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When coal or natural gas is burned with the addition of normal ambient air, a great deal of flue gas is created. These volumes can be reduced by removing the nitrogen present in air upstream from the furnace, leaving only oxygen. Engineers use this principle for the oxyfuel process.
Here, coal is combusted with pure oxygen instead of atmospheric air. To obtain this oxygen, air separation plants are used. Cryogenic (i.e. very low) temperatures are used to separate air into its constituent parts and remove the nitrogen before feeding the oxygen into the furnace where it is used to combust the coal. To regulate the temperature of the flame, most of the flue gas is fed back into the furnace. The advantages of this include a lower flue gas volume because no nitrogen is present, and a high degree of combustion efficiency thanks to the high temperature. In addition, the CO2 generated by the oxyfuel process is relatively pure and can be easily separated from the steam by means of cooling.
The Linde Group and the Vattenfall Europe Technology Research GmbH have created a comprehensive technology partnership to advance the oxyfuel process. The aim of this partnership is to explore the combustion process for lignite and pit coal and advance the technology to industrial-scale maturity. In September 2008, Vattenfall commissioned a 30-megawatt pilot facility on the site of the Schwarze Pumpe lignite power plant in Brandenburg, Germany. Linde supplied many of the components required to build this pilot plant. Linde is also involved in the collaborative OXYCOAL-AC project organised by RWTH Aachen, which is dedicated to investigating and advancing oxyfuel technology and applying it to coal dust combustion.
New procedure saves energy and increases efficiency
A power station’s output normally fluctuates in sync with energy demands. This is not an issue for conventional power plants that are fed with air. In the case of oxyfuel plants, however, the oxygen stream must be aligned with the power cycle as too much of this valuable gas would otherwise be wasted. To accommodate rapid changes in load, Linde engineers have optimised the entire design of an oxyfuel plant and even accelerated the process control technology. Such optimisation measures play to the company’s core competencies – Linde has been specialising in cryogenic air separation for many decades and can deliver mature solutions capable of supplying a sufficient oxygen stream even for large-scale oxyfuel plants. A single line produces 100,000 cubic metres an hour. But an oxygen stream also has a downside. Calculations thus far have shown that oxygen accounts for a third of investment costs in an oxyfuel plant and almost a quarter of total energy requirements. So the ten percentage points or so of efficiency losses associated with Carbon Capture and Storage (CCS) are largely attributable to the oxygen bill.
Linde has responded to this challenge by adapting the cryogenic air separation process for oxyfuel applications that do not require high-purity oxygen. The adapted solution can reduce energy consumption by around 25 percent compared with conventional air separation methods. This gives plant operators a new way to produce oxygen with a reduced energy bill and lower costs. Without this innovation, retrofitting modern power stations with CCS technology would throw the German power park back to efficiency levels typical of the 1960s, for example.
How oxyfuel combustion works
Schwarze Pumpe pilot plant:
Capturing, purifying and liquefying CO2
Vattenfall Europe constructed a pilot facility to separate CO2 using oxyfuel technology at its Schwarze Pumpe power plant site. This is a small-scale demonstration of how CCS will ultimately work for fully ramped up power plants. For each 5.2 tonnes of lignite burnt every hour, the 30-MW facility releases nine tonnes of CO2. The initial test phase commenced in July 2008 and the plant officially went on stream in September 2008. Linde is supporting this project by delivering numerous components and contributing its extensive expertise in cryogenic air separation and CO2 capture (cooling, pre-compression, dehumidification, liquefaction, separation, post-compression). The company also brought its wide-ranging process and design know-how to bear. The Vattenfall pilot plant does not generate electricity, but provides process steam for the power plant.
Contractor - Vattenfall
Location - Schwarze Pumpe / Germany
Processes - Dehumidification, compression, liquefaction, rectification with recycling and liquid storage of CO2
Capacity - 7,000 Nm3/h flue gas from oxyfuel boiler, 240 tpd liquid CO2
Purity - > 99.7 volume % of CO2
Scope of service - Turnkey facility
On stream - 2008