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

Chapter 3
the actual current density.
The number of influencing factors explains why energy consumption data of chlor-alkali
installations vary significantly, even if they are using the same cell technique [ 63, Euro Chlor
2010 ].
3.3.4.2 Energy consumption for preparation and purification of raw
materials
The energy required for the extraction of brine or rock salt, for the preparation of vacuum or
solar salt as well as for the purification of solar salt or salt from potash mining wastes is outside
the scope of this BREF. Nevertheless it is important to keep in mind that the type and quality of
raw material used will have an influence on the energy consumption of the electrolysis process
itself. Since all salt sources differ in purity and, as the brine purity requirements differ according
to the cell technique used, simple or complex raw material preparation and purification are
applied.
For example, the production of vacuum salt uses either electricity of approximately
155 – 175 kWh per tonne of salt produced via mechanical vapour recompression or steam, in a
variable quantity between 0.7 and 1.4 tonne low pressure steam per tonne of solid salt produced,
depending on the number of vaporisation stages installed. There is also some additional
electricity consumption for the auxiliary equipment (approximately 30 kWh/t salt) [ 63, Euro
Chlor 2010 ], [ 66, Ullmann's 2010 ]. Some installations produce their own vacuum salt from
solution-mined brine.
3.3.4.3 Energy consumption for the electrolysis
3.3.4.3.1 General considerations
The operation of a chlor-alkali plant is dependent on the availability of huge quantities of direct
current (DC) electric power, which is usually obtained from a high voltage source of alternating
current (AC). The lower voltage required for an electrolyser circuit is produced by a series of
step down transformers. Silicon diode or thyristor rectifiers convert the alternating alternative
current electricity to direct current for electrolysis [ 3, Euro Chlor 2011 ], [ 10, Kirk-Othmer
2002 ] [Kirk-Othmer, 1991]. Direct current is distributed to the individual cells of the
electrolysers via busbars. There are energy losses across the transformer, the rectification
equipment and the busbars. In 2010, the efficiency of rectifier and transformer units varied from
approximately 94 % (older units) to 98 %. To remove the dissipated heat, the units are cooled
by circulated air or by special water circuits [ 63, Euro Chlor 2010 ].
Connections between cells/electrolysers along with the corresponding energy losses have to be
considered for the measurement of the total energy requirement per tonne of chlorine produced.
The definition of the exact measurement point is necessary for an appropriate comparison of
energy consumption figures of different plants.
WORKING DRAFT IN PROGRESS
For the usual operating conditions, the specific electricity consumption w (in kWh/t Cl2
produced) which is the electricity consumed divided by the production rate is proportional to the
cell current density j (in kA/m 2 ) [ 63, Euro Chlor 2010 ]:
Equation 1: w = A · U0 + A · K · j
A is a coefficient which depends on the overall electrolysis efficiency including both the
efficiency of the electricity conversion (from high voltage alternating to lower voltage direct
72 December 2011 TB/EIPPCB/CAK_Draft_1