Rotary boiler technology: paving a new way for the combustion of solid wastes

Article posted on 08.01.2021

Horizon+ boiler

We’re the UK’s sole distributor of Sugimat’s Waste-to-Energy (WtE) boilers, and the latest technology – the Horizon+ rotary boiler – is capable of handling any type of biomass and most fuels, particularly problematic solid waste fuels such as RDF and SRF.

Our MD, Simon Webb, recently spoke to The Energy Industry Times about the global Waste-to-Energy (WtE) landscape, the drivers behind the new development, and how this will impact the UK’s waste and recycling industry. If you missed the original article, you can catch up below…

When it comes to the adoption of WtE technologies, the picture varies greatly from one country to another. While European nations such as Germany, Austria and Sweden have been utilising alternative fuels – including Refuse Derived Fuel (RDF) and Solid Recovered Fuel (SRF) – for many years, and have advanced national waste infrastructures to mirror this, the degree of uptake has a lot further to go in fellow European countries such as Romania.

However, regardless of geography, there is one constant at the heart of the entire energy and power generation industry – innovation.

And as the spotlight shines – from both a legislative and societal perspective – firmly on harnessing the resource potential of more of the globe’s wastes, equipment that is able to process the more ‘challenging’ fuels which generate significant levels of slag and ash, is now occupying a higher place than ever before on the waste management agenda.

This was one of the primary catalysts behind the development of Valencia-headquartered Sugimat’s Horizon+ rotary boiler – equipment that is designed and manufactured to work with flue gases from the combustion of solid wastes, for either a hot water or thermal oil application.

Development drivers

One of the biggest difficulties often facing WtE plant operatives is keeping systems running continuously – driving the maximum operational hours possible while simultaneously optimising combustion efficiency and maintaining equipment cleanliness. This is because, traditionally, a boiler’s efficiency starts to drop within a few hours of it firing up, and in order to achieve peak productivity levels, plenty of manual intervention is typically required.

Xavier Mas, technical service director and R&D at Sugimat and developer of the Horizon+ patent, adds: “When using solid fuels that generate high volumes of slag and ash – especially those with ash content over 6% – this can quickly contaminate the boiler, with sticky residue clogging up the heat exchange ducts. As a result, this reduces the continuous operation hours possible and ultimately, results in unplanned plant downtime.

“This kind of technical failure is likelier to occur in conventional boilers, due to the ease of ash build-up when there’s no rotating, self-cleaning and ash-extraction elements in place.

“And it was the operational difficulties and cost of maintenance of normal systems apparent in the market, that catalysed the development of the Horizon+ innovation.”

Horizon+ rotary boiler

A change in tide for the UK market

As the UK continues to struggle exporting its end-of-life waste overseas – due to evolving legislation, various waste import bans, and most recently, the logistical impacts caused by the pandemic – an increasing number of organisations are turning their attention to the value of RDF and SRF, with many looking for more sustainable and circular ways to utilise them in line with the European Union’s Waste Hierarchy model.

And as the country’s economic recovery looms ahead, it could perhaps be argued that it has never been more important, or timely, for the UK’s private and public sectors to begin – or ramp up their efforts – to harness the potential of these waste streams, and close the loop.

The implementation of more decentralised energy recovery facilities is perhaps the way forward in helping to halt the shipment of our valuable energy sources – or ‘wastes’ – overseas for recovery.

But with the increase in demand for sustainable WtE technologies, comes the need for machinery which is built upon innovation, reliability and efficiency.

Further elaborating on how the rotary boiler operates with productivity and sustainability in mind, Xavier explains: “The Horizon+ technology has a self-cleaning capability, which allows it to cope with the high ash content and low melting point sticky ashes, typically associated with ‘dirty’ fuels like RDF and SRF – meaning there is no loss of efficiency.

“The rotating element of the equipment enables fluid to easily circulate within the sealed system and causes the ash to fall from the boiler tubes – keeping them clear of residue and therefore elevating performance and maximising plant uptime. For particularly sticky ashes, a recyclable cleaning medium can be introduced to further improve the self-cleaning capabilities.”

