Optimising heat recovery with economisers in EfW and biomass plants

Written by: Matthew Crewe | Published:
Green’s in Wakefield has been manufacturing economisers for nearly 200 years

Given the need to reduce CO2 emissions, protect our environment and cut fossil fuel consumption, waste heat recovery is increasingly important.

Recently we have completed waste heat recovery projects that have required us to design, manufacture and install for a range of applications globally.

These range from sugar cane processing plants in the warmest parts of Australia to pulp and paper mills operating in the coldest parts of Finland, plus chemical companies and petrochem companies.

We have also supplied to Energy from Waste and biomass plants around the world such as LondonEnergy, Jersey Waste, HOFOR’s Amagervaerket in Copenhagen, Birmingham Biopower’s Tyseley plant, Ence’s plant in Huelva, Spain and – the Energy from Waste Conference’s site visit – Viridor’s Ardley ERF plant for Oxfordshire County Council.

The products supplied were dependent on the plant’s requirements and were custom-made. For example, for the Amagervaerket biomass plantwe designed and manufactured economisers, which make the process more energy-efficient, and air heaters that consisted of elements weighing almost 2,000 tonnes, before shipping them to Denmark.

The Birmingham Biopower plant also posed challenges as we had to design, manufacture and install replacement tube banks, plus remove 1,296 tubes that equaled over 8km in length. The new economisers were built in just 10 weeks, with on-site work completed in just 12 days.

The science behind economisers

An economiser unit is typically installed on the exhaust system from a steam and hot water boiler to reduce the flue gas temperature and increase the thermal efficiency. With the increased number of combined heat and power (CHP) units, thermal oil plants, biomass boilers and EfW plants in the UK, there has been an increased demand for economiser units.

Green’s in Wakefield has been manufacturing economisers for nearly 200 years, initially supplying units to coal boiler manufacturers. Green’s proprietary H-fin tube design lends itself perfectly to new renewable fuel systems, which have a high ash content and can cause erosion to tube surfaces and blocking issues to standard economiser units.

A heat recovery steam generator recovers heat from hot gas streams that have high energy content which would otherwise go up the chimney and out into the atmosphere, and instead use it to generate steam and power.

Capturing the heat from waste gasification or pyrolysis process by reducing the flue gas temperature from, for example, 900°C to 150°C, allows EfW and biomass plants to maximise energy usage and increase production efficiency.

For a heat recovery capital investment project, the client would need to consider a series of factors to calculate the return on investment (ROI).

These include available waste energy in the exhaust gas system, the manufacturing and installation cost of the heat recovery unit and physical space available in the plant for the heat recovery unit.

There are many benefits of installing a heat recovery system as they increase efficiency and fuel cost savings (based on plant operational hours), while reducing carbon dioxide and harmful emissions.

Heat networks can be used in pre-heating combustion air for boilers, ovens and furnaces and fresh air used to ventilate buildings, hot water generation, direct steam generation, space heating and drying.

However, before a waste heat recovery system can be designed for an EfW or biomass plant to obtain these benefits, 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 and data about the flue gas including flow rate, composition and inlet temperature. It is, of course, 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.

Using this information, the supplier will work on the optimum thermal design which will include outputs such as the best working fluid, ie, water, saturated steam, superheated steam or thermal oil, flow rate, 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 and length and, in some cases, seismic calculations and wind loading calculations.

Challenges to the process

Use of systems in EfW or biomass plants brings particular challenges for their waste heat recovery systems. The flue gas containing SO2 and chlorides, low water temperatures as well as high flue gas ash content need to be taken into account when determining materials and the types of tubes that are used and which fins are employed.

Generally, the fins need to have relatively large gaps between them, which is known as a wide pitch specification. They also need to have as high a heat transfer surface as possible which makes Green’s proprietary H-finned tube design an ideal solution.

EfW and biomass 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 and is best checked by visiting their facilities to see what they offer first-hand.

Having invested in a waste heat recovery system with the aim of making energy usage more efficient and reducing costs, it is highly recommended to initiate a planned maintenance programme to ensure that its lifespan is extended as much as possible and that unplanned outages are minimised.

While our products leave Yorkshire in perfect condition, the environments in which they operate, varying quality of feed water and rigorous operating patterns can result, over time, in efficiency reductions or even total machine failure.

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

Green’s supply an Aftersales service support package for customers, so that the internal surfaces of economiser’s and WHB’s can be examined during planned outages to ensure fouling and erosion issues are not occurring and online cleaning equipment such as soot blowers, are operating correctly.

Typically, a comprehensive report is submitted to the client after the examination work, stating the condition of the examined surfaces and recording corrective measures needed. Should any unforeseen failures occur within the plant, we offer 24/7 support worldwide with mobilisation 365 days of the year at very short notice.

Matthew Crewe is managing director of Green's.

The Energy from Waste Conference 2019 will return to etc. Venues, London on 26-28 February.


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