Operation of a Coal Fired Power Stations

The coal is delivered to the stockyard from where it is supplied by conveyor to the coal pulverizers. The pulverizers crush the coal to a fine dust, and from here it is blown into the boiler furnace along with the optimum combustion air, is combusted, and the heat energy is used to raise steam in the boiler.

The steam is superheated and supplied to a three stage steam turbine which drives the power generator.

The combustion gasses passed through a gas treatment plant where the SOx, NOx, and particles are removed, with the remaining gasses including CO₂, passing out of the flue into the atmosphere.

Methods Used to Remove CO2 From Coal Fired Power Plants.

There are several methods for removal of CO2,

• Removal before combustion, such as in coal gasification
• Removal during combustion, using an enriched oxygen combustion process
• Removal from the combustion gasses by absorption tower or by using a PTFE micro membrane

In our example of a coal-fired power station we will use an absorber tower process to remove the CO2 from the combustion gasses.

Removal of CO2 Using a Chemical Absorbent

The process of the removal of CO2 from the combustion gasses forms part of the fume treatment unit of a conventional coal-fired power station. This process consists of first removing the NOx, SOx and particles from the gas (see my article on coal-fired power stations) then passing the combustion gasses through a packed absorber tower where the CO2 is removed. Different types of chemical absorbents can be used in the tower, the most popular being monoethanolamine (MEA).

MEA Absorber Tower Operation

The absorber tower is a tall cylindrical pressure vessel, containing several sections of packing. The packing is normally made from ceramic rings, which are inert, providing a good contact between the MEA and the fume gasses. The fumes enter the tower moving up through the packed sections. The MEA is introduced at the top of the tower being dispersed downwards over the packed sections by a distributor plate. The MEA meets the gasses passing upwards which bubble through the packed sections removing the CO2 from the gas which drops to the bottom suspended in the liquid MEA. The resultant liquid mixture then gathers at the bottom of the tower from where it is withdrawn and pumped into the desorber tower.

Desorber Tower

This is another pressure vessel of similar design to the absorber tower including the ceramic packed sections. The liquid from the absorber tower is fed into the top of this tower, falling downwards through the packed sections. Steam is introduced into the bottom of the desorber, bubbling up through the liquid mixture cascading downward, and passing through the packed sections. The CO2 is stripped from MEA, and passes out the top of the desorber tower for dehydration and cooling and then is sent to storage facilities or to further processing. The MEA gathers at the bottom and is pumped into a reboiler where it is regenerated, and from here it is recycled into the absorber tower along with make-up MEA.

Uses of Captured CO2

• Dry ice manufacture
• Carbonated beverages
• Inert gas welding
• Oilfield enhancement
• Raw material used in chemical industry
• Firefighting as a blanketing gas and fire extinguisher medium

Note: The various methods for storage of CO2 will be covered in the next article on fossil fuelled power plants.

Sketches of CO2 Capture Operation

CO2

https: //www.brighthubengineering.com/power-plants/64165-capturing-power-plant-co2-emissions-for-long-term-storage/

 

Electricity from Burning Coal

Energy & Power Plants / By Willie Scott / Mechanical Engineering

Introduction

Electricity, in particular thermal power generated using fossil fuels, produces the largest amounts of industrial greenhouse gas emissions. This is because fossil fuels contain carbon which when burned in air produces CO².

In this series on power stations we look at three different types of fuel supplied to the thermal power stations, this first article being on coal fuel, then natural gas and ending with biomass fuel. We will examine the different methods of producing electricity and, fume treatment, both the present and future methods.

Coal Supply

The UK coal industry was dismantled by Maggie Thatcher’s Government in one of the bitterest disputes between a government and the trade unions ever witnessed. Since then mines have been continuously shut down until now we have only four surface mines and four deep mines operating in Britain today.

However we still use coal in our homes and for power generation fuel, so now most of the steaming coal used in the power stations is imported from as far away as Russia, South Africa, and Australia.

The coal arrives in Britain by ship and is transported by rail to the power station stockpile.

Power Station Stockpile

It is distributed into heaps in the stockpile by a bucket wheel machine. This is a device with large buckets arranged in a circle on the end of a rotating head. From the heaps the coal is loaded onto an enclosed conveyor.

Coal Pulverisers

From the stockpile a conveyor system conveys the coal to the pulverisers.

The coal pulverisers are really large ball mills having numerous hollow steel balls weighing over a ton each and about 750mm diameter. The pulveriser drum rotates causing the balls to move in a circular motion inside as the coal is fed into the drum. The coal is not large lumps like we use at home, but in small pieces of between 25 and 50mm known as steaming coal.

Water Tube Boilers

The pulverised coal, now really coal dust is blown by the primary air fan to the boiler coal burners and into the furnace. Forced draught fans propel the combustion air from the top of the boiler room through an air heater into the furnace combining with the coal dust. The mixture ignites and boils the water in the boiler tubes, turning it first to wet steam in the steam drum and then to superheated steam, in the superheater, which is now used to power a steam turbine.

Steam Turbines

The steam turbine can be one of several mechanical arrangements; the one shown is an in-line, reheat, single casing arrangement which has three stages. Superheated steam enters the High Pressure (HP) stage; the expanded steam is returned to the boiler and reheated before it enters the Intermediate Pressure (IP) stage. From here the exiting steam passes onto the dual Low Pressure (IP) stage where it expands before exiting to a vacuum condenser and hotwell from which the condensate is pumped into the boiler feed system.

This steam turbine drives a generator producing electricity which after passing through transformers is fed to the National Grid.

Ash and Fume Extraction

The furnace ashes are gathered and sold to the construction industry, with any surplus usually going to a disposal site.

The furnace combustion gasses pass through the superheater, economizer, and air heater before being discharged out the flue after being purified by the fume extraction system.

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