Gasification Systems: Winning popularity

Norman Z. Shilling, GE Power Systems, Schenectady, NY, USA
Douglas M. Todd, Process Power Plants LLC, Galway, NY, USA

Gas turbine technology development has hugely contributed to the increased use of Integrated Gasification Combined Cycle (IGCC) technology worldwide. Along with environmental factors and gasification processes, this development will continue to lead the way towards improving performance and lowering costs.

Figure 1. Economic impact of candidate design improvements
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Several specific areas of development have resulted in 24 IGCC plants currently on line or in construction. Major advances have been made in combustion and environmental validation, systems operability design and controls, improved ratings and performance, and reliability, availability and maintenance (RAM). New records have been set yearly in each of these areas to extend the viability of IGCC technology.

General Electric (GE) has been at the frontline in developing new projects and making technological advances. Recent highlights include the Motiva-Delaware 9 ppmvd NOx, 240 MW plant running on Pet Coke which first used syngas in September 2000; the Sarlux 540 MW Vis breaker tar plant which first used syngas on August 13, 2000; and the Exxon Singapore 180 MW which has now completed start-up. Notably, this plant has already operated 500 hours on distillate oil, which is one of 42 different fuel combinations it is designed to handle on the fly.

Additionally, four new projects have been awarded totalling 1995 MW. These plants will be put into operation in 2003, 2004 and 2005. The Gonfreville plant is a tri-fuel dual gas design operating at a rating 20 per cent above natural gas operation. Piemsa will operate at a rating 14 per cent higher than natural gas while simultaneously providing partial air integration with the air separation unit (ASU). Three confidential 7FA units will use the next generation design based on Motiva-Delaware at a rating eight per cent higher than natural gas with partial air extraction.

The technology for each of these plants has been developed step by step in cooperation with other industry suppliers, illustrating how IGCC is a technology partnership rather than a simple grouping of gas turbine, ASU, and gasifier equipment.

System validation

The core of IGCC design is based on combustion development through laboratory testing. With the potential to be the process bottleneck in the plant, combustors must be designed for a wide range of operating conditions with backup and co-firing fuels. They also have to meet ever-tightening environmental requirements.

Experimenting with these parameters in the field can be very costly because they can affect all the related technologies. Typical gasified syngas outlet constituents are not suitable for meeting today’s NOx requirements. However, dilution of the syngas with nitrogen, water, CO2 or combinations can be used to meet the desired operating conditions. By combining combustion technology with output enhancement capability, plants can determine the most optimum and economic mix of diluents.

The ability to use nitrogen economically is derived from elevated pressure ASU improvements and can be very important in obtaining flat ratings at high ambient temperatures. Many of these plants produce co-products from the syngas and the gas turbine must be able to accommodate the resultant tail gas. GE has completed testing for hydrogen-only and CO-only fuel to complete the combustion map. The H2-only case is based on removal of CO as CO2 for enhanced oil recovery (EOR) or sequestration and allows CO2-free power plants. Ratios of 50/50 hydrogen/nitrogen can have very low NOx capability as well as enhanced output in a modern IGCC.

With this new combustion technology it may be possible to use IGCC for CO2-free plants at about 15 per cent extra cost.

System operability

GE has worked on both air-blown and oxygen-blown systems. Most of the recent efforts have concentrated on oxygen due to the large size of plants and particularly the co-production of chemical products.

Figure 2. Syngas Reliability/Availability/Maintenance (RAM)
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To achieve optimal results for each system a different type of air integration can be designed. Tampa Polk is designed for air-side integration at zero per cent, which means no air extraction to the ASU but full nitrogen return to the GT. Piemsa is designed for partial integration where some air is extracted for ASU. Sierra Pacific is designed for full integration where all air for the ASU is extracted.

Various gasifiers require from 11 per cent to 20 per cent of the gas turbine air flow for full air integration. Mixed fuel operation or co-firing has become an important operating mode to enhance project economics. It was first used at the Texas El Dorado IGCC plant in Kansas where the gasifier was only a third of the size of the plant electrical load. Since starting in 1996 the Eldrado gas turbine has performed at better than 97 per cent power availability. Many later plants such as Exxon Singapore, Gonfreville and Piemsa also have this feature. For a dual gas design the operability standard is in the 70/30 per cent range. This range is dictated by combustion system controllability requirements at low fuel flow rates.

A tri-fuel gas system has been designed for the Gonfreville plant. This 90/10 system incorporates a single fuel nozzle for natural gas operation and splits the fuel flow into all six nozzles in each combustor on the syngas side of the bumpless transfer zone. Both nitrogen and air from the compressor discharge (CPD) are used to purge during various operating modes.

In keeping with the gas turbine packaging concept the complete system is modularized. Both NOx and power augmentation benefits can be realized by using diluents to lower the heating value of the syngas.

Figure 3. GE “F” Class combustion testing
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For example, nitrogen can be injected into the combustor in the same manner used for steam injection rather than mix it with the fuel. In this manner the nitrogen pressure can be lower than the syngas pressure and the fuel control valve can be reduced to half size since they are controlling only the fuel.

Combinations of N2 and moisture are frequently the most cost effective, considering the aero limitations in a standard gas turbine for additional flow. Another area of concern to the operator is the method of overall control. IGCCs can be designed to follow the gasifier and use only the syngas fuel that is made. They can also be designed to meet electrical load demand either by forcing the gasifier to follow the demand or by co-firing the back-up fuel. The latter is more frequently used. A part of the design normally incorporates a gas turbine simulation for the benefit of the team.

Improved performance

For natural gas machines, normal gas turbine full load operation over the ambient range follows air density and falls off considerably at high ambient temperatures. These can be counteracted with IGCC designs.

GE frequently designs the system to utilize the full winter capability, even at 32à‚°C (90à‚°F). In some cases this can result in an extra 43 per cent power in the gas turbine. An important new system developed over the past year incorporates the latest gas turbine improvements for a High Efficiency Quench (HEQ) design. This plant incorporates all the features developed from previous plant and includes an additional output enhancement of an expander turbine in the fuel system. It provides an additional four per cent output.

RAM performance

Cost of electricity is affected by reliability availability and maintenance (RAM) performance in a manner similar to plant cost and fuel efficiency. Generally, RAM performance is improved during the design stage by incorporating lessons learned in previous plants. Gas turbine suppliers can do some of this based on their experience in other fields but it is very important to have IGCC experience.

The lessons learned have been reported before and incorporated in all current designs. The Tampa plant has reported that power availability is running at 96 per cent. The fleet leader is El Dorado, which has a power availability average of 97 per cent for the last four years. From this experience GE concludes that a syngas combined cycle can have the same performance as a natural gas combined cycle, although several specific conditions first need to be met by the supplier.

Experience has shown that higher hydrogen content, which produces more water, and the increased flow of syngas tend to increase metal temperatures in the hot gas path. GE has developed a control system to mitigate this effect by lowering firing temperatures to maintain metal temperatures consistent with natural gas machines. On this basis GE has been willing to provide long term service agreements (LTSA) at a cost basis similar to natural gas machines along with availability guarantees. Many current IGCC plants have found this to be a cost-effective way to make IGCC viable.

Current production gas turbines can be used for IGCC applications but some modifications are desirable to enhance the IGCC economics. Gas turbine improvements have come from operations in many applications and from industry competition. In addition, specific attention to IGCC needs in an emerging market hasgenerated a host of new technology needed for IGCC viability. When combined with the same type of efforts by ASU, cleanup, and gasification technologies, we can see an exciting trend.

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