Waste heat recovery units in the coal industry

A waste heat recovery unit/heat exchanger recovers heat from hot streams, gasses that still have relatively high energy content in them. This would otherwise go unused into the atmosphere. The most common examples of these units are those taking the exit gases from boilers in the world’s largest coal fired power stations.

The diagram show waste heat recovery using an economiser application, technology that was first developed by Green’s founder back in the mid 1800s and has been developed considerably over the years.

How the heat recovery process works

Waste heat found in exhaust gas is extracted by the following equipment:

• Economiser – waste heat in the exhaust gas leaving the boiler is passed over the economiser’s heating surface (often comprising finned tubes) heating the feed water that passes through the tubes before it enters the boiler. This increases boiler efficiency and results in less fuel being required to generate steam and overall thermal efficiency can be improved around 10%. In addition, economisers allow boilers to react more readily to loads by using heated feedwater and the boiler is protected from thermal stresses caused by relatively cold water entering the hot boiler so the economiser also helps to prolong the boiler’s service life.
• Waste heat from other sources such as diesel engines or gas turbines can also be used to pre-heat boiler feed water and generate steam. Waste heat boilers (WHB) or exhaust gas boilers (EGB) – can be used to create steam without the need for a separate boiler. Steam from these units is then used in another process or, if superheated, used to drive a steam turbine to generate electricity.
• Flue Gas Coolers are similar to economisers and cool down flue gas from various boilers and utilise waste heat to heat up water which is then used in the plant.
• Air-Preheater – a unit responsible for preheating combustion air and also for drying fuel. It can be applied at the very end of boiler exhaust systems as a flue gas and air heat exchanger In all cases, the heat that would ordinarily be lost from the steam generation process is used for producing energy and increases efficiency of the plant thereby reducing costs.

Obviously waste heat recovery comes with a capital cost for the design and manufacture of the system and the various solutions on the market need to be assessed in terms of their track record in meeting the specific needs of the coal industry. However, taking steps to determine whether a valuable heat source can be used rather than wasted is always worth the effort.

How this applies to coal fired power plant

Most power stations burn many millions of tons of coal per year which can come in various forms and quality from very dirty coal in some regions to very pure coal in others. This dictates the heat produced and the plant and equipment needed to optimise use of the heat.

Once at the power station the coal is stored, then pulverised before being burned in the power station. The heat taken from this process then heats the boilers. As the boiler is heated the gasses need to exit of course. These gasses are usually still at significantly high temperatures, so capturing the heat from the boiler using economisers and reusing it to heat water in tubes means it can be utilised to preheat the feed water and increase efficiency overall.

Throughout this stage, the coal (depending on its quality/consistency) creates ash when it is burned. The type and consistency will depend on the quality of the coal. Combined with the intense heat this ash is often highly abrasive which means that the boilers, burners and ancillary equipment at the power station need to be particularly robust if they are to perform effectively and provide acceptable system life.

To gain the maximum benefit from the waste heat from the boiler there are essential criteria that need to be considered with regard to selecting the most effective economisers. These include ensuring that the correct and durable materials are used and that the design and positioning of the economiser itself are appropriate to achieve the required heat transfer.

One design solution, to optimise heat transfer is by using extended surface tubing. This is done by adding fins to the tubing as shown below in a similar way to a radiator. This ensures the heat transfer into the tube is maximised by using the fin to conduct heat into the tube.

The use of H finned tube technology, rather than plain tubes, is particularly suited to power stations which want to maximise waste heat recovery as efficiency can be significantly increased within the space available. This usually results in reducing the number of tubes, the economiser size and in turn the heat of the exhaust gasses needed and input fuel used.

H fin applications – based on their design are well suited to both light and heavy fouling, high ash content, applications with most modern stations using this application. Indeed our proprietary steel H finned tubes are used in more than 70% of UK coal fired power stations and many more around the world.

Recent coal fired power station case studies

Projects include:

• Drax power station, UK’s largest power station – 6 660MW units, serving around 8% of the UK’s electricity requirement (around 5 million homes). We replaced the economiser equipment we had manufactured for Drax in the 1960s. Some of the units continue to use coal and some have been recently converted to biomass applications - all using Green’s economiser equipment.
• Intergen - Millmerran power station, an 850MW supercritical power station, serving 1.1 million homes – uses around 4m tonnes of coal a year and is one of the world’s most energy efficient coal fired stations – We installed ‘H’ Fin tubes as part of a system redesign to increase efficiency.
• Lynemouth Power – Coal fired power station. Project to replace and upgrade equipment to suit biomass application.
• Pha Lai Thermal Power Power Station, largest coal-fired plant in Vietnam. Replaced complete ‘H’ fin economiser unit as this station further develops using locally available coal reserves.
• Hwange and Munyati Power Stations, Zimbabwe – using Zimbabwe’s coal reserves and providing additional and stable power sources locally.

Full economiser replacement.

Green’s has been designing and manufacturing waste heat recovery systems and economisers since 1845. Our heritage is within the coal market and our expertise is still very much in demand where this resource is plentiful alongside increasing interest from the biomass sector.

Gaining the greatest efficiency from the coal reserves that are used is clearly still a very important factor in the industry, the use of waste heat recovery further aids this through the use of economiser equipment.

Submitted by waste heat recovery solution specialists, Green’s.

Read the article online at: https://www.worldcoal.com/coal/20042017/waste-heat-recovery-units-in-the-coal-industry/