There’s a reason why China has become known as “the world’s factory.” The country now manufactures 20% of the world’s goods by value, and its exports are now worth around $2.5 trillion per year. But the country’s recent explosive economic growth has come at a cost: Emissions from its power plants contribute to smog thick enough to block out the sun in many cities. In 2017, the government-owned newspaper China Daily reported that a sharp increase in lung cancer in the country, especially among women and nonsmokers, could be related to long-term exposure to air pollution.
It’s no surprise that the Chinese government is trying to clean up the air with smog-busting programs like the Blue Sky plan, which aims to quickly cut pollution in China’s largest cities. One of the targets of the plan is nitrogen oxide (NOx), a pollutant emitted by power plants that burn fossil fuels. Several gas-burning power stations in the booming city of Shenzhen are soon expected to meet the local government’s tough new NOx standards, thanks to an upgrade to the combustion systems of their GE 9E gas turbines. The upgrade consists of new hardware augmented with smart software.
Those recently upgraded heavy-duty 9E gas turbines are owned by five utilities in Shenzhen. The upgrades are expected to cut their annual NOx emissions by up to 80%, which would help drive the NOx emissions from the power plants well below the Blue Sky limit of 7.5 parts per million. The reduction is equivalent to the annual NOx emissions from hundreds of thousands of gasoline-fueled cars.
The five power generation enterprises — Shenzhen Nanshan Power Co., Shenzhen New Power Industry Co., Shenzhen Datang Baochang Gas Power Generation Co., Shenzhen Yuhu Power Co. and CNOOC Shenzhen Power Co. — will be able to keep hundreds of megawatts of gas-fired capacity online, says Zhaosheng Li, a senior manager at GE who worked on the project. The generators also met the city’s Oct. 31 deadline to lower their NOx emissions with several months to spare. “It helped that we were very quick in providing our customers with the solution: It took us less than six months from receiving the request to complete the project.”
Li explains that Shenzhen’s power producers face strict and firm requirements to lower the NOx emissions within the target deadlines. In the worst cases, authorities can suspend operations of a power plant or completely shut down the facility.
The people who live in and around Shenzhen, an industrial dynamo located in China’s heavily populated Pearl River Delta region, will enjoy cleaner air, says GE Power’s Bruno Monetti, who has overseen strategy for the 9E gas turbine for eight years. He explains that NOx reacts with volatile organic compounds in the air and sunlight to produce ozone. Unlike the good ozone found in the earth’s stratosphere that soaks up the sun’s powerful ultraviolet rays, the bad ground ozone can trigger many health problems. “A reduction in NOx will cut ‘bad’ ground-level ozone, which can inflame and damage the airways.”
Monetti says the secret behind lowering the gas turbine’s NOx emissions involves reducing the temperature of the flame inside the combustor. A leaner fuel mix, whereby the natural gas burns in a higher volume of air, can deliver this cooler, low-NOx flame, he explains.
However, there’s a catch: The leaner mix has to be made up in the turbine’s fuel nozzle before igniting and burning the mixture. It’s similar to a carburetor that mixes the air and fuel for injection into a car’s engine. It’s a delicate balance. If there’s too much natural gas in the mix, the flame will be too hot. If there’s too much air, the flame could blow out, like a birthday candle.
Engineers improve this air-to-fuel ratio with modifications to the turbine’s combustion hardware. This includes the addition of a venturi shape to the liner, the pipeline that feeds natural gas into the front end of the turbine’s combustor. A venturi looks like the pinch point in an hourglass, and it restricts the flow of natural gas into the mix. “This is the ‘pièce de résistance’ in terms of hardware,” Monetti says. He explains that the venturi is dual purpose: It tightly regulates the entry of natural gas in the air-fuel mix, and also prevents backdraft, or the flame blowing back into the front end.
Engineers have also punched a pair of dilution holes — which pump in compressed air to achieve a leaner fuel mix — farther downstream to what engineers call a transition piece that links to the combustion chamber. This allows for the last-second addition of air to obtain the optimal temperature before the hot gases are burned. It’s almost like having a soda fountain on hand to perfect the spritzer just before serving.
The hardware is complemented by software and human intelligence. Monetti says that software, Corrected Parameter Control (CPC), gathers mountains of data about the turbine’s environment. This includes ambient humidity and temperature readings, as well as inlet and exhaust pressure, which are all factors that can affect the production of NOx and other pollutants. These inputs are always changing, so the software is constantly correcting the air-fuel mix to deliver the lowest-possible NOx emissions.
The upgrade, DLN1+ in GE parlance, where the acronym stands for “Dry Low NOx,” dramatically reduces NOx emissions. The previous versions of the 9E turbine models, which were upgraded to the older DLN1 standard, emitted 15 parts per million (ppm) of NOx. But the DLN1+ upgrade, which is receiving its first global application in China, can bring that down to 5 ppm, Monetti says. That is comfortably below the Chinese limit of 7.5 ppm.
It means the annual NOx savings from the nine upgrades will run into the hundreds of metric tons compared to the previous models in operation. The 9E DLN1 turbine emitted approximately 204 metric tons of NOx emissions per year from 6,000 hours of operations. The DLN1+ upgrade should bring the annual NOx emissions of all nine turbines below 100 metric tons per year.
China’s environmental drive demands serious action from cities such as Shenzhen, because the Blue Sky plan focuses on heavily populated regions that propel the country’s mighty economy. Yang Dan, the president of GE Power China, says the city’s inhabitants can look forward to cleaner air and seeing more sunshine.
“We admire the Shenzhen Municipal Government’s adherence to the greener and low-carbon development and its efforts made to improve the living standards of its citizens,” she says. The project will help to protect Shenzhen’s “magnificent blue skies,” she adds.