Civil Engineering Reference
In-Depth Information
technical facilities exist, e.g. by catalytic reduction. Shaft kilns can be fired with
wood. The raw materials in cement also emit large amounts of acidifying sul-
phur dioxide and the greenhouse gas carbon dioxide.
Sulphur dioxide can, in principle, be cleaned out by adding lime to the exhaust
gases. This is more difficult with the carbon dioxide which results from the cal-
cination of limestone. This amount of carbon dioxide is a much larger proportion
of the total carbon dioxide emissions from cement production than that caused
by the firing processes, even though coal is the main fuel. The extremely high
temperatures suggest that heavy metals are also emitted.
The problem of dust has previously received the most attention in connection
with cement production. Today the dust problem is often much reduced as a
result of closed systems for handling the clinker, more efficient dust filters, etc.
A similar pollution situation arises when calcining ordinary lime in charcoal-
kilns, even though the temperatures are somewhat lower and the use of wood as
an energy source gives a lower level of energy pollution.
The most effective step towards reducing pollution in the production of
cements lies in the increased use of pozzolana mixtures in both hydraulic lime
and Portland cements. In this way the amount of lime can be reduced, with a
reduced emission of sulphur dioxide and carbon dioxide as a result.
On building sites the use of cements can produce dust problems. Wet Portland
cement can cause skin allergies. In the construction process, cement products are
relatively free of problems, though if setting is not effective, chemical reactions
can occur between it and neighbouring materials, e.g. with PVC floor coverings.
As waste, cement products are relatively inert.
Cement production and energy use
Energy consumption in cement production varies according to the type, but is
mainly somewhere between the energy consumption levels of timber and steel
production. Portland cement has a relatively high energy consumption, largely
due to the high temperatures needed for production (up to 2000°C in the firing
zone). The cement industry is usually very centralized, and the use of energy for
transport is high.
It would be a significant achievement to reduce energy consumption in both
production and transport. A decentralizing of cement production could save a
great deal of energy, not only in transport, but also because smaller plants can be
as efficient as larger plants. Today rotary kilns are used, but smaller, more efficient,
modern shaft kilns could reduce energy consumption by 10-40 per cent. Rotary
kilns are very specialized - shaft kilns have a greater variety of possibilities. They
can be used for both calcination and sintering of most cement materials.
There are many ways of utilizing the heat loss, e.g. by production of steam, elec-
tricity or district heating. It is also possible to preheat the clinker in a pre-calcination.
This process has been developed in Japan and has saved energy in the process.
