Gasification Research & Development
Combustion (gas) turbines are used to convert a gas’ chemical energy into electricity. They accomplish this by first compressing the gas and passing it at high speed and pressure to a combustion system. The gas is then combusted, producing a high temperature, high pressure stream. The gas expands, driving a turbine by spinning the turbine blades. This spinning drives a generator to produce electricity. In smaller plants, a recuperator captures waste heat exhaust and uses it to preheat combustion air and boost efficiencies. In larger plants, a "heat recovery steam generator" (HRSG) is installed to generate steam for a steam turbine-generator from waste heat. This configuration is called a "combined cycle." Gas turbine and HRSG are integrated components of an integrated gasification combined cycle (IGCC) system. An IGCC plant equipped for high level carbon dioxide (CO2) capture typically “shifts” the synthesis gas (syngas) composition through the water-gas shift (WGS) reaction, where carbon monoxide (CO) and steam react to form hydrogen (H2) and CO2 (for more on this reversible, equilibrium reaction, see the section on Water-Gas Shift). This increases the concentration of CO2 for easier removal, but also increases the concentration of H2, providing the impetus for H2 turbine development. With its development, IGCC equipped for CO2 capture can be a near-zero emission, coal-based power plant.
Advanced Turbine Program
Research is being performed by the Advanced Turbines Program at the National Energy Technology Laboratory (NETL) with an objective to design and develop a fuel-flexible (coal-derived H2 or syngas) advanced gas turbine for IGCC applications that meets DOE turbine performance and CO2 Capture and Sequestration (CCS) goals. Hydrogen turbines face more significant challenges for combustion than that for hydrocarbon fuels (like natural gas). Hydrogen fuel varies in a few important ways:
- Lower density and energy density than other gases
- Diffusivity is much higher than other gases
- Hydrogen has a wide range of volume concentrations over which it is flammable
- Laminar flame speed of H2 is much higher compared to other gases
Turbine performance and design is based upon accurate models of flame characteristics and heat and air flow. Cooling of turbine components (like gas nozzles and the turbine blades) is important to prolong the life of the turbine and minimize downtime. An accurate model of temperatures in the turbine guides cooling system designs. Hydrogen and mixtures of gases (like syngas) deviate from established models built on hydrocarbon fuels and have limited data available for modeling. In addition, variation in fuel composition can be high between plants, gasifiers, and even feedstocks, dictating the need for more flexible turbine design.
Areas for turbine research and development include improving modeling, combustor technologies, materials research, enhanced cooling technology, and coatings development. These improvements will help to meet goals of the Advanced Turbines Program:
The first targets of the turbine program (operating on syngas) are to:
- Increase turbine efficiency by 2-3% over the current baseline achieved by the F-frame turbines at the Tampa Electric and Wabash River IGCC facilities
- Reduce NOX exhaust emissions to 2ppm at 15% oxygen
- Reduce capital costs of the power generation system in IGCC by 20-30%
By 2012, the targets of the Turbines Program (operating on syngas) are to:
- Develop a hydrogen-fueled turbine that can integrate with carbon capture systems (water-gas shift increases H2 and CO2 concentrations in the fuel)
- Demonstrate fuel flexibility, allowing for operation on conventional syngas or 100% H2 (on a heat input basis)
- Maintain efficiency gains (2-3%) of the previous program goals
- Reduce NOX exhaust emissions to near-zero
- Maintain capital cost reductions (20-30% over current baseline)
- Reduce the cost of CO2 compression by reducing auxiliary power needs by 30-40%
Two major turbine manufacturers are working in partnership with NETL, General Electric and Siemens Power Group. The goals for these two projects are the same: to develop a fuel flexible (syngas or hydrogen) gas turbine for IGCC applications that are capable of 45-50% HHV plant efficiency, near-zero emissions and competitive capital costs.
GE’s program aims to adapt their turbine technology (like the 7FB natural gas turbine) for firing high-hydrogen fuels. As of December 2006, GE has tested an advanced combustion concept for their turbines which showed “strong operability” and significantly lower NOX emissions. The project has constructed models for several turbine designs and completed a study of the impact on the turbine of 90% carbon removal. A full summary of accomplishments and further planned activities is in the General Electric Hydrogen Turbine Project factsheet.
Siemens Power Group’s goals are similar: increase efficiency, lower risk by taking a platform and proven component approach, and design flexibility. As of December 2006, Siemens has designed models for syngas and hydrogen-fueled variants of existing SGT6 turbine models. Studies have been done for CO2 capture and overall plant costs as well as experiments on turbine parameters, aerodynamics, diffusion flame combustion, and turbine cooling. Future activities and more detailed program notes can be found in the Siemens Power Group Hydrogen Turbine Project factsheet.