With interest in IGCC technology gathering momentum, Thermoflow’s latest software is expected to accelerate progress by including design routines for such plant, based on current gas turbines and gasifiers.

Eric Jeffs

Twenty years ago Dr Maher Elmasri quit the Massachusetts Institute of Technology, where he had been Professor of Mechanical Engineering, to launch Thermoflow a software development company that produced a computer programme for the design of combined cycle and combined heat and power plants.

This programme, GT PRO, was one of the first to be introduced for gas turbine power plants and when the programme was first developed there were no gas turbines over 100 MW in the 60 Hz market and boilers were at most 2-pressure designs with a high pressure output of about 80 bar. Now, the largest gas turbines in the 50 Hz market are about 280 MW – with units over 300 MW under development – and tri-pressure reheat steam cycles with maximum steam conditions at 160 bar, 560°C are available.

Furthermore, interest in Integrated Gasification Combined Cycle (IGCC) technology is growing while the costs are rapidly falling towards that of supercritical coal fired steam sets. In consequence, the last ten years have seen some seventeen such projects come into operation around the world, the majority converting oil residues and asphalt to supply power and steam to oil refineries. In recognition of this growing interest in the technology, the latest edition of the GT PRO software, Version16, has been further extended to include routines for the design of IGCC-based plant.

Designing the plant

An IGCC plant is a standard combined cycle with a modified gas turbine and external heat recovery back to the steam cycle from the syngas coolers. Current design effort for the first commercial IGCC plants in the United States assumes that there will be no air bleed from the gas turbine to an air separation unit (ASU), but that the burners and the turbine will be reconfigured to handle the low energy synthetic gas, which has a calorific value of the same order as blast-furnace gas – about 130 Btu/ sf3. The interaction with the combined cycle is on the steam side with the heat transfer from the syngas coolers into the high pressure stream of the heat recovery boiler.

To design such a plant GT PRO prompts the selection of ‘gasification’ as a first step and specification of the size of plant and the number of pressures in the steam cycle followed by detailed site conditions, the method of cooling and the configuration of the steam cycle.


Figure 1: GT PRO’s graphic output screen showing results of calculation for a single-shaft block of SGT5 4000F linked to a GE Texaco gasifier.
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If gasification has been specified the programme automatically increases the turbine inlet nozzle size by a default (but user adjustable) 6 per cent for a low Btu gas and the gas turbine input page specifies the fuel as syngas. So far the gas turbine library, from which the unit is chosen, does not include any version of a machine specifically modified for IGCC application but makes appropriate modifications to standard machines.

The next programme steps follow standard GT PRO routines to define the steam cycle in terms of pressure and temperature but because of the large amount of steam generation in the gasifier’s syngas coolers, the boiler pinch points have to be set higher than for a natural gas fired plant to hold up the stack temperature and prevent dew-point corrosion.

Gasifiers

Only three oxygen-blown gasifiers are currently offered commercially: the GE Texaco, Shell, and Conoco/Philips E-Gas systems. Of these GE has adopted the Texaco gasifier as its generic design, while Siemens has used the Shell and Texaco gasifiers on some European projects. Siemens has also designed a 600 MW reference plant for the US market using two E-gas systems powering a combined cycle with two SGT6 5000F gas turbines.

The gasifier page within the GT PRO application allows either selection of one of the three standard gasifier types or a more flexible user-defined air blown gasifer. This is the preferred choice of Mitsubishi since the air-blown gasifier has a smaller auxiliary load and the gas turbine can therefore use the standard low Btu gas combustor. However, the air-blown concept is still under development and the prototype IGCC based on it with the smaller M701D gas turbine is still under construction 160 km north of Tokyo and expected to go into operation early next year.


