Waste Fuel Data

As has been pointed out elsewhere, the development phase of the British cement industry was characterised by - and critically influenced by - the availability of energy so cheap that energy-inefficient and highly-carbon-polluting plant became the accepted technology. From the 1970s, it began to dawn on the industry that fossil fuels in the future would be more expensive and harder to get, both for geopolitical reasons and because they were becoming physically depleted. This led to a move to explore alternative sources of energy, and in particular, waste materials that had zero or negative price (i.e. they incurred disposal costs). Because such wastes are generally inherently inferior fuels (otherwise they would not be thrown away), considerable work was needed to modify the process to make use of them.

The use of energy-bearing wastes was occasionally practiced before the 1970s. Both Cousland and Padeswood used coal-rich colliery waste as a rawmix component. The latter has cleared the entire waste tip of Llay Main colliery, and is embarking on clearance of that of Bersham colliery. Apart from these, waste fuel usage was on a very small scale, if it was used at all, consisting of such practices as adding used lubricants to the main fuel tanks on oil-fired plants.

Domestic Refuse

Sometimes referred to as refuse-derived fuel (RDF). From the 1970s, the first attempts at using waste streams consisted of installing plants to process municipal refuse which would otherwise be landfilled. This consisted of separating metals and other inorganics on site, and shredding the rest, consisting mainly of paper with some plastic, into pieces sufficiently small to be blown into the kiln alongside the main firing pipe. This system operated at Westbury from 1977 to 1992, in co-operation with the local authority. The process ceased to operate when co-operation was withdrawn. Since then, the introduction and escalation of landfill taxes has put more pressure on authorities to co-operate in such schemes, and a number of plants are now installing plant, typically with the preparation process contracted out. Because the combustible portion of domestic refuse is mainly paper, the fuel ranks as very low-grade, and large usage rates can result in considerably reduced output due to the reduction of flame temperature. This can be ameliorated – at some cost – by enriching the combustion air with oxygen.

The plastic and paper components of the waste contain large and variable amounts of mineral fillers of which the most common is calcium carbonate, but also kaolin, rutile, gibbsite and talc. To control its effect on clinker chemistry, a considerable degree of blending of the waste is necessary.

Domestic waste from a large pooled collection facility, divided into its conventional sub-categories, has the following properties:

Landfill Gas

Aside from the lack of suitable land, one of the main reasons that landfills are today discouraged is that organic material – both paper and plastic – is decomposed by bacterial and fungal action to produce methane – a gas with 20-100 times the “greenhouse effect” of carbon dioxide. In general, the methane percolates out into the atmosphere, although in some circumstances it may disappear into joints in the surrounding rock and re-appear some distance away. The problem can be ameliorated by covering the landfill with an impermeable layer, penetrated at regular intervals with flare-pipes, at which the methane can be burned.

Many cement plants have adjacent landfills as part of their quarry restoration programme, and in some instances, gas that would otherwise be burned is collected and used as fuel. Kiln fuel was supplemented by this means at both Norman and Swanscombe from 1983 onwards. The landfill must be carefully compacted and sealed to allow anaerobic decomposition to be established: even then, at least one third of the carbon ends up as CO₂. Progressively more CO₂ (rather than methane) is produced if aerobic conditions persist. The source must be put under suction in order to obtain the gas, and the result contains fair amounts of air as well, so the gas is a low-grade heat source, providing heat, but tending to reduce flame temperature.

Tyres

A much better fuel is tyre rubber. The mixture of natural and synthetic polymers used to make tyres has combustion properties similar to (or better than) those of coal. Tyres that have been worn beyond legal use have long been landfilled. Because they remain hollow and flexible, they can’t be compacted into a landfill in the normal manner, and when buried they have a peculiar tendency to work their way to the top of the landfill pile and re-emerge at the surface. Here they become mosquito-nurseries and homes for vermin. Because of this, they have become unpopular at landfills, while tyre-only dumps are of course a major fire hazard. Destroying tyres by ordinary incineration is difficult because of the production of polluting combustion products and black smoke. On the other hand, in a cement kiln, extremely high temperatures and oxidising conditions allow tyre rubber to burn almost instantaneously, and the alkaline conditions scrub out such combustion products as SO₂.

