Thermal power plants must have a consistent power output if they are to achieve a steady baseload role in the power generation mix. This, however, is a particular challenge for EfW plants, which use a fuel – municipal waste – that does not always yield the same amount of energy per tonne. The last thing an operator needs, while closely monitoring the boiler temperature and turbine speed, is an unplanned stoppage.
Yet, planned and unplanned stoppages remain frequent at EfW plants to replace worn parts, or to mitigate emissions. And as the European Union tightens its rules around burning municipal waste, EfW operators must ensure their plants are efficient and help support their nations’ power infrastructure.
Castolin Eutectic is playing a major role in addressing these issues, building on more than a century’s experience of innovation focused on improving the lifespan and efficiency of industrial facilities.
Unpredictable abrasion and wear in waste handling
Most EfW plants burn mixed municipal waste – food, paper, textiles, wood and plastic – that would otherwise be sent to landfill. There are also more abrasive materials like metal, glass and electronic waste. Because the different materials present yield widely different amounts of energy, the waste must be mixed well before incineration, to achieve a stable boiler temperature. However, this causes significant wear on the hoppers the waste is delivered into, the crusher rotors that mix and break it up, and the systems that convey it to the incinerator.
The wear on fuel handling parts is also unpredictable because the fuel loads are not uniformly abrasive – the composition of the waste varies depending on the habits of people in the feeder municipalities. However well mixed the waste is, some loads will contain more glass, metal and hard plastic, and some will contain less. All these factors have historically posed an economic conundrum for EfW plant builders, owners and operators.
The case for up-front investment in harder wearing parts would be more obvious if the plant only handled the most abrasive materials: it would be unable to operate for long without them. Because not all the fuel is significantly abrasive, it can be tempting to gamble on less hardwearing parts.
At the same time, a planned stoppage in six months’ time can become an unplanned stoppage tomorrow. This brings up the challenges of quickly and safely shutting the incinerator and turbine, and later ramping back up to a stable boiler temperature.
A typical example illustrating the difference wear protection can make is provided by fuel crusher rotors, and their casings, which need to be rewelded on a yearly basis because of high wear. An effective solution is to refurbish the rotor using welding wire to rebuild it to its original dimensions. This increases the lifetime of the crusher significantly, resulting in a life 2 to 10 times that without wear protection. The ideal welding wire will provide excellent wear resistance while preventing health and safety risk. For example, the absence of chromium and nickel in Castolin Eutectic’s EnDOTec SafeHard 600 meets the strictest European Exposure Occupational Limits (EOL), while carbon and carbide content boosts wear resistance by providing toughness and hardness.
The abrasive fuel in EfW plants also creates wear issues for hydraulic garbage grabs. This results in more waste falling back into the bunker and leads to more lifting movements and more energy and time required for fuel filling. Fitting wear plates on the inside of a hydraulic garbage grab guarantees a 200% increase in lifetime.
Ash handling – time to raise the industry standard?
EfW operators must also look out for non-combustible materials. Non-ferrous metals and some other materials form slag, which must be removed from the incinerator along with ash.
In most EfW plants, the chutes, chain conveyors and sieves of the ash handling system are manufactured from manganese steel. These are well-suited to ash but without protection from additional plating or specialist coatings, these standard parts are susceptible to high abrasion wear from the non-combustibles in the slag. For example, manganese steel de-slagger chutes and slag-sorting sieves usually need replacing after only a year in operation.
Why are EfW plants still using standard parts that cannot endure the normal stresses of their operations? Using harder-wearing materials or adding wear-facing solutions for materials like manganese steel would undoubtedly increase the up-front investment needed to construct an EfW plant – but the historical performance of these standard parts suggests that these up-front costs would pay off over the lifecycle of the plant.
Some areas for improvement are the gliding strips, chain and bars on chain conveyors. The solution is to use cut ultra-hard wearplates for the strips and bars. Chromium carbides and borides in these plates provide protection against abrasion, and reduce friction and consequently wear of the chain. The result is that the lifetime of gliding strips increases up to 10 times, for the bars up to six times.
The influence of attitudes and regulation in flue gas cleaning
As well as the engineering challenges outlined, EfW plants must also deal with changing attitudes to waste among regulators, businesses and the public.
Although these trends are not uniform everywhere, with operators around the world facing different challenges, there are common goals: reduced carbon dioxide and other emissions; and a circular economy featuring increased recycling and reuse.
Monitoring the plant’s emissions is a priority. The European Union has specific requirements about allowable emissions from incinerating municipal waste. For example, if the levels of particulates emitting from an EfW plant’s flue exceed the allowed limit, the plant must shut down to investigate and deal with the cause.
The flue gas cleaning systems employed in EfW plants process hot, acidic exhaust gases. These systems are therefore prone to various forms of corrosion and erosion by fly ash carried in the flue gas. Wear and damage can reduce their effectiveness, potentially leading to regulatory stoppages on top of the stoppages needed to replace damaged parts.
A key area for attention is the flue gas deflectors. These experience erosion due to fly ash particles in the flue gas at around 170˚C and may require annual repair consisting of covering the deflectors with a laser clad LC8 half pipe.
Extending service life is a priority for EfW plants
It is more important than ever for EfW plants to maximise their efficiency – extracting the maximum energy possible from each hopper of fuel to compensate for decreasing plastic content and calorific value as more plastic is being recycled. It also means operating at a stable boiler temperature, turbine speed and energy output for as much of the year as possible, keeping stoppages to a minimum.
To achieve this, operators must prioritise extending the service life of parts deployed on site. Longer-lasting parts will reduce stoppages for maintenance and repairs. Parts that perform at peak for longer will also maintain the efficiency of systems and reduce the chance of regulatory stoppage and damage to the environment.
The key to supporting the EfW industry in its mission to reduce landfill and protect the environment is to take a whole life cycle approach to operations and maintenance. This maximises the efficiency of systems, the service life of parts, and long-term cost-effectiveness.
When it comes to repair, products like ultra-hard wearplates can extend the life of waste and ash handling systems, while heat- and corrosion-resistant coatings avoid outages in boilers and flue gas treatment systems. By adopting these solutions, operators have seen the service life of waste grabbers double, and fuel ducts and grate bars last for eight years and counting with very low wear.
Staying power
EfW plants already in operation can make the most of upcoming scheduled maintenance periods to apply wear-facing solutions to extend the life of parts in service. And for any new EfW plants currently in the planning stages – now is the time to rethink the accepted industry standards, drawing from real-life experiences of existing plants, and build systems that last.
For further information: https://www.castolin.com/recycling
Case study: ChromeClad boiler coatings
In March 2021, Castolin Eutectic performed a boiler coating job at a chemical plant in Germany, applying wear protection on 740m² of boiler panels. More than a year later, the exposed surfaces remained visually flawless. The solution, applied via Twin Wire ARC Spray, was a nickel-based coating called ChromeClad XC, which provides superior resistance within extreme corrosive environments. The proprietary alloy top coat is applied over an extremely dense bonding layer which seals the weak ferrous substrate and ensures excellent bonding.
Subsequently, a protective ceramic layer called TubeArmor was applied over this coating to densify it and prevent build-up of slag, which would restrict air flow and literally choke the boiler.
A distinguishing feature of this coating is its green colour. This provides excellent contrast for easy identification of issues during drone inspections.
