the costs for the gas cleanup system equipment.

The levelized COE results for the greenfield cases is directly related to the investment cost and the cost of fuel. The results indicate that Case CMB-5 is the most economical from a COE basis. Compared to case PC-5 it requires significantly less weight of very expensive Ni alloy tubing.

As the cost of fuel increases, as shown in Figure 3, the cost of electricity increases also and at the same time the economics continue to shift towards the more efficient power plant systems. The ultra-supercritical CMB-5 continues to offer the lowest COE among the power plant cycles analysed.

For the repowering case, CMB technology was selected because of its economic advantage for high temperature cycles. The calculated thermal efficiency (HHV basis) is improved from 35.70 per cent for the existing unit to 38.40 per cent. The investment costs for all the equipment required for this repowering project is $413/kW net and the resulting incremental cost of electricity is ¢0.47/kWh. Incremental cost is calculated relative to the unmodified existing unit. This difference may quickly disappear if the price of NOx credits continues to increase and/or a major capital investment is required to refurbish the existing boiler or if there is loss of availability caused by the aging equipment.

Figure 3. Cost of electricity comparison for 1.25 and 1.80 $/MMBtu fuel cost – all cases
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From the results of the study the evaluated power plant systems fall either into the near term or long term category with respect to technology implementation. All combustion technologies can achieve low levels of pollutants and comply with the CURC 2010 targets. Technology is available today to facilitate construction of all the PC cycles, except for the ultra-supercitical steam conditions of PC-5, and all the CFB steam cycles. Technology is being developed to enable market introduction of PC-5 and both CMB cycles in a 10-15 year timeframe. The very high steam temperatures of the ultra-supecritical steam cycle do not appear to be practical for conventional CFB technology where combustion temperature is generally limited to 843°C. CMB technology with the combustion temperature of 1093°C allows greater latitude in selecting the range of steam cycle parameters. The ultra-supecritical CMB design offers the lowest COE.

The evaluation indicated that CFB power plants are the technology of choice for high sulphur coals. For low sulphur coals the PC power plants that require no back-end NOx and minimal or no back-end SOx control would be favoured. The supercritical CFB plants have the lowest cost of electricity and its cost continues to improve for higher steam conditions.

Based on reliability, investment costs, emissions and cost of electricity a coal fired steam power plant will continue to be a good investment for power plant owners – especially compared to other options such as IGCC for coal powered electric power production.

Commercial technology

Alstom has already bid several projects using an approach it believes will boost overall plant efficiency from about 35 per cent to more than 40 per cent. Contending that two of its existing and well-proven commercial technologies can be successfully married, Alstom is now offering supercritical CFBs in the 400-600 MW range.

This premier technology application combines the improved efficiency of supercritical steam cycles with the superior fuel flexibility and inherently low emissions of CFBs. And by increasing plant efficiency by five per cent, an even greater emissions improvement will be achieved since about 15 per cent less fuel will likely be consumed.