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Optimising coal combustion

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World Coal,

Carbolite Ltd, UK, describes the use of coal ash fusibility furnaces to help optimise solid fuel combustion in electricity generation.

Reliable and adequate power generation is a cornerstone of every industrialised country: an essential ingredient for successful economies and comfortable lifestyles. While other technologies continue to be developed, solid fuels are still extremely important, accounting for 40% of electricity production in the UK in 2013, for example.

The propensity of coal to cause problems, such as slagging and fouling, in the furnaces is well known and power generators need to understand the behaviour of coal during combustion in order to optimise the performance of their plant. Recently, in pursuit of a greener fuel, biomass (vegetable-based material) and recovered fuels have been increasingly used, and these materials must be tested for the same reasons.

Carbolite coal ash fusibility (CAF) furnaces

Ash is the remaining residue after the combustion of coal under specified conditions (ASTM D-1374 and ISO 1171) for which Carbolite offers a dedicated range of ashing furnaces.

Slagging, fouling and clinkering difficulties have been found to correlate with the fusibility of the ash. In the ASTM D-1857M-04 (2010) and ISO 540:2008 tests for the fusibility of coal ash, the temperatures are observed at which triangular cones prepared from the ash begin to deform and then pass through specified stage of fusion when heated at a specified rate. The test procedure provides for performance of the test in a controlled atmosphere.

The Carbolite CAF digital furnace is designed to provide a complete integrated solution for assessing coal and coke ash fusibility in accordance with the above standards. It also meets the recognised standards required for solid recovered fuels and solid biofuels, for which ISO, BS or ASTM standards have not yet been finalised.

Ash is mixed with dextrine to form test pieces. Several test pieces are then positioned in the furnace on a sample stand, which allows them to be viewed by a digital camera through a porthole in the furnace door. During the automated process, which doesn’t require the attendance of an operator, the furnace is initially purged with nitrogen. The samples are then heated in reducing or oxidising gases, at a constantly increasing temperature from 815°C to a maximum of 1600°C.

The process is recorded as a series of digital images that are saved to a PC, together with the temperatures at which specific physical changes take place:

  1. Deformation temperature: when the corners of the test piece first become rounded.
  2. Softening (sphere) temperature: when the top of the test piece is spherical. This is the critical temperature most commonly referenced in the evaluation of coal ash properties.
  3. Hemisphere temperature: when the entire test piece takes on a hemisphere shape.
  4. Flow (fluid) temperature: when the molten ash is flat on the sample tile.

When the test is complete, the images are viewed to identify at what temperatures the changes took place and to archive the appropriate files.

  • Up to 12 samples can be tested simultaneously and sample loading is designed to be as convenient as possible Carbolite supplies all necessary sample holders, tiles and loading tools.
  • The temperature ramp rate is user defined as is the image capture rate, up to a maximum of one image per °C.
  • The software supplied is Windows 7-compatible and allows operators to generate test reports of the transition temperatures.
  • Gas systems can be specified to suit different reducing, oxidising or purging atmospheres as defined by the test standards.

Testing low carbon alternatives

Biomass obtained from plant material is viewed as having a critical role in the transition to a low-carbon economy in line with reducing carbon dioxide emissions. Burning biomass in place of coal is calculated to reduce CO2 emissions by as much as 80% over the full life cycle of the fuel.

Drax power plant in North Yorkshire was the largest coal-fired power plant in the UK when it was completed in 1986. Total generating capacity when burning coal is 4 GW. It is currently transforming its generating process by converting three of its six generating units to burn sustainable biomass in place of coal. The first unit has been running successfully since April 2013, the second is due on stream in 2014 and the third in 2016. It will then be the largest single electricity generator using renewable fuels in the UK.

The biomass burned includes waste forestry and timber products, straw, seed husks and purpose-grown energy crops such as oil seed rape. When all three converted units become operational, they will use 7 million tpa of biomass.

As with coal, biomass is assessed for its ash fusibility to allow it to be burned at maximum temperatures while maintaining ash flow. Specific adaptations to the Carbolite CAF furnace allow tests to start at 550°C, which is the starting temperature for ash fusibility for biomass.

Written by Carbolite Ltd. Edited by

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