BGL GASIFICATION OF LOW-COST FUELS COMPETITIVE WITH NATURAL GAS COMBINED CYCLE

In the last year a number of significant events have occurred in the gasification technology marketplace. New Integrated Gasification Combined-Cycle (IGCC) projects have come onstream with commercial operation, other new IGCC projects have been announced and started in development, environmental issues have gained emphasis, and energy prices, notably natural gas, have escalated dramatically. Directionally, all of these events appear to have created a more favorable atmosphere for IGCC projects.

Related to an ongoing IGCC project currently in development, a joint analysis has been performed by Global Energy, General Electric Power Systems, and Praxair to evaluate technical and economic elements for the performance of British Gas Lurgi (BGL) gasification technology based on solid hydrocarbon fuel feed to an IGCC for power generation.

Results of the analysis provide a picture of the relative economics in today’s environment for electrical power generation by conventional natural gas-fired combined-cycle power systems compared to using BGL gasification technology in an IGCC configuration. These results were reviewed by R. Olliver of Global Energy at the 2000 Gasification Technologies Conference in San Francisco, California, in October.

A single design case was developed and analyzed for the BGL gasification technology application, considering use of Pittsburgh #8 coal as the solid hydrocarbon feed to the Gasification Island.

The overall project configuration includes the Gasification Island, comprised of the BGL gasification units, Air Separation Unit (ASU), and syngas cooling and cleanup units, and the Power Island, which consists of two General Electric 7FA gas turbine generators and Heat Recovery Steam Generators (HRSG) and one steam turbine, all optimized for firing on syngas, but capable of operation on natural gas. At site design, ambient conditions of 59ºF, 14.28 psia and 60 percent relative humidity, gross and net electrical power output are approximately 586 megawatts and 538 megawatts, respectively, and net heat rate is 8,072 BTU per kilowatt-hour, higher heating value. Plant capital cost is assumed to be $1,000 per kilowatt.

Gasification Island

The BGL gasification process is a fixed-bed gasifier that uses a lockhopper system to admit dry feed to the pressurized reaction vessel. The gasifier units are refractory lined and water jacket cooled. As the feedstock descends it is heated by rising high temperature gases. Moisture and volatile light hydrocarbons leave the coal soon after the feed enters the gasifier unit and exit the gasifier with the syngas stream. Oxygen and steam are injected near the bottom of the unit and react with devolatilized coal to provide thermal energy needed for the formation of syngas components. The high temperature also converts the inert ash content of the coal into vitreous frit or slag.

Power Island

The Power Island is based on two trains of dual-fuel General Electric 7FA gas turbines with hydrogen-cooled generators. Each train is coupled to its own HRSG, which together will provide superheated steam for a single steam turbine generator. The steam enables transfer to natural gas should syngas flow be interrupted. This provides for Power Island availability equal to that of conventional natural gas-fired powerplants.

Prior to entering the gas turbine combustor, the syngas is saturated with water and is then superheated. Additionally, nitrogen from the ASU is moisturized, superheated and injected into the turbine combustor, effectively diluting the fuel to reduce NOx emissions. Saturating the syngas and the addition of saturated nitrogen also increases the mass flow to the gas turbine, resulting in increased electrical power generation.

Results

The results of the analysis are presented in detail in Figure 1 and show that IGCC power generation systems with solid hydrocarbon feeds can be competitive with Natural Gas-fired Combined-Cycle (NGCC) systems. The results show an equivalent Cost of Electricity (COE) for IGCC and NGCC systems at certain natural gas and gasifier feedstock prices. For example, natural gas at about $3.75 per million BTU and coal at $1.00 per million BTU will both yield a COE of $0.049 per kilowatt-hour. While these electrical power prices are not likely to stimulate consideration of the large capital investment required to build a self-sufficient project-financed powerplant, rising prices for natural gas clearly make IGCC increasingly attractive as an option for power generation.

figure1Figure 1


An important factor, which has the potential to directly improve today’s IGCC economics, is the utilization of the BGL gasifier’s ability to handle a wide variety of fuel (feedstocks), including Refuse-Derived Fuel (RDF). For example, a mixture (50/50 by heat content) of coal at $1.00 per million BTU and RDF at $0.00 per million BTU equates to a gasifier feedstock price of $0.50. This places electricity generated from a BGL-based IGCC on par with electricity from an NGCC if the price of natural gas is $3.00. These numbers are within the range of annual average fuel costs considered reasonable by developers motivated to build an electric powerplant, say Olliver et al.

The specific results of the analysis indicate that:

Furthermore, the following prospects have the potential to further improve IGCC economics:


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