Commercial Power Production Based on Gasification
Combustion (gas) turbines are used to convert a fuel gas' chemical energy into electricity. They accomplish this by first compressing combustion air through axial, bladed compressor stages and passing it at high speed and pressure into a combustion zone, where fuel gas is injected or otherwise mixed in, and where combustion of the fuel gas produces a high temperature, high pressure stream of exhaust gas. The exhaust gas is expanded in an axial bladed turbine, spinning the turbine shaft which drives a generator to produce electricity. (Also some mechanical work is transmitted to drive the coaxially mounted air compressor stages at the front of the turbine). 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 the substantial quantities of 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 explanation for water-gas shift). This increases the concentration of CO2 for easier removal, but also increases the concentration of H2, which results in higher combustion temperatures than unshifted syngas or natural gas, in turn tending to cause more NOx formation and higher operating temperatures of turbine blades. Other characteristic differences of hydrogen fuel from conventional gaseous fuels exist (see discussion below). These challenges provide the impetus for H2 turbine development. With its successful development, IGCC-based plants equipped for CO2 capture can reach the ideal of a near-zero emission, coal-based power plant.
DOE R&D: 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 targets of the Hydrogen Turbine Program for 2012 were:
- Demonstration of a hydrogen-fueled combined cycle gas turbine (previously fueled with syngas) that maintains the same CC efficiency performance improvement realized in 2010 (2–3 percentage points above the baseline).
- Reduce NOx exhaust emissions to 2ppm in CC, and single digit NOx emissions in simple cycle exhaust, at 15% oxygen.
- Reduce capital costs of the power generation system in IGCC by 20-30%
Currently, the Hydrogen Turbine Program is focused on further advancements to turbine technology to attain the ultimate performance targets for IGCC power plants with CCUS. By 2015, the program plans to demonstrate:
- Hydrogen-fueled turbines with 3 to 5 percentage points improvement in CC efficiency (total above baseline).
- At least 4 percentage points improvement to the overall IGCC plant performance with CCUS.
- Competitive cost of electricity for near-zero emission systems.
- Hydrogen-fueled IGCC with 2 ppm NOx at the power plant exhaust.
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. In recent work, GE has been improving fabrication methods for advanced high-hydrogen premixing fuel nozzles; testing demonstrated high operability on hydrogen fuel, and showed a further reduction in NOx emissions. Improvements made to the high-hydrogen combustion system in the areas of aerodynamic, thermal, and mechanical design were targeted at further reduction in emissions, robust operability, better part life, and reduced costs. Testing is being performed at a full chamber combustion test rig in GE Energy's Gas Turbine Technology Laboratory in Greenville, South Carolina. The system has now been operated for more than 100 hours in the lab at full gas turbine conditions (in excess of F-class) on H2 fuels. In ongoing work, the main focus will be on (1) expanding demonstrated performance to the 2015 conditions, (2) further reducing NOx emissions, and (3) further addressing requirements such as reliability, manufacturability, and durability. A full summary of accomplishments and planned activities is in the General Electric Hydrogen Turbine Development Project factsheet.
Siemens Energy, Inc.
Siemens Energy's goals are similar: increase efficiency, lower risk by taking a platform and proven component approach, and design flexibility. In past years, 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.
Under current funding through the American Recovery and Reinvestment Act, Siemens Energy is focusing on turbines specifically designed for operation on hydrogen and syngas fuels derived from industrial processes that capture a large percentage of CO2. In latest progress, prototype parts are being produced using novel manufacturing techniques, and tested in a full-scale engine at the Siemens Berlin Test Facility. Modular vanes, produced using new manufacturing techniques, are undergoing a two phase test. Innovative advanced cooling designs have been successfully manufactured into Row 1 blades and undergone full-scale engine testing, and local metal temperature reductions were predicted relative to current production parts. Future activities and more detailed program notes can be found in the Siemens Energy Hydrogen Turbine Project factsheet.