Taking the heat

Introduction

Given the need to reduce carbon emissions, protect the environment, and reduce fossil fuel consumption, waste heat recovery (WHR) is becoming increasingly important. Green’s has completed recent WHR projects that have required the design, manufacture, and installation of applications in all corners of the world. These have included sugar cane processing plants in the warmest parts of Australia, pulp and paper mills operating in the coldest parts of Finland, and projects for energy producers and petrochem companies. Green’s has also supplied to cement plants around the world, including Shree Cement in India, Cadcime in Switzerland, Tosoh in Japan, and DG Khan Cement in Pakistan.

Green’s products

WHBs

A WHB recovers heat from hot gas streams, which still have a relatively high energy content and which would otherwise go up the chimney and out into the atmosphere. Reducing the flue gas temperature from 400°C to 180°C captures the exhaust energy from diesel or gas engines. This allows plants, including cement factories, to maximise energy usage and increase production efficiency.

For a heat recovery capital investment project, the client would need to consider the following factors for the return of investment:

• Available waste energy in the exhaust gas system.
• The manufacturing and installation cost of the heat recovery unit.
• The physical space available in the plant for the heat recovery unit.
• Efficiency savings based on plant operational hours.
• Fuel cost savings from increased plant efficiency.
• Reducing carbon and harmful emissions.

Common applications for Green’s products include the following:

• Preheating combustion air for boilers, ovens, and furnaces.
• Preheating fresh air used to ventilate the building.
• Hot water generation, including preheating boiler feed water.
• Direct steam generation for process or power generation.
• Space heating.
• Drying.
• Other heating or preheating for industrial processes.

However, before a WHB system that would allow a cement plant to obtain these benefits can be designed, managers need to provide the supplier with information that will help to inform an effective system. This includes the type of fuel that will be used, as well as data about the flue gas, including flowrate, composition, and inlet temperature. It is also essential to know where the system will be located and the space available, which will be used to determine the best materials and tube arrangements.

Optimal design

Using this information, the supplier will work on the optimum thermal design, which will include outputs such as the best working fluid (i.e. water, saturated steam, superheated steam, or thermal oil), as well as flowrate, thermal duty, waterside pressure drop, gas side pressure drop, working fluid outlet temperature, and operating pressure.

All of this information will then be used to formulate pressure parts calculations and develop the mechanical and structural design that specifies factors such as the system’s weight, width, length, and, in some cases, seismic calculations, wind loading calculations, and pressure part calculations.

The use of systems in cement plants brings particular challenges for their WHB systems. The need to keep the environment dry must be carefully managed and a key issue here is to use air heaters rather than soot blowers to clean the boiler. The dusty atmosphere also influences the types of tubes that are used and which fins are employed. In general, the fins need to have relatively large gaps between them, known as a wide pitch specification. They also need to have as high a heat transfer surface as possible, which makes Green’s patented H-finned tubes an ideal solution.

Cement plant managers should also ensure that their chosen supplier has the required experience, track record, and industry accreditations to be able to design and manufacture a system that meets all regulatory and insurance needs. This can only be achieved by companies that have stringent quality and technical assessment processes in place, which is best checked by visiting their facilities to see what they offer first hand.

Conclusion

Having invested in a WHB system with the aim of making energy usage more efficient and reducing costs, it is recommended that a planned maintenance programme is initiated. This will ensure that its lifespan is extended as much as possible and that unplanned outages are minimised.

Green’s has been designing and manufacturing boilers and WHB systems in the UK since 1845 and is well known around the world for its engineering and manufacturing skills. While Green’s products leave Yorkshire in good condition, the environments in which they operate, the varying quality of feed water, and rigorous operating patterns can result, over time, in a reduction in efficiency, or even total machine failure.

To avoid this, it is vital to commission a proactive inspection service to identify potential problems with WHR equipment so that they can be addressed. This will maintain optimum efficiency before problems become unnecessarily costly or result in avoidable downtime.

Published by Lucy Stewardson, Editorial Assistant, World Cement