Substituting fossil fuels with waste-derived alternatives has been a focus for the cement manufacturing industry over many years as companies seek sustainable solutions.
One of the latest to go live is at Hope Construction Materials’ cement works in Hope, Derbyshire, one of the few active UK plants operating twin kilns.
Until recently Hope works was achieving substitution rates of around 35% for the coal used to fire its kilns. But the company has progressive environmental plans to extend this substitution rate to more than 50%, so Saxlund International was brought in to collaborate on the development of a system for the reception, storage, transportation, weighing and pneumatic injection of solid-waste fuel (SWF) to the main burner in each kiln.
SWF, much like solid recovered fuel, is derived from commercial waste, mainly paper, plastic and other non-biodegradable materials that have been processed to remove metal, glass and heavy plastics, then is dehydrated and shredded to <30mm, suitable to feed cement kilns. The fuel is supplied to an agreed specification.
But waste-derived fuel is potentially sticky, dusty and non-free flowing with a tendency to bridge, obstructing flows and downstream processes. High moisture content also makes it liable to compact and sweat.
A key requirement for the team was to provide stable and reliable process conditions that would avoid any unmanageable build-up of material in the pre-heater tower, along with a future-proof solution capable of handling changing fuel characteristics, even different types of fuel, should suppliers change.
The solution incorporates a 350cu m fuel reception and Saxlund’s Push Floor storage solution with hydraulically operated reciprocating ladders fitted into the silo floor. The ladders’ motion pushes fuel towards the discharge end of the bunker, achieving a mass flow with a ‘first in, first out’ fuel delivery, which effectively designs-out compaction issues. The shearing action of the ladders also agitates and breaks up the material as it moves downstream.
Discharged fuel is conveyed into a process tower, passes through a drum magnet to remove ferrous metal, then via a presentation conveyor with a controlled feed into a policing screen to remove out-of-specification material. Here, the aggressive action of a star screen breaks down any matted or oversize material to minimise the risk of blockages. The system is unique in that the screen size can be changed at the push of a button, and acceptable fuel particle size can also be changed from 10mm to 50mm.
Oversize material is diverted into a waste recycling skip, while the in-specification fuel falls through the gaps and is collected by a heavy-duty twin-screw feeder. Two outlets feed the weighing systems which accurately dose the fuel, which passes into the pneumatic injection system feeding each kiln.
Reliability is a key factor, and the design emphasis has been on minimising potential restrictions or blockages. Key maintenance areas are external to the system where possible to minimise disruption. This applies especially to the Push Floor system which can be overhauled even when the fuel silo is full.
Pipework for the pneumatic injection system, which extends some 70m from the process tower to the main burner in each kiln, is also designed with long radius bends to reduce friction and wear-resistant basalt liners.
With commissioning successfully completed, Hope works is now operating with a larger volume of waste-derived fuels, diverting up to 80,000 tonnes of bulk solid waste from landfill each year. That is a significant carbon saving to the business, and forms part of the major investment announced in January to boost efficiency and improve sustainability at Hope works.
Waste-derived fossil fuel substitution at hope works
2001 – Tyre chips introduced to the pre-heater tower
2006 – Meat and bone meal added to the front end of the kiln
2011 – Processed sewage pellets introduced
2013 – SWF introduced via a trial system
2015 – SWF installation managed by Saxlund goes live, expected to take fossil fuel substitution from 35% to above 50%
Matt Drew is managing director at Saxlund International