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23rd April 2015

Posted By Paul Boughton


A reverse osmosis unit is one of the technologies in a bespoke solution for improving water quality for CHP
Small increases in scale thickness within a boiler result in large increases in energy use

Think about water quality early for long-term CHP efficiency, advises Jim Baker

Combined heat and power (CHP) is becoming an increasingly attractive technology. A raft of government financial incentives offset the initial high capital expenditure and initiatives including reduced business rates and exemption from Carbon Price Support make long-term operation attractive too. However, the operating efficiency of any CHP unit depends on the efficient generation of electricity and the effective use of the associated heat, requiring effective use of boilers. Highly efficient long-term operation and reduced maintenance can be accomplished if attention is given to the quality of water used in the system and considerable savings can be made if this is considered early in the design stage.

Culligan is a water treatment specialist with an extensive track record of treating water upstream of CHP units and district heating systems across Europe. “All process industry works hard to deliver high-quality products with a keen eye on reducing overheads. When it comes to being energy efficient, CHP makes sense together with considering using alternative water sources to mains. Early consideration to achieving good water quality feeding the CHP and the design of a cost-effective treatment plant will pay dividends,” says Chris Smith, director of Culligan UK.

At the heart of CHP systems is a more efficient way of generating power. In a conventional power station electricity is generated by heating water to produce steam that drives a turbine, which in turn drives a generator. Huge inefficiencies come from needing to cool the water. CHP also relies on raising steam, but uses the waste energy to heat process plant, factories and homes.

Although a CHP unit is designed to run efficiently, water quality can have a major effect on long-term operation, even when using a mains supply, so effective design of an upstream water treatment unit prevents damage and inefficient operation. “I would advise anyone looking at CHP to take time considering what quality of water they require and how best to achieve that. In the UK, BS2486 details these standards depending on the specification of the boiler and its pressure rating. Talking to a specialist early will result in a CHP system that provides you with the operating efficiencies you expect and the associated financial savings,” recommends Smith.

So what should you consider in the design of an upstream water treatment unit? “The first thing is to understand the peak flow rate and the average flow rate. Inexperienced designers may size the unit on peak flow, but incorporating a storage tank to buffer the peak flow rather than running an oversized water treatment plant can make considerable savings.

“The next variable to understand is water hardness. Manganese and silica are of particular interest as they precipitate out as the water is heated in the boiler and produce scale with resultant hot spots. Scale increases energy usage with a 10% loss in efficiency with scale just 1.6mm thick rising to a staggering 60% loss for scale more than a centimetre thick.”

The manufacturer’s specification for the boiler and its pressure rating needs to be considered to understand the level of water treatment required and the best combination of technologies to accomplish this. High-pressure boilers, often used in CHP systems, will require water to be treated with reverse osmosis systems as a minimum. Reverse osmosis treats the dissolved solids in the water, ensuring they do not exceed the manufacturer’s specified maximum levels. Lower pressure systems or higher quality raw water may only require a simple softener; again considerable savings both in OPEX and CAPEX can be realised with correct system selection.

Aim to reduce blowdown

Dissolved solids in the boiler feed water are concentrated as steam is generated, so boiler blowdown vents hot water through a valve to waste when impurities in the water build up to set levels. The lost hot water is replaced with an equivalent amount of cooler water, thus diluting the dissolved solids concentration. Clearly the water in the boiler is dramatically hotter than the feedwater so blowdown is a huge (and to some extent avoidable) energy loss to the system. For any given system, the better the feedwater quality, the less blowdown is required.

“Considering the design of the water treatment unit as integral to the design of the overall CHP scheme delivers capital savings on the unit and both its operation and the effective operation of the boiler. Blowdown on a boiler is necessary but excessive blowdown brings with it huge inefficiencies. Minimising the dissolved solids upstream to create better quality feed water to the boiler is financially prudent,” concludes Smith.

For more information visit www.engineerlive.com/epe

Jim Baker is with Culligan.









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