burns waste gases to generate on-site power
Steel plants often produce one or more combustible ‘waste’ gases that can be put to use on site to generate electricity. GE is expanding its large frame gas turbine product line to burn these, often ultra-low calorific value, gas fuel blends – as this application in China illustrates.
Strong growth in the steel industry in recent years and mounting concerns globally over climate change are driving steel mill operators around the world to install more reliable and fuel-flexible on-site power equipment to boost steel production capacity while also reducing the environmental impacts of their manufacturing activities.
A case in point is the Wuhan Iron & Steel Group Corp (WISCO) steel mill near Wuhan City in Hubei Province, China. Founded in 1958, WISCO company’s main steel products include hot rolled coils/sheets, hot rolled section steel, hot rolled heavy-duty rails, medium plates, cold rolled coils/sheets, galvanized sheets, tinned sheets, cold rolled (oriented and non-oriented) silicon sheets and high speed rolled wires. The firm is China’s third-largest steel producer based on output.
To comply with China’s goals to reduce energy consumption and emissions, WISCO installed a combined-cycle power plant – powered by two GE Energy 9E gas turbines – at the Wuhan mill. The first phase of the plant entered commercial service in December 2009 while the second phase began operating in August 2010.
Reusing the mill’s own ‘blast furnace’ and ‘coke oven’ waste gases (BFG and COG) as ‘free’ fuel, the two GE 109 combined-cycle systems each generate 164 MW of on-site power to support the mill’s activities. Currently, the power plant’s annual output is 1 TWh/year, with a guaranteed electrical efficiency greater than 42% (LHV).
Key benefits of this project for WISCO include lower emissions from waste gases created during the steel production process and new revenues generated by the sale of some of the power plant’s electricity to the local grid. Also, steel mills typically buy a great deal of power from the public grid for production each year. Implementing BFG-fired power generation solution is helping WISCO realize significant savings compared with buying power from the public grid and may offer additional benefits from CDM (Clean Development Mechanism) emissions trading.
‘As we evaluated our technology options, we decided that GE’s combined-cycle technology would be the best fit to help us achieve our operational goals, since gas turbines offer better efficiency and lower NOx and CO emissions than steam boilers and other alternatives,’ said Liu Handong, Wuhan Iron & Steel Corporation technical director. ‘GE rose to the challenge of meeting and even exceeding its contractual commitment with its robust, reliable energy solution that maximizes the use of our mill’s waste gases. This has the combined benefits of reducing emissions and enhancing our revenues.’
|Night-time view of the Wuhan plant, which burns blast furnace and coke oven waste gases to generate 164 MW of on-site power|
Approved under China’s 11th Five-Year Plan as an energy saving and environment protection project, WISCO’s alternative energy combined-cycle power plant also received support from the World Bank.
After Wuhan signed its combined-cycle power plant order with GE in September 2007, GE shipped its components to China between late 2008 and early 2009. Following installation and commissioning, the plant’s first combined-cycle unit began operating in December 2009 and the second entered service the following August.
The order represented GE’s first BFG-fired combined-cycle power generation project in China. For the power plant featuring two 92 MW power trains, GE’s scope of supply consisted of:
- the two 9E gas turbines and two 9A5 generators supplied by GE Energy;
- MCL 1805 and MCL 1407 fuel gas compressors supplied by GE Oil & Gas; and
- fuel cleaning equipment.
GE also supplied training, technical advisory services and performance testing on its systems. GE’s flexible configuration allowed the customer to supply local combined-cycle boilers and a steam turbine to achieve 164 MW per train. Featuring GE’s new 9E power train for 50Hz BFG applications, the Wuhan power plant leverages a proven gas turbine/generator/fuel gas compressor power train configuration that was originally developed in the 1990s and first deployed in Italy and later in China.
The heart of the power train is the frame 9E gas turbine, GE’s 50 Hz industrial gas turbine workhorse with more than 530 operational units, 23 million operating hours, and a typical rating of 126 MW on natural gas. More than 50 of the 9E units have been installed in China in support of the country’s energy supply and environmental goals.
Designed to offer customers fuel flexibility, the 9E gas turbines at Wuhan required a few modifications to burn Wuhan mill’s ultra-low heating value BFG gas – see Figure 1. Principal hardware changes to the core engine are limited to the combustors, and an enlarged throat area for the stage one stationary nozzle segments.
Compared with plants that use traditional fuels, BFG-fired power plants require a larger volume of fuel to run through the gas turbine expander due to the relatively low heating value quality of the gas. The first-stage turbine nozzle (S1N) has therefore been enlarged and the gas turbine also makes use of uprated, third-stage turbine nozzle (S3N) and third-stage bucket (S3B) components that provide superior performance in high-flow conditions.
The fuel gas compressor is double ended to handle the large compression loads associated with compressing ultra-low heating value fuel. The fuel compressor is a non-geared fully centrifugal two-casing MCL unit from GE’s Oil & Gas division, a proven product developed for the stringent reliability requirements of the petrochemical industry. The large centrifugal compressor provides robustness for gas variation and low life-cycle cost due to its full-life hardware and long inspection interval with no blades to be replaced.
The combustion system employs the multi-nozzle quiet combustor (MNQC), derived from decades of syngas experience, and validation testing at both GE Energy’s combustion development facility in Greenville, South Carolina, US, and at field installations – see Figure 2.
The fuel used is a mixture of BFG and COG and can reach a typical heating value of 1050 kCal/Nm3 for improved performance, or 950 kCal/Nm3 for reduced COG consumption. Typical composition of BFG is ~2.5% hydrogen, ~25% CO and inert gases – mainly CO2 and nitrogen. COG is made up of ~60% hydrogen and ~20% methane, with 20% comprising CO, CO2 and nitrogen.
|Figure 1. Hardware modifications to burn Wuhan mill’s ultra-low heating value BFG gas|
The environmental benefits of GE’s combined-cycle technology include a reduced emissions profile of 25 ppm oxides of nitrogen and 25 ppm for CO.
|Figure 2. Multi-nozzle quiet combustor cross section|
BUILDING ON 50 YEARS’ EXPERIENCE
To date, there are more than 40 GE turbines operating on low-BTU fuels and these turbines have accumulated nearly 2 million operating hours, with 18 of them in steel mills.
Before Wuhan, another site in China was completed in 2008 featuring a 9E multi-shaft combined-cycle power plant using Corex gas. This medium heating value off-gas fuel from the Corex steel making process comprises about 20% hydrogen, 45% CO and 30% CO2, having a typical low heating value of about 1950 kcal/Nm3. The plant had nearly run for 24,000 hours on Corex gas by mid-2011. GE’s latest steel mill waste gas project announced in March with Handan Iron and Steel Group also features a single 109E multi-shaft combined-cycle power plant similar in design to the Wuhan plant.
These projects stand as examples of a growing trend, in China and around the world, as steel mill operators seek new and innovative ways to reduce emissions and increase their on-site power generation capabilities.