Waste-derived fuels
The overall priority for cement makers is the safe manufacture
of high quality cement. The UK cement industry is committed to
achieving this objective in a sustainable way: environmentally,
socially and economically. To achieve greater sustainability, it is
essential that all available resources are used efficiently and
effectively. The cement industry considers wastes, with some
exceptions, to be alternative resources awaiting an appropriate use
and is actively pursuing beneficial use within its manufacturing
processes. The UK Government has committed itself to the EU
Landfill Directive and other international agreements that aim to
reduce disposal of wastes to landfill and to recovering energy and
materials from used tyres, packaging wastes, solvents and many
other waste streams. The UK cement industry is, therefore, playing
a vital role in helping to achieve the UK's environmental
objectives by utilising appropriate wastes as alternative fuels and
raw materials in the manufacture of cement, via industrial ecology.
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Traditionally, fossil fuels (coal, petroleum coke [petcoke]
and a limited amount of gas and oil) have been burnt to generate
the temperatures of around 1,450oC needed to produce
cement. Throughout the world alternative fuels have been used for
many years to partially replace fossil fuels in cement manufacture.
In the UK, alternative fuels are carefully specified and sourced
from amongst: used tyres (whole or chipped), secondary liquid fuels
(recycled inks, solvents, thinners, oils and residues), packaging
wastes (non-recyclable paper, cardboard and plastics), and, to a
limited extent, sewage sludge (pellets). On a mass basis, fuel
accounts for only 10% of the 'throughput' of a cement kiln; the
rest comprises raw/mineral materials. On a thermal basis, the level
of replacement of fossil fuels by alternatives is only 6% averaged
over the UK whereas it is up to 50% in some EU countries.
Alternative fuels, like fossil fuels, are predominantly
hydrocarbons which, when burnt during cement manufacture, are
almost entirely decomposed to carbon dioxide and water with
insignificant amounts of more complex carbon-based compounds
1. A very small residue or ash is formed from liquid
fuels, larger quantities will be formed from solid fuels but all
are chemically bound in cement clinker. In particular, tyres burn
to give a residue that is mainly iron, thus reducing the need for
additional iron ore to be added as a raw material component.
Portland cement, the major type of cement, is made from
mixtures of mainly natural mineral materials, principally
chalk/limestone and clay/shale. These raw materials can be
partially replaced by wastes and by-products from other industries
in order to 'fine tune' the overall chemical composition.
Conversely, additional amounts of natural minerals would have to be
added to achieve this 'fine tuning' requirement. Alternative raw
materials currently in use are: power station fly ash, foundry
sand, mill scale (steel production) and iron from used tyres. Less
commonly, china clay wastes and colliery shale have
been used. Work is also underway to introduce wastes rich in
silica, iron, alumina and lime minerals from, for example, the
water and automotive industries. The alternative raw materials used
are mainly inorganic in nature, carefully sourced and subject to
specifications to which suppliers must comply. Alternative raw
materials are processed through the high temperature kilns (solids
at 1,450oC, flames at 2,000oC) in exactly the
same way as natural materials.
The cement industry is primarily regulated by the European
IPPC Directive 2 implemented in England, Wales and
Scotland through the Pollution Prevention and Control Regulations
2000, together with the Hazardous Waste Incineration Directive
3 and now the Waste Incineration Directive 4.
Additionally, the Environment Agency (EA) places specific demands
on the cement industry through its Substitute Fuels Protocol (SFP)
as modified by the Tyres Protocol. The
Environment Agency is responsible for issuing licences or 'permits'
to cement kilns in England and Wales whereas the Scottish
Environment Protection Agency (SEPA) is responsible for Scotland.
In order for a 'permit' to be granted for burning a non-fossil fuel
the EA/SEPA requires an 'environmental assessment' to be carried
out to demonstrate that the impact of the factory when using the
waste will be no greater (and in reality less) than when using
traditional resources.
The introduction of changes to long-established operations
such as cement works can cause interest, or sometimes concern,
amongst communities and other stakeholders. The cement industry
engages with all interested stakeholders through regular, open
communications about any aspect of its operations. The cement
industry's key stakeholders include the neighbours/local
communities, employees, customers, shareholders, regulators,
'green' issue interests and those who depend on the industry for
their livelihood. However, experience has shown that the
stakeholders who become most involved are the local communities and
the regulatory bodies.
