Briquetting is the densification (agglomeration) of an aggregate of loose particles into a rigid monolith. In a typical briquetting process, the application of low to moderate pressure is used to compact the loose material and reduce the space between the particles.
Potential crops and forest residue for briquetting include; barley straw, rice straw, rice husk, flax straw, rape straw, sunflower stalks, sunflower seed husks, maize stalks, maize cobs, cotton crop residue, cotton stalk, wheat straw, beech, water hyacinth, wood chips, wood bark residue, pine needles, groundnut shell and grass straw.
However, for the purpose of this write-up, we shall look at the process of the production of saw dust and charcoal briquette.
Types Of Processes
An authority in briquetting, Mendis has divided the processing into two broad categories – extrusion and batch processes.
Extrusion processes, which include screw and piston extruders, pellets mills, and cubers, force the feedstock through a hole or die. The sliding friction between the material and the walls of the die heats the material and provides the backpressure needed for briquetting.
In the batch processes, which include roll briquetting, bailing and manual presses, the walls of the compression chamber provide the pressure and there is little or no friction heating.
These types of briquettes namely sawdust briquettes and charcoal briquettes are produced by pulverising sawdust which is compressed together under low or high pressure into desired shapes and sizes revealing a high grade incombustible product.
On the other hand, sawdust/charcoal briquettes are produced by carbonising or pyrolising sawdust into elemental combustible carbon and other chemical compounds that are then pulverised and briquetted into the desired shapes and sizes.
Briquetting is the physico – mechanical conversion with or without an additive, of a dry, loose wood waste of fine particles size into a solid state characterised by a regular shape and a high density.
The two main processes are distinguished by the degree of pressure exerted and the introduction of an external binding agent through basically low pressure.
Machines require binders unless heat is applied from an external source. In the high-pressure process, lignin (which is a complex chemical that impregnates and cements the wood cell wall together) softens at temperatures between 135oc and 195oc and binds the particles together. There are two distinct products: sawdust briquettes and carbonized sawdust briquettes or charcoal briquettes. Their processes are the same except that there is an addition of one process in the carbonized briquettes which is the carbonisation process.
The carbonisation process, otherwise known as pyrolysis is a destructive distillation of the raw material in a closed reactor devoid of oxygen. Here, sawdust and fillers are fed into a chamber called a carbonizer for heating to particular level before quenching. The carbonized materials are then crushed and passed through the normal production processes.
Sawdust briquetting comprises the following processes: Sawdust gathering, Screening, Drying, Pulverizing, Mixing, Extruding and Packaging.
(a)Gathering – Gathering of sawdust from the sawmills and other wood processing plants;
(b)Screening – The sawdust gathered are screened of strange materials e.g. sand and metallic components;
©Drying – Adequate control of the moisture present in the materials is necessary before briquetting. High moisture content reduces cohesion and increases energy requirement while an excessively dry feedstock requires much energy to density. Obviously, this increases the rate of depreciation of the die.
A very common mechanical drying system utilised in the briquetting process is the rotary dryer;
(d)Pulverisation – The sawdust, wood chippings, slabs etc must be reduced to the same size before briquetting; hence they are crushed into a fine powder form. However, some manufacturing processes use the shaker;
(e) Mixing -The pulverised raw materials are now mixed together with a binding agent (this may not be required in the high pressure process). Some processes injects steam to rise the temperature of the feedstock thereby reducing the energy needed for briquetting the materials while improving its heating value as well;
(f) Extrusion – The conditioned materials are then fed into the extruder, which compresses, compacts, and extrudes the product into the desired shapes and sizes; and
(g) Packaging – This is done according to market demand and specifications as they vary in shape, size and density. They could be packed in watertight plastic bags of 10, 25, and 50, 100kg weight and covered with cardboard boxes. It can be supplied in trevira bags or low-density poly bags of 500 – 1000kg weight. All potential exporters may need to consult packaging specialists.
Uses And Market Potential
Briquettes are essentially manufactured to provide cheap energy to the various sectors that need it. Most importantly, the domestic households that are finding it difficult to purchase the other sources of energy because of their costs and scarcity.
Kerosene in recent time has been scaring the local populace because of its occasional explosions, which has bereaved many homes of their children, wives and loved ones. Coupled with this, is the high cost of kerosene, many houses go through harrowing experiences which include, queuing at the filling stations for a long time, as well as having to bribe the stations’ attendants.
Charcoal on the other hand, is not always available and even where it is available, can only serve a small segment of the populace. Electricity which should have been the saviour in terms of energy supply has undergone various metamorphoses in service and ownership. This has not only increased the cost of energy supply, but has also been irregular because of the dearth of maintenance fund and a bad culture of asset maintenance which has beleaguered its ownership.
One other great fact is that sawdust briquettes will complement all these other energy sources and most importantly, substitute firewood, especially in areas where there is no regular firewood supply.
Advanced countries of the world import sawdust briquettes as a cheaper source of energy for their domestic households – for heating the home during winter in furnaces, barbecues, and small industries – like ceramics, bakeries, gardens, restaurants and other various processing plants.
For the international market, sawdust must be from identified wood specie and must be homogenised in terms of size, density, mass and moisture content. The demand requirements will be determined by the end to which they will be put.
The importance/benefits of briquetted fuel cannot be over-emphasised. The future of briquetting is very bright in a country like Nigeria. It can be a great avenue for export earnings. On the overall, briquettes could provide cheap energy to the various needs of the economy.