Handle with care: why fuel handling is critical for renewables

Matt Drew, managing director of Saxlund International, discusses a new fuel-to-boiler processing line at one of the UK’s largest biomass plants, and explains why fuel handling is critical to a range of renewable power generation projects.

Despite some high profile failures, notably the closure of two plasma gasification sites in the Tees Valley earlier this year, confidence in Energy from Waste and biomass power generation, whatever the selected combustion technology, remains strong with a growing number of plants either fully operational or under construction across the UK.

A critical area, and one that is sometimes not given due focus, is the automated bulk-material-handling solution at the front end of most plants. This is designed to receive, store, convey, dose and feed the alternative fuel into the selected combustion process.

A typical solution involves reception and storage for incoming fuel, with discharge and reclaim conveyors plus screening to remove contaminants and out-of-spec material. Pneumatic injection, together with weighing and dosing systems may also be required.

However, biomass and waste derived fuels (WDF) composed of municipal solid waste, plus commercial and industrial waste and waste wood, are notoriously tricky to handle, non-free-flowing with a tendency to bridge and compact. This means the flow can easily be disrupted.

Blockages invariably result in system shut-downs, both timeconsuming and costly with processes idle while the issue is resolved. Furthermore poor material handling means that some plants may never reach their full potential, so it’s important to get this right.

Understanding material behaviour is therefore crucial to system design and plant reliability. Factors covering particle size, mass flow properties, density variations, moisture content and compressibility are all important considerations. The composition of the biomass/WDF and its source may also change over time, and this needs to be factored into the system design to ensure a robust, enduring solution. In this respect the strength of the relationship with the fuel supplier can be paramount and all of this can be challenging for mechanical and process engineers unless they are working with these systems on a continuous basis.

Saxlund Group is currently delivering the fuel handling systems for ten renewable energy power stations in the UK with more in the pipeline. A critical element is the push floor or sliding frame technology incorporated in the design of the fuel reception and storage silo. Continuously developed by Saxlund since the 1960s, this is based on ‘first in, first out’ design principles to ensure fuel stocks have minimal chance to compact or degrade. The technology is designed to break up the material, preventing any build up or bridging, to ensure a consistent, homogenous delivery to the discharge point.

One highly successful and long standing example is Slough Heat & Power. Owned and operated by SSE Enterprise, it is one of the UK’s largest dedicated biomass fired Combined Heat and Power (CHP) plants, supplying steam and hot water to major businesses on the Slough Trading Estate and generating in the region of 100GWh electricity per annum.

A new fuel-to-boiler supply system, to broaden the fuel envelope at the site and increase fuel flexibility, is now running at full capacity feeding shredded waste or waste wood chips at a rate of 25 tonnes per hour to an existing boiler.

This is a second Saxlund Push Floor that can feed the boiler and the additional fuel stream was designed to upgrade the fuel reception storage and screening capacity. Lead contractor for the project was O.Kay Engineering, a specialist in waste processing technology and turnkey recycling plants.

The Saxlund element within the plant incorporates a new 720m3 fuel reception area with live storage for approximately six hours of operation using the company’s well-tested push floor discharge solution.

The overall system was designed and project managed by O.Kay Engineering’s in-house team of designers and project managers. The Saxlund discharge system is at the front-end of the O.Kay system and delivers the material onto outtake conveyors which feed the material in the fuel screening system before onward delivery into the fuel hopper of the boiler.

The system can handle both woodchip and RTU (Ready to Use) fuels and incorporates screening of the fuel stock and removal of metals to ensure correct particle size and quality of fuel. Material is continually weighed and the system can also be fed from an existing back-up feed stock via an existing Saxlund Push Floor and discharge system.

The main handling and screening system was designed and installed to run in an ATEX 21 (Explosive Atmosphere directive) environment. This is another important consideration for biomass CHP projects and dust suppression including covered conveyors may be required in some installations.

The installation presented some challenges in that three of the existing four feed lines had to be removed in stages while the plant was still running. The removal of the final line and main installation could then be carried out with the resulting installation providing Slough Heat & Power with a simpler, more efficient and reliable fuel feeding system.

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