Cement Kilns
The production of cement utilizes waste materials in two primary ways, firstly using specific Industrial Waste and Mining & Quarrying Waste (and waste from the kiln part of the process itself) as additives and replacement for traditional raw materials, and secondly as a fuel to substitute for the use of traditional fossil fuels in a cement kiln. In 2018 1,414,195 tonnes of waste was used as raw materials and fuel in the production of cement, with just over 43% of kiln thermal input replaced by Waste Derived Fuel. The five companies manufacturing cement together supplied 78% of the cement consumed in the market in 2018. [1].
The Process
There are six main stages of cement manufacture[2]:
- Stage 1 - Raw Material Extraction: raw ingredients include limestone, sand and clay, shale, fly ash, mill scale and bauxite. Materials are mined, crushed and reduced in size.
- Stage 2 - Grinding, Proportioning and Blending: the production of a fine homogeneous mix with additives that comprises primarily around 80% limestone and 20% clay.
- Stage 3 - Pre-heating Raw Material: a pre-heating chamber using kiln 'exhaust' gases to reduce energy consumption and pre-condition the raw material into oxides.
- Stage 4 - Kiln Phase: the principle phase which produces a clinker, exiting the kiln in marble-sized nodules - it is the binder that is used in cement products.
- Stage 5 - Cooling and Final Grinding: the clinker is rapidly air-cooled and gypsum (and other active ingredients as required) is added to the clinker and ground to a fine power to produce cement
- Stage 6 - Packing and Shipping: the cement is conveyed from grinding mills to silos where it is then packed in bags.
Not all cement kilns have the total 'end to end' process on site, with some sites producing the clinker for subsequent grinding, blending and packing at a different site. This might include, for example, the introduction of other materials, such as Blast Furnace Slag to produce a Blast Furnace Cement - a highly sulphate resistant cement that can have up to 65% granulated Blast Furnace Slag mixed with the clinker.
Use of Waste - Historical Context
The introduction of waste materials - or 'Alternative Raw Materials' as coined by the industry - can be as part of the kiln phase or the final grinding stage and it can be used as an additive or as a fuel in the process.
In 1995 there were 20 cement factories in the UK producing around 15 million tonnes a year of cement each year, with the cost of energy in the process representing 40 to 50% of running costs. There was a drive to substitute traditional fuels (coal and petroleum coke) with cheaper fuels slightly before this date, and trials were still running at this time with the ramifications of these trials captured in a Parliamentary Briefing Note[3]. Waste Derived Fuel had already started to play a more significant role in UK cement processing, in line with other countries (especially in Europe). It started with the use of solvent based hazardous wastes, named by some operators 'Chemfuel', which was being trialed at a substitution of 50% of traditional coal fuel (although at the time 100% substitution occurred in Belgium). At around the same time shredded/chipped waste tyres were starting to also be used in some cement kilns as a Waste Derived Fuel. By 1998 the MPA reports that 446,511 tonnes of waste by-products were used as fuel and raw materials, accounting for 4% of cement production and the proportion of fuel comprising waste was 5.7%[4].
By 2005 the range of alternative fuels had expanded in Europe to include tyres, plastic, paper, Refuse Derived Fuel, dried sewage sludge, meat and bone meal and some other waste streams, with fuel costs typically accounting for 30 to 40% of operating costs[5]. This was driven by the cement industry looking to reduce its costs and carbon emissions, and by changes in the waste sector around the Landfill Directive and the introduction of the Landfill Tax which made cement kilns an alternative outlet for suitably prepared waste materials/Waste Derived Fuels.
By 2010 the MPA reports that 1,528,315 tonnes of waste by-products were used as fuel and raw materials in UK cement manufacture, accounting for 7.2% of cement production and the proportion of fuel comprising waste was 39.7%[6].
By 2018 the MPA reported the percentage of recycled content of cement was 1,414,195 tonnes in UK cement manufacture, equivalent to almost 10% of cement production 2018. This same report notes that since 2013 the total waste used has not varied by much more than +/-150,000 tonnes per year, and the proportion fuel comprising waste was 43% in 2018 and has not changed by much more than +/- 1.5% since 2013[7].
In addition to the use of Waste Derived Fuel in UK cement kilns, RDF and more commonly SRF has been increasingly exported from the UK to Europe since 2009. This has been driven by demand in Europe, combined with a stagnant demand in the UK cement kilns, and an increasing Landfill Tax that has has made export commercially viable/attractive.
Use of Waste - Current Context
The UK cement kilns, on the whole, have waste permits and complete returns of the waste handled each quarter. The most recent data for each site is listed in the site specific page per cement kiln, although no returns have been made/are available for Cookstown, Purfleet and Dunbar. An analysis of the waste returns for 2018 is summarized as follows:
As Blast Furnace Slag is considered a primary input for this type of cement, it is not defined as waste and therefore is not normally captured in UK Waste Tonnage figures or waste permit returns.
List of UK Cement Kilns
More detailed information can be found by clicking through to the site-specific page:
List of UK Grinding and Blending Sites
| Site | Company | Process |
|---|---|---|
| Tilbury | Cemex | Grinding and Blending |
| Dagenham | Breeden | Blending only |
| Dewsbury | Breeden | Blending only |
| Theale | Breeden | Blending only |
| Walsall | Breedon | Blending only |
| Barnstone | Tarmac | Grinding only |
| Celtic Ash | Tarmac | Blending only |
| Northfleet | Tarmac | Blending only |
| Seaham | Tarmac | Blending only |
| Scotash | Tarmac | Blending only |
| West Thorrock | Tarmac | Blending only |
[8].
References
- ↑ MPA Cement Sustainability report 2019
- ↑ https://civiltoday.com/civil-engineering-materials/cement/106-cement-manufacturing-process
- ↑ Parlimentary Briefing Note 60 April 1995
- ↑ MPA Cement Sustainability report 2019
- ↑ Environment Agency Science report on Solid Waste Derived Fuels in Cement Kilns, an International perspective
- ↑ MPA Cement Sustainability report 2019
- ↑ MPA Cement Sustainability report 2019
- ↑ MPA Cement Sustainability report 2019