When any proposal is made to use an alternative fuel the
cement manufacturer will include it in its open dialogue with
stakeholders. This is done at the earliest possible opportunity
through established 'open door' policies, formal open days, liaison
committees and newsletters. During trials, reports on progress may
be published weekly, supported by opportunities for interested
groups to see the fuel being used at first hand.
The elements that make up the regular dialogue develop with
the communities' involvement, which brings advantages to both
parties. Manufacturers are better able to incorporate feedback into
their plans and the transparency of the process means that the
community is involved, consulted and reassured.
The main indicators of cement quality are the requirements for
mechanical, physical and chemical properties and are standardised
in European/national product specifications 5. These
requirements must be met regardless of either the type of fuel used
or the nature of the raw materials. These properties are measured
and monitored continually, under independent third party scrutiny,
in order to ensure that cement conforms to its specification and
can be legally placed on the market. The use of wastes has no
effect on these major properties because any potential effects will
have been accounted for by making compensatory adjustments to the
chemical composition of the raw material fed to the kiln.
Does the industry go beyond regulation to ensure safety?
The UK cement industry goes beyond properties and 'qualities'
that are not specification requirements in order to fulfil its own
commitments to health and safety. For example, the 'heavy metals
profiles' are measured using the most sophisticated analytical
instruments available (e.g. inductively-coupled plasma techniques
such as ICP-MS and ICP-AES). Although the quantities of individual
heavy metals contributed from different wastes and different
traditional materials can vary, the levels in the cement are always
in the 'parts per million' (ppm) to the 'parts per billion' (ppb)
ranges and of little consequence.
In the case of any organic/carbon-based substances, there are
no known compounds, whether present in fuels or as contaminants of
raw materials, which can survive thermal decomposition at the
temperatures of cement kilns and still be found in measurable
amounts in the final cement.
Does using wastes as fuel and raw materials have any effect on concrete performance? Is durability affected?
The recovery of wastes as alternative fuels and raw materials
in cement manufacture is a mature practice in many countries e.g.
United States, Germany, Belgium, France and Switzerland. To date,
there have been no problems of concrete performance related to
their use. Indeed, it is extremely unlikely that any durability
problems will ever emerge given the tight specification of 'kiln
inputs' and 'kiln outputs' and the compositional congruence of
cements made with or without waste materials.
The UK cement industry is not, however, complacent and in any
durability investigations that are undertaken, cements made using
alternative fuels and alternative raw materials are included.
Does leaching occur?
Irrespective of the cement type or its process of production,
the quantities of heavy metals, or organics, leached have generally
been at, or below, the limits of detection (LOD) of the sensitive
analytical instruments used. Where they have been detected, they
have been present at orders of magnitude less than any regulatory
'maximum admissible concentrations' 7.
Leaching tests, of various types, carried out on 'monoliths'
(solid specimens with formed surfaces) of hardened mortar and
hardened concretes in the UK 6,
USA 7, and France 1 have shown that it
is not possible to distinguish between cements made with
traditional fuels and raw materials from those made with partial
substitution of alternatives.
Further information
For more information on waste-derived fuels, contact the British Cement
Association.
References and background reading
1. Incinerating waste in the kiln of a cement works does not
affect the use of cements, Association Technique de L'industrie des
Liants Hydrauliques (ATILH), 1994
2. Council Directive 96/61/EC concerning integrated pollution
and control
3. Council Directive 94/67/EC hazardous waste incineration
[until superseded on 28 December 2005]
4. Council Directive 2000/76/EC on the incineration of
waste
5. British Standards Institution/European Standards
Organisation BS EN 197-1 : 2000, Cement-Part 1: Composition,
specifications and conformity criteria for common cements
6. Improved materials testing, Leaching tests on ready-mixed
concrete, Report to the UK Department of the Environment, Transport
and the Regions, DETR/DWI No. 4551/1, 1998
7. A comparison of metal and organic concentrations in cement
and clinker made with fossil fuels to cement and clinker made with
waste derived fuels, Colucci M, Epstein P and Bartley B, NSF
International, Portland Cement Association, 1993