The obvious difficulty in using tyres as a conventional fuel is that they can’t be ground sufficiently finely to be used in the front-end of the kiln. Tyres can – at some expense – be shredded into pieces a few centimetres across, but blowing these into the front of a kiln has much the same effect as firing similar-sizes lumps of coal: clinker mineralogy is destroyed by gross localised reducing conditions. The first successful use of tyres (Hope 1981, Cauldon and Plymstock 1982, Weardale 1984), consisted of adding them – either whole or shredded, at the rear of preheater kilns. The gas temperature at this point is typically above 1000°C, allowing rapid and complete combustion and the resulting energy is used in the preheater. Subsequently, a similar philosophy was employed in long wet and long dry kilns by injecting tyres into the kiln at a point about half-way along. The use of whole tyres has the obvious disadvantage that the fuel is fed in intermittent dollops, giving rise to “CO spikes” in the exhaust gas composition which upset the volatile cycle and may trip electrostatic precipitators. More recently, purpose-built off-set firing chambers have been designed to burn whole tyres as part of precalciners, and these probably represent the optimum technology. The attractiveness of tyres as fuel is such that most kilns now use them, at anything up to 40% replacement of their conventional fuel, depending on availability.

Tyres consist of three main components: rubber of largely hydrocarbon composition, webbing which is typically nylon or other synthetic fibre, and steel wire. All this burns: the wire is sufficiently small in diameter to readily burn to iron oxide, which is incorporated into the clinker. The rubber component consists of a wide variety of polymers (in descending order of occurrence):

• natural Hevea rubber
• styrene/butadiene co-polymer ("SBR")
• polybutadiene
• 2-methyl propene/styrene co-polymer ("butyl")
• acrylonitrile/butadiene co-polymer ("nitrile")
• poly(2-chlorobutadiene) ("neoprene")
• polysulfide rubbers

It also contains fillers in the form of carbon black (10-25%) and clays and pigments, and hydrocarbon oils (5-15%). It contains a significant amount (~2%) of zinc oxide as a vulcanisation accelerator. This is totally absorbed in the clinker, and being for most clinkers the sole source of zinc, a clinker’s zinc content is a good indicator of the amount of tyre rubber burned in its production. Tyre rubber contains 1-2% of sulfur, mainly from vulcanisation: this is no more than is present in most fossil fuels. Nitrogen derives from the nitrile content, and chlorine from the neoprene.

The steel component varies little: it is iron with around 0.7% carbon and 0.3% manganese.

The textile consists of nylons (contributing nitrogen), polyesters and cellulose.

Despite the scope for variation in the formulation of tyres, in practice their composition hardly varies, and they are thus one of the more dependable fuels.

A survey of 23 tyre sources (17 car tyre types and 6 truck tyre types) in the 1990s gave the following results:

Values of tyre GCV less than 30 MJ are occasioanlly reported. These are certainly in error. The correct procedure involves gathering a statistically valid number of tyre or tyre chip sectional slices, and grinding these by attrition in liquid nitrogen, typically in a ring-and-puck mill. This process renders the rubber as a fine powder that can be treated as a normal fuel, and quantitatively separates the textile and steel.

Meat and Bone Meal

Meat and bone meal (MBM) is the dried and ground residue of waste meat and carcass rendering, after most of the fat has been pressed out. Historically, most of it was used in animal feed as a protein source, but since the BSE scare, regulations have severely restricted that outlet, so that cheap MBM has become available as a fuel, and has been used at many plants. The mineral content, deriving largely from bone content, is high in phosphorus. Typical analysis of the ash:

The amount of phosphorus limits the amount that can be used to about 1 MJ/kg, because high levels of phosphorus destabilise alite. At high levels of fuel replacement, the variability of the MBM composition impacts clinker variability.

Biomass Sludges

These include both paper waste sludge and sewage sludge. Sometimes referred to as prepared sewage pellets (PSP). The sludge and filter cake remaining after bacterial decomposition and settling of sewage is dried and heat treated to obtain a sterile pelletised solid. The gross calorific value is given as 14-21 MJ/kg (nett/gross ratio about 0.93), with sulfur up to 1%, chlorine up to 0.3% and phosphate up to 2%. As of 2014, it has only been used at Cauldon, Dunbar and Hope.

Waste Liquids

Sometimes referred to as recycled liquid fuel (RLF), or substitute liquid fuel (SLF). The use of waste lubricants as a fuel has been practiced on a small scale since the 1950s. Since then, more specialised equipment has been developed to burn a wider range of liquids, including waste solvents, cooking oil and other liquid organics. All these are materials which are difficult or impossible to dispose of by other means, and in many cases the cost is negative - i.e. a charge can be levied for their disposal. Because most of these come from ad hoc sources, no generalised data are applicable. However, the properties of individual solvent components that might be present can be listed:

The combustion air used has 50% humidity at 20°C (see composition). "Gross" calorific value is otherwise known as Upper Heating Value. "Nett" calorific value is otherwise known as Lower Heating Value.