Pumps Pass Endurance Test in Pressurised Coal Gasification Process

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...the pumps’ hydraulic systems and geometry, particularly of the impellers, had to be carefully fine-tuned to the job. Moreover, all pumps had to have as much free passage as possible and impart as little shear as possible to the pumped fluid. “The gas liquor,” Wallner describes the difficulty involved, “contains sulphur and ammonia as well as dissolved oils that would tend to become so homogeneously distributed at elevated rotational speeds that they could no longer be separated out.”

Pumps by KSB also serve in the downstream subprocesses. All in all, 40 KSB pumps are employed in the core process, and another 60 units in such peripheral processes as coal handling, air separation and coolant circulation in the DRI process. The pumps’ nominal diameters range from DN 65 to a gigantic DN 350.

All From a Single Source

In addition to supplying the pumps and the auxiliary systems, the Pegnitz-based specialists were also responsible for sizing and selecting the motors and frequency inverters — and those side shows were not to be underestimated. For example: ambient temperatures of around 50 °C made it necessary to install the inverters in air-conditioned cabinets away from the plant. That put the extremely long distance of 450 metres between the pumps’ motors and the inverters. “This,” says Wallner, “gives rise to substantial EMC effects.” The job of connecting up the aluminium cables also posed quite a challenge, because they have much larger cross-sections than the customary European copper cables.

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Background

Coal Gasification — a Process with Perspectives

All in all, 149 plants with a total of some 430 gasifiers are in service around the world producing nearly 60,000 MWth worth of synthesis gas — and more are coming online all the time. While the synthesis gas produced in Angul is used for making pig iron, pressurised coal gasification is also attractive for use in fuel production in so-called coal-to-liquid plants, and for power generation in integrated gasification combined cycle (IGCC) power stations. The latter technology is generating lots of interest, because it produces electricity in a more ecologically viable manner than in conventional pulverised coal-fired plants. In addition, the environmentally relevant CO₂ can be captured at low cost. Finally, pressurised coal gasification plays a major role in the production of biofuels.

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Technics in Brief

The pressurised coal gasification process in detail

Lumpy coal and other solid fuels can be gasified in counter-current fixed-bed pressure gasifiers. The vertical cylindrical reactor has an external water jacket. The coal is fed into the gasifier from above, while oxygen and steam are blown in from below. A rotating grate ensures uniform distribution of the coal. The coal is dried and the gases desorbed in the upper part of the gasifier. The reaction takes place at the next-lower level, where high temperatures and pressures act on the coal in a reducing atmosphere to produce synthesis gas consisting of CO, H₂, small amounts of CO₂ and methane. Gas purification and desulphurisation equipment is therefore needed. The main advantages of this method are its reliability, relatively low initial cost and long-term operational reliability. Moreover, coal of inferior quality can be gasified. The only drawback is that the raw gas contains tar and that the throughput is limited by the maximum dimensions of the gasifiers. Hence, the only way to increase the capacity is to install additional gasifiers. However, new approaches are already in the offing.

Prospects

Coal gasification has lots of potential, and it opens up new perspectives independent of fossil gas and oil as the prevalent energy sources. To be cost-effective, the relevant facilities must operate uninterruptedly for years on end. That, in turn, calls for robust, reliable systems consisting not only of the pumps themselves, but also of things like frequency inverters and mechanical seals. Thanks to KSB’s long-established know-how in coal gasification applications, the pumps’ design and manufacture have been going smoothly. The first few pumps are already demonstrating their reliability on KSB’s own test rigs in Pegnitz, and all of the pumps will have been delivered to the customer on time by early April 2011. The entire plant is scheduled for commissioning in 2011. Related projects — like the installation of additional pressure gasifiers to increase the plant’s capacity — are already being planned. Again, the KSB engineers’ full range of know-how will be needed.

* The author works as a Technical Journalist.

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