Sustainable Power Generation: Coal Gasification

ByBernd Müller - Siemens, Pictures of the Future, siemens.com/pof

Raw Material Production instead of Power Generation: Ninxia Coal-to-Polypropylen-site (NCPP).

China is facing a dilemma: on many days, the air quality in cities like Beijing is so bad that it doesn’t seem possible to stay outdoors for long without a breathing mask and not suffer health effects. President Xi Jinping has promised that China’s CO2 emissions will decrease beginning in 2030. But the groundwork for that change has to be laid now. Whether or not the country can achieve this target depends largely on decisions made regarding coal, the fuel that still accounts for about 70 percent of China’s electricity and is the main culprit behind the country’s huge environmental problems. Although coal’s share of China’s energy budget is supposed to drop to 52 percent by 2030 in favor of renewables, nuclear power, and natural gas, the amount of coal burned — not only for electricity generation, but also for the production of raw materials for industry —is expected to continue increasing.

More coal, fewer emissions — just how to solve this dilemma is a question driving a great deal of research and development work, and Siemens, as a leader in efficient and therefore also low-emission technologies, is part of these efforts. The best solution is probably coal gasification, an established technology based on a simple principle: powdered coal — in other words, almost pure carbon — is gasified at a high temperature together with water. The result is a synthesis gas of hydrogen and carbon monoxide, which is converted using yet more water into carbon dioxide and more hydrogen. Integrated gasification combined-cycle (IGCC) plants use this process to supply high-purity hydrogen that can be mixed with natural gas in a gas turbine to produce electricity. Here, CO2 has already been separated out, and rather than being released into the atmosphere, it could be injected into an oil field to increase its yield, for example. Electricity generation would then be almost entirely CO2-free – a goal that will be achieved by an IGCC plant under development in the USA: the Texas Clean Energy Project. That plant is expected to produce electricity and fertilizer with gas turbines supplied by Siemens. The CO2 is injected into an oil field.

Coal: The Key Ingredient

The IGCC process is not just suited for the generation of electrical energy; it can also be used to produce chemicals for industrial purposes. It is the ideal way to use different carbon-containing fuels like coal, refinery waste, and even organic waste to produce many different materials — “basically, anything that contains carbon,” says Frank Hannemann, head of Technology and Innovation at Siemens Fuel Gasification Technology GmbH in Freiberg, Germany. Plastics, methane (natural gas), methanol, diesel, fertilizer — there are almost no technical or chemical limits to what can be produced from coal.

China is already taking this approach in many of its facilities. This is because electricity generation using coal gasification is usually the worst option for China, whose greatest coal reserves are located in remote regions like the northern Chinese steppe where there are very few customers for electricity. Together with coal-mining company Shenhua Ninxia Coal Group, Siemens has built a coal gasification plant near the Mongolian border. The plant has an output of five times 500 megawatts. For the five fuel gasifiers at the plant, Siemens is using an advanced design with a pneumatic coal conveyance system and a burner that requires less water to cool the hot synthesis gas and less high-pressure steam to convert the carbon monoxide it generates into hydrogen. All of these features result in greater efficiency and lower CO2 emissions.

Innovations from Siemens Corporate Technology in Beijing

Ready to Use: The SFG-500 gasifier developed by Siemens Fuel Gasification Technology in Freiberg, Germany.

This progress is due in no small part to Dehui Wang, Technical Manager at Siemens Corporate Technology in Beijing. “My team supports Frank Hannemann’s R&D group in developing new concepts of coal gasification for the Chinese market,” she says. Her team oversaw the startup of the first demonstration plant and optimized it. In Wang’s lab, an hour’s drive from her office at Corporate Technology in Beijing, there is an experimental plant that she uses to study the chemical reactions that occur when hot synthesis gas is cooled. Wang is also developing new processes for improving efficiency and minimizing CO2 emissions. Her team has created a new type of shift process that uses water to convert carbon monoxide into carbon dioxide and hydrogen. For the first time, a two-step process is being used, with an inhibiting catalyst in the first step. The catalyst slows down the reaction, causing only a small portion of the gas to be converted in the initial step. Not until the second step does the complete reaction take place with an active catalyst. What gives this solution an edge over other approaches is that, because the process is slowed down, it requires less steam for cooling the hot gas (which has a temperature of up to 1,400 degrees Celsius), and that makes it more efficient overall. Together with Siemens’ Fuel Gasification Business Unit, Wang is now looking for a customer for the first pilot plant in China to demonstrate this new technology.

The facility sets new standards for efficiency. Unfortunately, though, it isn’t as environmentally friendly as it could be. The CO2 is removed, but it is not stored underground. Shenhua Ninxia Group considers this to be too uneconomical. Instead, the carbon dioxide is still being released into the air.

Solar or Wind-Powered Coal Gasification?

Frank Hannemann and Dehui Wang, whose work has secured Siemens several patents and a technological lead, are working on an approach that is downright revolutionary for the coal industry. The idea is based on the fact that the IGCC process is energy intensive and generates a lot of CO2. What if surplus electricity from solar and wind energy were used to power these plants? In such a scenario, the IGCC process would be augmented with electrically powered modules for air separation and electrolysis. The first module would take oxygen from the air. Air separators of this sort have been in use for a long time, and the technology is mature. Electrolysis would separate the water into its constituents, hydrogen and oxygen. Siemens is currently developing modules of this kind. Both gases can be fed into the process, oxygen to convert the feedstock into synthesis gas and hydrogen downstream of the gasifier. Ideally, it would be possible to adjust the mixture so that the shift reaction for carbon monoxide could be eliminated in order to prevent the production of CO2. At the end of the process, all the carbon would then be contained in chemical products such as methane or plastics, while the hydrogen could be diluted with nitrogen and burned in a gas turbine.

A coal power plant that doesn’t generate any CO2, doesn’t have to take the roundabout route of CO2 sequestration, and even functions as a storage system for wind and solar energy? “It’s possible,” says Hannemann, “but it’s still a long way off.”
 

This article is republished by permission from Siemens, Pictures of the Future, siemens.com/pof.

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