Greenhouse Gases

2.1 What are greenhouse gases?
Greenhouse gases are those gases that have a “greenhouse” effect; that is, they retain heat and have a warming effect, just as greenhouses that are used to maintain warm/temperate conditions to nurture plants.

2.2 Principal greenhouse gases are:

1. carbon dioxide (CO2)

2. methane (CH4)

3. oxides of nitrogen (NOx)

Out of these, CO2 and NOx are the most relevant because they are an integral part of the process of production of cement. Carbon dioxide is released in the process of calcination and also in the process of combustion.

2.2.1. Carbon dioxide from the process of calcination
In the production of cement, the principal raw material, limestone, is calcined into calcium oxide, a major constituent of cement clinker. Portland Cement clinker contains about 65% CaO.
Limestone which is calcium carbonate in calcination releases carbon dioxide as per equation:

3=CaO+CO2

=56+44

For every kg of CaO, CO2 released is 0.79 kg.
Therefore for every kg of Portland cement clinker, CO2 released is ~ 0.51 kg.
(Intergovernmental Panel for Climate Change (IPCC) default value is 0.525 kg/kg clinker, corresponding to ~ 67% CaO in clinker)1
Expressed in relation to cement, it would be ~ 0.49 kg per ton of
Ordinary Portland Cement (OPC) which has ~ 5% gypsum in it, with a cement to clinker ratio 1.05:1.
In the case of blended cements, the ratio of cement to clinker is much higher.
In the case of Portland Pozzolana Cement (PPC), which can contain up to 30% fly ash, the cement to clinker ratio would be ~ 1.54 (percentage of gypsum remaining the same).
Therefore CO2 released per kg of cement would be ~ 0.33 kg. In the case of slag cement, which can contain up to 60% blast furnace slag, the cement to clinker ratio would be 2.86 and the CO2 released per kg of cement would be ~ 0.19 kg.
Thus the obvious way to reduce emissions of CO2 in relation to cement produced is to make blended cements.

2.2.2. Carbon dioxide from the process of combustion of fuel
In producing cement clinker, heat is first required to calcine raw meal fed to the kiln. Raw meal is then sintered at a temperature of approximately 1400 °C to produce cement clinker.
Presently most of the cement plants are dry process cement plants with 5/6 stage preheaters and calciners and high-efficiency clinker coolers.
The heat is supplied by firing fuel, mostly coal, sometimes oil or gas.
To derive maximum benefit of the calciner, fuel is divided between the kiln and calciner in the ratio 40:60.
Total heat consumed in making clinker is expressed as sp. fuel consumption in kcal/kg clinker. It takes into account various losses in the process.
Presently the representative value of sp. fuel consumption can be taken as 700 kcal/kg, though quite a few plants have achieved efficiency of 650 kcal/kg.
Coal used in making cement varies greatly in quality, depending on its source. The calorific value of coal is around 4500-5000 kcal/kg.
Coal often comes from different sources and therefore varies in calorific value, ash content, and moisture.
Most cement plants therefore install facilities for blending coal in the form of circular/linear stacker reclaimers so that they can fire in kilns and calciners coal of uniform quality.
Let us assume for the sake of this exercise that the representative value coal is:

useful calorific value: 4500 kcal/kg

ash < 30%

fixed carbon ~ 50%

For a supply of 700 kcalories, coal used would be 700/4500 = 15.6% or 0.156 kg per kg clinker. It would contain 50% or 0.078 kg carbon, which will produce carbon dioxide:

+O2=CO2

+32=44

Therefore the coal burnt would produce 0.29 kg of CO2 per kg clinker.

2.2.3. Total impact
Adding this to the CO2 released by way of calcination, total CO2 released = 0.51 + 0.29 = 0.80 kg/kg clinker emission in terms of kg/kg cement would be:

The beneficial impact on reduction of GHG emissions as result of making blended cements can clearly be seen from the above table.

2.3 NOx
It is possible to control generation of NOx by admitting feeds of raw meal, fuel, and tertiary air at multiple levels in the calciner to create reducing zones. A reducing agent with an ammonia content of about 5% can also be added in the reducing zone to bring down NOx concentration.

2.4 Potential for Reduction of Current GHG Emissions when making Blended Cements
Potential for reduction in current GHG emissions by making blended cements will differ from country to country.
The principal factors influencing it would be:

1. Proportion of blended cements already being made in relation to total volume of production.

2. Division between PPC and BFSC – is the proportions of these two types of cements made in the total quantity of blended cements produced.

3. The three industries involved—cement, steel, and power.
Suppliers of blending materials are the power and steel industries.
Growths of these three industries may or may not synchronize and hence slag or fly ash may not be always available in quantities required by the cement industry.
There is a limit to the proportion of fly ash that can be added.
Today the maximum permitted is 35%.
Unless this proportion can be increased there is little scope for increasing the quantum of PPC produced.

4. In India, presently the proportions of different types of cement made are2:

OPC 25%

PPC 66%

BFSC 8%

All available slag is used. At the current production level, therefore, the maximum scope is to make all OPC into PPC.
Further, if the proportion of PPC averages 27% it can be raised to the maximum permissible 35%.
The potential for reduction can be worked out.
Since the situations will be different from country to country, with standards prevailing also playing an important role, each country will have to work out its own potential for reduction of GHG emissions by making blended cements.

5. Fortunately no major changes are involved in beginning to make blended cements or enhancing the proportion thereof.
What is required is storage for the blending materials—fly ash or slag, and their withdrawal and feeding to cement mills.
This is easily possible in most cases in existing plants and can be planned in new plants.
In new plants there is also a...