Cement production is one of the world’s most fuel-intensive industries. Fossil fuels, mostly coal, have tended to be the industry’s main power source, although increasingly, cement manufacturers are replacing up to 80% of their fossil fuels with more sustainable waste-derived fuels (WDF). Using WDF reduces production costs and improves energy efficiency. Here Matt Drew, Managing Director of Saxlund International, discusses the challenges faced by Hope Construction Materials in the UK and how these were overcome.
Fuels continue to be a major part of the global cement industry’s cost base. In a competitive market, where profit margins are narrow and legacy systems costly to replace, the capital investment required for sustainable production has led to the cement industry, certainly in the UK and Europe, taking an incremental approach to fuel replacement. These incremental changes typically achieve a 10-20% reduction in the amount of fossil fuel used each time a change is introduced.
Fossil fuel substitutes
In the past decade or so alternative fuel systems were being designed to handle tyre chips, meat and bone-meal (MBM) and pellets from sewage sludge. Many plants still operate these. One of the UK’s largest cement manufacturers, Hope Construction Materials, introduced tyre chips in 2001, followed by MBM in 2006 and a processed sewage pellet (PSP) feed system in 2012. Hope Construction’s plant in Hope, Derbyshire, achieved a fossil fuel substitution rate of around 35%.
In common with other cement manufacturers around the world, Hope Construction Materials wanted to reduce its fossil fuel intake to cut costs, become greener and to operate in a more sustainable way. The company had progressive plans to increase its fossil fuel substitution rate to over 50%. In 2013, it contacted Saxlund about the introduction of a new, future-proof fuel management system to handle solid waste fuel (SWF) more efficiently. With the objective of replacing over 50% of its fossil fuel consumption with alternative, more sustainable fuels such as SWF to power its kilns, Hope Construction Materials had some real challenges to overcome.
SWF is difficult to handle, is sticky, abrasive and not free-flowing. It can bridge and build up easily, blocking feeder systems and halting production, causing costly down-time. If left to build up or held too long in storage, SWF can become compacted due to the amount of residual moisture it contains. It can sweat and even self-combust.
The greatest challenge for cement manufacturers in using SWF is in finding an efficient, future-proof system capable of handling different types of waste-derived fuel. Hope wanted a complete end-to-end solution including reception, storage, transportation, weighing and pneumatic injection of the SWF to its two rotating kilns. On-site storage was required for a minimum of 350m3 of fuel to maintain several hours of fuel storage for uninterrupted operation and fast turn-around for trucks on site.
The solution
Following close consultation, a turn-key solution was designed and delivered using a range of bulk-handling features. Foremost of these was a 350m3 silo, combined with leading-edge push floor technology. This technology, which was patented by Saxlund in the 1970s and has been implemented in hundreds of forms and in different onerous environments over the years, is key to managing the SWF.
The Saxlund push-floor solution incorporates a series of hydraulically-operated parallel ladders, fitted as a layer into the silo floor. The motion of the ladders pushes the fuel towards the discharge end of the bunker, achieving a mass flow with ‘first in, first out’ fuel delivery. New waste is added to the top of the silo, while the push-floor extracts it from the bottom,
effectively designing out compaction and self-combustion issues.
The shearing action of ladders helps to break up the SWF. The depth is monitored by sensors above each ladder to ensure an optimal feed rate. This minimises the risk of compaction against the back wall of the bunker, which could block the system. It also prevents an over feed of fuel to subsequent equipment.
The SWF silo and push-floor system feed material to a chain conveyor and in turn into a process tower. Metals and other ferrous materials are removed using a drum magnet to protect down-stream equipment and recover metal for collection and recycling. The SWF is then fed to a star-screening system, which breaks down oversized clumps. At the push of a button the particle size can be varied from between 10mm to 50mm to optimise the fuel supply. This future-proofs Hope Construction’s solution in terms of dealing with changes in fuel composition. Oversized material that is not broken down is passed over the stars and rejected into a skip. Right-sized fuel falls into the twin screw feeders and into respective FLSmidth Pfister weighing systems.
The Pfister systems comprise fully electronic weighing, controlling flow rates with a high degree of accuracy. As waste from each screw is passed into the pre-hoppers on top of each rotor-weigh feeder, the volume is controlled to ensure a constant filling level. From here the material is passed into the rotor-weigh feeder, facilitated by an agitator in the pre-hopper.
Control systems continuously measure the bulk material inside the rotor wheel, together with its angular velocity at the discharge point, ensuring that the mass flow of the fuel remains constant. The bulk material is then fed into the pneumatic feeding pipe via a blow-through rotary valve with feeding shoe and into the pneumatic injection systems feeding each kiln and subsequent combustion. It is 70m from the fuel tower to the injection systems. Kingfisher International delivered the pipework. The company has experience in these solid fuel types and implemented wear-resistant basalt liners, such that the life-time of bends is extended six times compared to that of just using mild-steel.
Fuel processing solutions are designed by Saxlund to avoid blockages and ensure reliability, high-availability and low maintenance. Key maintenance areas, where achievable, have been installed externally to minimise disruption. This is especially true of the silo and push-floor, where complete overhauls can be carried out with a full silo.
Sustainable cement production
The new waste-derived fuel system was fired up at the Hope plant in early June 2015. The increased proportion of waste-derived fuels will allow the use of alternative fuels to be increased to above 50%, resulting in a major reduction in greenhouse gases and a reduction in the overall fuel bill. Hope Construction Materials expects to recover its investment in the next few years, once other equipment upgrades have been completed. The benefit to the UK is that the company now diverts up to 80,000t/yr of bulk solid waste away from landfill.
This work for Hope Construction Materials is an important example of what can be achieved in the cement industry. The use of waste-derived alternatives is essential for the cement industry’s future. The carbon savings alone will be significant to any cement manufacturer that is committed to sustainable manufacturing and in working towards a zero-waste economy. Pressure to change is gaining momentum and there seems little to be gained in waiting to see how things develop while the more innovative manufacturers become greener and more profitable.