Figure 2: The gasifier screen showing settings for the Conco/Phillips E-gas gasifier linked to a 3-pressure steam cycle. Gasifier coolant feeds HP flow and clean fuel preheating is taken from the IP section.
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There are variations of each gasifier according to the number of syngas coolers, the quenching arrangement, and the preheating of the syngas for the gas turbine combustor. GT PRO also allows the designer to specify different levels of radiant and convective syngas cooling for recovering heat to the combined cycle, or the less efficient but simpler and less costly quenching process. The Coal Library page contains a convenient global list of coal types according to their calorific value and chemical composition, how much sulphur, ash or moisture is in them. The default coal is Pittsburgh 8, a high volatility bituminous coal with a heating value of 27680 kJ/kg at 25° C and with 6 per cent moisture, 2.9 per cent sulphur and 10 per cent ash.

In addition, the air source for the Air Separation Unit (ASU), which could be either from an external air compressor or the gas turbine compressor, may also be specified.

Design studies

With the GT PRO software evolving from the 1987 DOS-based Version 1 to now include design capabilities for an IGCC plant, when combined with the Preliminary Engineering and Cost Estimation (PEACE), the latest version of GT PRO also provides preliminary cost estimation capabilities for IGCC plant, although these estimates must be treated with caution since IGCC is still a developing technology. Nonetheless, Thermoflow state that they have endeavoured to make the cost estimate rationally dependent on the key variables for the main IGCC equipment. Two well proven systems serve as the basis of a suitable illustration of the application.

A combination of a single-shaft combined cycle block based on the latest version of Siemens’ SGT5 4000F gas turbine and the GE Texaco gasifier. The natural gas fired plant has an output of 410.15 MW (397.8 MW net) at an LHV efficiency of 58.4 per cent at the generator terminals. This is for a plant in a typical configuration 115 m above sea level, with a wet mechanical draught cooling system, and a tri-pressure steam cycle with the high pressure output at 135 bars, 560°C.


Figure 3: Energy balance for GT PRO design of an IGCC scheme modelled on Siemens proposal for the US market with two SGT6 5000F gas turbines with E-gas gasifiers and one steam turbine
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The IGCC design sends none of the intake air to the ASU but receives 20 per cent of the available nitrogen as dilution air into the combustor for NOx reduction. The GE Texaco gasifier receives fuel as a pulverized coal slurry. It is a two stage gasifier with radiant and convective cooling of the syngas but for this design only the radiant cooler is used. The fuel is a British coal from Bilston Glen colliery, which like Pittsburgh 8 is a high volatility bituminous coal with a calorific value of 26574 kJ/kg but with 11.5 per cent moisture, 0.76 per cent sulphur and 5.8 per cent ash. The heat recovery streams from the gasifier coolers are connected between the high pressure economizer outlet and the evaporator drum, effectively acting as an additional evaporator augmenting the heat recovery boiler’s evaporator.

The resulting design has a gross output of 462 MW at 51.6 per cent efficiency on the generator terminals, but the net output is near to that of the natural gas fired plant at 409 MW with net LHV efficiency of 45.7 per cent from coal to electricity.

A second example, a 60 Hz design based on the Siemens 2-on-1 reference plant which uses the Conoco/Philips E-Gas gasifier also with a two-pressure steam cycle sends no air to the ASU and receives back 20 per cent of the available nitrogen. But with two gas turbines and a larger steam turbine the gross output is 704 MW at 50.04 per cent efficiency and 621.8 MW net at 44.15 per cent LHV efficiency from coal to electricity.

Design for the future

With much of the world’s coal fired generating plant fast approaching the end of its life, and most of it operating at an efficiency of 35 per cent or less, IGCC clearly has the potential to offer an acceptable coal-based energy scheme with higher efficiency and lower emissions. And, with costs approaching those of supercritical coal fired generation, support for such clean coal generation technologies is unlikely to wane anytime soon. The tools are now available to study and plan such new systems and while the final judgement will always be made on the basis of the cost of operation and maintenance – and its effect on the price of electricity so produced – efficient design tools will inevitably bring this cost down still further.