Methodological issues

CO2 AND CH4 FROM SILICON CARBIDE PRODUCTION

Silicon carbide is produced from silica sand or quartz and petroleum coke, which is used as a carbon source, according to the reactions (Austin, 1984; p. 262):

The formula describing the overall reaction is given below but in practice it does not proceed in the stoichiometric proportions indicated:

The formula describing the overall reaction is given below but in practice it does not proceed in the stoichiometric proportions indicated:

In the production process, silica sand and carbon are mixed in an approximate molar ratio of 1:3. Some carbon, about 35 percent, is contained in the product and the rest is converted to CO2 in excess oxygen and released to the atmosphere as a process by-product.

The petroleum coke used in this process may contain volatile compounds, which will form methane. Some methane will escape to the atmosphere, particularly during start-up.

CO2 FROM CALCIUM CARBIDE PRODUCTION

Calcium carbide (CaC2) is made by heating calcium carbonate (limestone) and subsequently reducing CaO with carbon e.g., petroleum coke. Both steps lead to emissions of CO2. Around 67 percent of the carbon from petroleum coke will be contained in the product.

The basic reactions are:

The CO gas will be used as an energy source in most plants.

The CO gas will be used as an energy source in most plants.

Box 3.4

Double counting

To avoid double counting, CO2 emission from combusting CO gas generated in the process of CaC2 production should be accounted in the IPPU Sector, and should not be included in the Energy Sector. Petroleum coke used in the production process should be deducted from the Energy Sector as a non-energy use of petroleum coke.

The most important application of calcium carbide is producing acetylene (C2H2) by reacting CaC2 with water. A substantial use of acetylene is welding applications. Acetylene is also used in chemical synthesis for the production of acetaldehyde, acetic acid, acetic anhydride, and as a feedstock to manufacture 'acetylene black', a form of carbon black. Frequently, acetylene is not produced within the same plant that produces CaC2 and this needs to be taken into account when applying higher tier methods to estimating CO2 emissions from CaC2 use.

Use of acetylene in chemical synthesis and production of acetylene black results in carbon contained in the products reducing the total emissions of CO2 that are related to CaC2 use. Acetylene may be produced from partial oxidation of natural gas as well as from CaC2. The approach to accounting for acetylene in these uses is outlined in Section 3.9 of this volume.

Production and use of acetylene for welding applications is summarised by reaction:

CaC2 + 2H2O ^ Ca(OH)2 + C2H2 (+ 2.5O2 ^ 2CO2 + H2O)

Where acetylene is used in welding applications, emissions can be derived from the quantity of CaC2 used in the production of this acetylene on the assumption that the acetylene will be used in a relatively short period of time after production.

Box 3.5

Allocation of emissions from CaO production

CaO (lime) might be produced in-house or at a plant other than the carbide plant. In either case, the emissions from the CaO step should be reported as emissions from lime production (Section 2.3 of this volume) and only the emissions from reaction of CaO with petroleum coke and use of the product to produce acetylene for welding applications should be reported as emissions from calcium carbide.

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