Briquetting agricultural residues

by Andrew Russell, Consultant Engineer, Renewable Energy and Appropriate Technology; 86 Grosvenor Road, Rugby, Warwickshire CV21 3LE, UK.

Agricultural residues, often discarded or burned as wastes, occur in large amounts and have the potential to be an important source of fuel for many people in rural areas, especially in areas of ever-increasing wood scarcity.

However, it is often assumed that because agricultural residues are wastes they are freely available, but this is not always the case. Initially the residues may be free, but once a market has been established the residues may well acquire a monetary value which will increase the cost of the briquettes. In some cases the residues may already be used by the local community for various purposes: fuel, animal fodder etc. Share-cropping systems are common, in which part of the crop as well as residues are divided between the landowner and tiller. Landless people often have access to residues on common lands.

Although it is possible to use them directly, many residues are loose, low-density materials which makes them difficult to burn in a controlled and effective manner. Converting them to a more dense form, such as a briquette, could provide a means of producing a fuel that is more convenient to burn.

Briquetting is essentially a mechanical process requiring investment in equipment and training to ensure a product of reasonable quality that will perform the task for which it is intended. Although it is often seen as a relatively high-cost high-pressure technology, it is possible to use low-cost low-pressure techniques to produce acceptable briquettes.

Low-pressure briquetting

The main advantages of low-pressure briquetting are low capital costs, low operating costs and low levels of skill required to operate the technology.

Low-pressure techniques are particularly suitable for briquetting green plant waste such as coir or bagasse (sugar-cane residue). The wet material is shaped under low pressure in simple block presses or extrusion presses. The resulting briquette has a higher density than the original material but still requires drying before it can be used, as shown in Figure 1. The dried briquette has little mechanical strength and crumbles easily.

Figure 1: Low pressure bagasse and molasses blocks set out to dry.

Binders can be added to this process to improve mechanical strength and also allow dry materials to be briquetted. Binders such as molasses and vegetable starch (from maize, cassava etc.) add to the calorific value of the briquette. Materials such as clay, ash and cement can be used as a binder but they inhibit combustion, producing more ash and smoke.

In the Sudan two different techniques have been developed: one producing bagasse blocks and the other producing a charcoal substitute from carbonized cotton stalk. In the first case, wet bagasse is mixed with waste molasses as a binder, and compacted under low pressure into blocks using a standard cement building block press. The blocks are then air dried for three to four days before use. In the second case, fine dust and particles of carbonized material are fed into a rotating drum (a modified cement mixer) and molasses is poured over to bind them. The rotating action causes small balls to form which, after drying, can be used as a charcoal substitute.

In Sri Lanka a simple technique has been developed to produce corrugated briquette sheets made from coir. These are shown in Figure 2. A small amount of binder, lime, is mixed with the wet coir which is then placed into a tobacco bailing box in alternate layers between sheets of corrugated metal until the box is full. Pressure is exerted on to the layers, pressing water out of the material and also helping a chemical reaction to occur between the coir and the binder. After the briquette has been formed it requires drying for a number of days before being used. The resulting briquettes can be cut easily or broken along the corrugations to produce small logs suitable for use in small boilers and stoves.

Figure 2: Corrugated coir briquette.

High-pressure briquetting

Three types of press are in common use: piston presses, pelletizers and screw extrusion presses.

Piston presses work best with dry (15% moisture content maximum) cellulose material which is fed into a compression chamber. A reciprocating piston then forces the material through a tapered die to form a long briquette. Typically flywheel drive machines produce between 300 and 500kg of briquettes per hour while hydraulic machines can produce up to 200kg/hour.

While it may be possible to achieve the claimed service lives of between 500 and 1000 hours using relatively clean material such as sawdust, use of agricultural wastes containing high levels of silica (sand) will reduce operating hours considerably. The initial cost of this type machine is high and the briquettes are prone to breaking.

Pelletizers produce cylindrical briquettes between 5mm and 30mm in diameter and of variable length. They have good mechanical strength and combustion characteristics. Pellets are suitable as a fuel for industrial applications where automatic feeding is required. Typically pelletizers can produce up to 1000kg of pellets per hour but initially require high capital investment and have high energy input requirements.

Screw extrusion presses are common throughout Asia for briquetting. The waste material is fed into a screw which forces or extrudes the material through a tapered die, forming long briquettes (Figure 3).

Figure 3: High pressure screw-press

Material needs to be relatively dry (12% moisture content maximum) and particle size should be uniform. Briquettes produced by this method tend to be strong. They have very good burning characteristics and do not fall apart during combustion. In addition these briquettes can be carbonized which means that they could be a substituted for wood charcoal.

Screw presses are much simpler than other high-pressure briquetting equipment and cost less than piston presses. The problem with a screw feeder is that its form and pitch is designed to suit a particular particle size, so if this alters the screw is libel to jam. Also the screw form is susceptible to wear, especially when using materials with high silica content, necessitating regular maintenance.

Setting up briquetting projects

The potential for briquetting loose low-density agricultural residues into a more usable form of fuel is enormous. However, projects set up in the late 1980s and early 1990s often failed due to high capital, high maintenance and operational costs, and the failure to take into account the other uses and possible costs of the residues available.

It is important, therefore, to understand the nature and availability of the residue, the fuel market in which the briquettes are going to compete, and the type of technology being considered in terms of maintenance, operation and energy requirements.