(BAT) Reference Document for the Production of Chlor-alkali ...

Chapter 3
sector. Because of the electrolytic process in itself, emissions are not linked to production in a
linear way. The majority of the emissions are from the cell room where the absolute amount is
far more related to the equipment, plant design, maintenance requirements, pressure and
temperature of cell and denuder. However, it could be assumed that if half of the cells are
switched off this reasoning may be wrong. The industry gives two main reasons for reporting
mercury emissions in terms of chlorine capacity. The first one is economic and the second
technical. For economic reasons, a plant will always prefer to run all its cells because it is the
cheapest way of operating and minimising costs. This is particularly true in countries like Spain
or the United Kingdom where electricity tariffs can vary a lot during the year or even the day.
Running at lower current densities is cheaper than switching off the cells. The second reason
given is the design of the electric circuit. The rectifier is specified for a certain voltage and the
electrical equipment may not support a voltage drop, especially for processes using a
combination of diaphragm and amalgam technologies or amalgam and membrane. In Europe,
however, the figures reported refer to 90 % running capacity.
The industry also reports that production figures on a plant by plant basis are confidential data
for ‘competitive reasons’.
Mass balance calculation
A mercury balance consists in comparing all mercury inputs to and outputs from a chlor-alkali
plant during a specified time. Conducting periodic mercury balances is a useful method to better
understand mercury consumption and emissions levels. In theory, inputs should equal outputs,
but there are two disturbing factors: the measurement uncertainty and the accumulation of
mercury in equipment [ 86, Euro Chlor 2010 ]. As a consequence, the mercury balance for an
individual plant varies considerably from one year to the next, is frequently positive but
sometimes negative, and is often much higher than the total emissions to air, water and via
products. In 2009, the 'difference to balance' ranged from -35 – 210 g/t annual chlorine capacity
for individual chlor-alkali plants in OSPAR countries, the median being 2.6 g/t annual chlorine
capacity [ 85, Euro Chlor 2011 ]. In parallel with mercury consumption, the average difference
to balance of chlor-alkali plants in OSPAR countries was reduced by 85 % from 1977 – 2008,
due to improvements in technology, analytical methods and operating procedures (Figure 3.6)
[ 96, Euro Chlor 2010 ].
WORKING DRAFT IN PROGRESS
Source: [ 96, Euro Chlor 2010 ]
Figure 3.6: Trend of the average difference to balance for chlor-alkali plants in OSPAR
countries
128 December 2011 TB/EIPPCB/CAK_Draft_1