Fired clay products and reduced energy consumption

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Energy consumption in the manufacture of fired clay products is very high and thereby also energy related pollution; above all, emissions of the greenhouse gas carbon dioxide where firing is based on fossil fuels.

8.12

Section through a Pakistani mobile rotating kiln for the production of expanded clay pellets. The kiln is about 5 m long with an inside diameter of 500 mm. The rate of production is about 125 kg per hour. Source: Asfag,1972.

8.12

Section through a Pakistani mobile rotating kiln for the production of expanded clay pellets. The kiln is about 5 m long with an inside diameter of 500 mm. The rate of production is about 125 kg per hour. Source: Asfag,1972.

The brick industry uses large amounts of oil-based energy to dry the unfired brick before firing. The required temperature here is relatively low, which means that solar energy and recovered waste heat from the kilns could be used. Recovered heat could also be used to preheat the kilns.

As noted, the consumption of energy in the kilns depends on the type of kiln, the roller kiln being the most effective. Perforated and hollow products also require relatively less energy, as do products where biomass is added. However, there is probably most to gain by using products fired at lower temperatures. Many bricklayers will remember the routine of using low and medium-fired bricks as the internal leaf in cavity walls and well-fired bricks on the outside. Only vitrified and well-fired bricks are commonly available on the market today unfortunately. Since the use of energy increases by about 0.2 MJ/kg for every 100 °C increase in the firing temperature, the brick industry could reduce its total energy consumption substantially by returning to differentiated firing. This could go a step further by using unfiredbricks for internal or rendered non-loadbearing walls. There is no technical barrier to this, even in large buildings (see Figure 14.12). The unfired brick also has exceptionally good moisture regulating qualities.

Energy consumption is also related to transport. Fired clay products are heavy and industries producing them are relatively centralized. It is worth considering whether it is ecologically sensible to use brick in an area with no local brick factory. This is especially relevant for areas that cannot be reached by water, since transportation by boat uses less energy.

Simple technology and the relatively widespread availability of clay gives brick and clay tile production many potential advantages for local manufacture. For light expanded clay products it should also be possible to have competitive manufacture at the local or regional level, as well as mobile manufacturing plants.

Recycling must also be considered, since the energy consumption in manufacture is so high. Fired clay products are very durable. For bricks fired at high temperatures, a lifespan of 1000 years seems feasible (Gielen, 1997; Minke, 2006). The energy needed to remove and clean these materials after use only represents 0.5% of the energy required for manufacture. However, the re-use of bricks is only possible if a weak or medium strength mortar has been used. Products such as

roof tiles that have no mortared joints have a very high reusability potential. Bricks can also be ground to pozzolana powder, if they were originally fired at temperatures no higher than 600 °C.

Light expanded clay that is free from mortar, such as in insulation underneath a shallow foundation, can be easily re-used in the same way if it has been protected from roots, sand and earth.

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