The intricacies of the Horizon+ technology

The newly designed Horizon+ system – available in models spanning 300 kW to 12MW outputs – has been constructed with ‘fuels of the future’ in mind. This encompasses the fuels that have been overlooked or excluded to date because of the challenges associated with their combustion.

The horizontal heat exchanger is composed of several connected coils – through which the heating fluid for the installation circulates. And the heat exchanger inlet and outlets are connected by rotary couplings, with seals designed to work at high temperatures.

The boiler itself is mounted on a frame – equipped with casters – and driven by a gear motor, which enables it to rotate on the axis of the coils at a slow speed up to four revolutions per minute. As a result, the rotary movement – coupled with the screw action of the coils that form the heat exchanger body – allows suspended particles in the gas flow to be discharged through an outlet.

Additionally, unlike conventional boilers that are often unable to manage flue gases with high fly ash content, the continuous granular cleaning system of the Horizon+ removes any surface ash on the heat exchanger coils – negating the need to stop the operation and maintaining heat exchanger optimum performance at all times. In turn, this prevents lengthy shutdown processes, whereby the system would typically have to cool to fix the problem, before being fired back up.

But what exactly makes the equipment unique?

In conventional vertical and horizontal boilers, combustion gases are not distributed uniformly within the heater – creating dead spots and areas with high gas flow – and in static heat exchangers, gases do not behave consistently across the surfaces.

The constant movement of the coils in the Horizon+ affords a more homogenous and consistent transfer of heat, which eliminates the dead spots and ash deposit build-ups, and prevents coil abrasion traditionally caused by areas of high flow. It’s this slow revolving movement which encourages in-furnace impurities to move along the surface of the heat exchange tube – comparable to the thread of a screw – until they end up at the bottom of the boiler, where they are automatically extracted.

Therefore, the rotary boiler’s anatomy works to achieve an increase in the performance and overall lifespan of the heater. And as the machine is turning at a slow rate, mechanical maintenance issues are not only rare, but the technology is able to guarantee 8,000 operational hours per annum.

This contrasts with traditional static boilers, which use air to blast the surfaces clean – causing the ash to be mixed with the flue gases. This results in suspended solids contaminating analyser outlets and flue gas filters, preventing regulatory compliance, and shortening the longevity of the filtration equipment.

The optimised, rotary design is what makes the equipment perform more efficiently for longer periods, and in order to develop it effectively, this complex feature required more attention during the nascent conceptual and design phases.

Another benefit – and carefully considered element – is the size of the machinery. For electricity generation, boilers generating 12 MWth typically require a 20-metre-high building to house the system, but the horizontal design of the Horizon+ means it can be sited in a building less than 10m high.

As a consequence, this not only helps to reduce potential planning issues and construction costs, but also makes it more viable for the equipment to be used in areas where space is at a premium. This includes decentralised WtE facilities and smaller-scale sites close to urban areas, where the heat and power generated can be used for surrounding residential areas or industrial estates.

Projects in the pipeline

Regarding market application of the rotary boiler technology, the first machine will be installed at an international reusable pallet company – creating its own on-site, closed loop system. This will see the waste from the pallet repair operations used as fuel within the boiler, generating heat for the line which dries the painted pallets.

Also, the second rotary system is set to be installed in 2021 at a well-known event venue – where it will be operating on a mix of waste straw bales and waste wood as well as RDF, to supplement its fuel source. It will provide heat and power for the entire site – comprising hotel, farm, cottages and more – in a bid to reduce its carbon footprint and become a self-sufficient cycle of energy generation and consumption.

Regarding what the future holds for the rotary equipment, and the WtE scene more broadly, Tidy Planet has received multiple enquiries in the UK for applying the Horizon+ technology within decentralised Energy-from-Waste facilities. And we anticipate the appetite and demand for UK-centred recovery solutions to only increase over the next year, as the country continues to navigate the unknown waters ahead following Brexit and the aftermath of the pandemic.

Tidy Planet Energy is a Waste-to-Energy solutions specialist.

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