Designer Aggregate

Manufactured “Designer” Sintered Aggregate from waste materials & process by-products.

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Background

Pulverised Fuel Ash (PFA) from a coal fired power station producing electricity has long been established as a prime material for the manufacture of high strength lightweight aggregate using a Sinterstrand. Providing the ash quality is consistent, a high quality product is made.

The coal used will change as new contracts are placed in the world market. Different coals produce different ash which will also vary depending on the power station load and output consistency, the carbon content being one of the major variants. These variations can be disastrous for the Sinterstrand process.

Clay and shale based lightweight aggregates have traditionally been manufactured using cement style rotary kilns. The clays and shales used are chosen specifically for their natural bloating quality. However these materials are not widely available.

The Trefoil Process

The Trefoil process is based on rotary kiln technology. The Trefoil kiln, with its unique features, has double the capacity of a conventional rotary kiln of the same size. The Trefoil kiln motion ensures even firing of the aggregate. Combustion air can be introduced at zones along the kiln giving greater control over the combustion process.

The Process incorporates the burning out of organic and carbon fuel from within the pellet which makes the aggregate lightweight. The organic fuel is in the form of waste materials, typically sewage cake, the biodegradable part of municipal solid waste etc. These wastes supply the fuel for the thermal part of the process, make the aggregate lightweight and revenue is earned from their disposal.

The bulk of the aggregate filler material is a combination of ash and clay. PFA is the material which produces the aggregate strength. It has to be blended with an amount of clay to enable the PFA/clay/sewage cake to make a pellet durable enough to be transported through the process. The proportions of materials can be varied to give different aggregate qualities and also allow the introduction of other materials. The aggregate produced is therefore designed to meet its end need rather than be a resultant product of the input material.

Designer Aggregates

The blending of materials enables aggregates of differing densities and strengths to be produced The energy for the process comes from the waste materials used which earn revenue for their disposal and make the aggregate lightweight The process enables the operator to have the financial choice between aggregate strengths and revenues from both the aggregate and the
input materials.

Materials Used

For all materials a balance has to be determined between waste revenue, effect on product and resulting product revenue, cost to incorporate into process and effects on emissions.

Bulking materials

Any material which can be handled, dried to powder and will sinter below 1200°C including:

• PFA
• Clay
• Other ashes from MSW, CHP, sewage incineration etc.
• Shales
• Aggregate crushing / washing fines
• Glass fines (also acts as flux)

Fuel Materials

Any biodegradable material which can be handled and can either be dry powdered or with slurry including:

• Sewage cake
• Biodegradable part of MSW
• Carbon ash / slag from pyrolysis / gasification
• Paper pulp cake
• Farming slurries and chicken / turkey wastes / animal by-products
• Effluent sludge

Designer Aggregate Options

The mixing process used to form the aggregate pellet allows the ratios of the materials used to be varied. This enables choice between strength, density and revenue. The strength of the aggregate is given by the bulking material used; PFA is an excellent base material to give high strength. The process does not facilitate the manufacture aggregate only from PFA. The dried green pellet (before entering the kiln) has to have sufficient strength to withstand the transport between the pelletising process, the dryer and delivery to the kiln and then the thermal shock of entering the kiln, which is not possible with PFA alone. To give the pellet green strength when dried a binder must be added. The simplest binder to use is a non-bloating clay.

Depending on the clay, usually a 10% minimum is needed, although some clays will work with as little as 5%. The aggregate with the 10% clay content is very similar to an all PFA aggregate produced on a Sinterstrand. The clay ratio can be increased up to 30% with virtually no noticeable changes in characteristics of the final aggregate. This additional clay can be very useful when balancing against a higher carbon content in PFA. This would otherwise require an extended burn out time in the kiln thus reducing output. By increasing the clay content high carbon in the PFA level can be compensated for in the overall pellet mix. This PFA and clay mix, without organics, will produce an aggregate with a loose bulk density of around 900kg/m3. The process would require burners to be used to achieve the sintering.

One of the basics of the Trefoil Process is the addition in the pellet mix of sewage cake or other organic materials. The burning out of this material from within the pellet creates voids which makes the aggregate lighter in weight. This process starts at the pellet stage. The green pelletisation process needs around 20% moisture. The pellets are then dried with the process maintaining the pellet size. This effectively loosens the compaction within the pellet enabling easier combustion of the volatiles. The volatile material combustion starts immediately on the green pellet being fed from the chute entering the kiln helping the bed temperature rise quickly. The feed end of the kiln has a bed temperature of 500°C. This rapidly rises to 800°C which is the temperature at which the carbon burn out from the PFA starts.

Temperatures are measured along the kiln length with the profile shown on the control screen. The optimum position for the 1170°C sintering is ¾ of the way through the kiln. The kiln runs with a fixed bed and the sintering position is adjusted by altering the feed rate. Combustion air is fed into the kiln in two locations, at the discharge hood and the close to the feed end.

Limiting the combustion air at the discharge end will control the PFA burn out. When balanced, the kiln will be operating entirely from the energy within the pellet mix. The volatile material in the pellet entering the kiln requires excess combustion air and this is fed into the kiln a ¼ of the way along from the feed hood.

The density of the aggregate is governed by the bulking materials used and the burnout of the biodegradables. By increasing the biodegradables in the pellet mix the aggregate manufacture will be lighter. The introduction of the sewage cake to the basic PFA/clay mix adds important fuel to the process, and reduces the aggregate density from 900kg/m3 to 850kg/m3.

It is the moisture in the sewage cake which determines the amount of cake which can be put into the pelletising process. The pelletisers work at around 20% moisture. If a lighter aggregate is desired then more organics need to be added to the mix. To enable this to happen the incoming material needs to be dryer than the 22-24% dry solids sewage cake. The biodegradable (BMW) part of municipal solid waste (MSW), when separated using a conventional materials recycling facility (MRF), will be around 40% dry solids. Therefore for the same moisture content considerably more organics, of similar energy values to that contained in the undigested sewage cake, can be added to the aggregate with resultant reduction in density. In operation it will probably be a mixture of sewage cake and BMW used. Not all manufactured aggregates will need the high strength of the traditional PFA aggregate. Other bulking materials which are fee earning can be used to dilute the PFA and manufacture an aggregate of a lower strength aimed at specific fit for purpose uses.

Excess Heat Energy

A stand alone Trefoil Plant uses its kiln exhaust for the clay drying process. Exhaust leaves the kiln at around 1150°C and cooled (water cooled) to around 500°C before entering the drying process, so dumping valuable energy, therefore the Trefoil Plant has the potential for excess heat energy for an alternative use.