Chemistry Reference
In-Depth Information
0.06
25
ACA-SR
ACA-SR
0.05
20
CO
CO
CR
0.04
CR
15
BO
BO
0.03
10
0.02
0.01
5
0
0
0
500
1000
1500
-
0.01
0
500
1000
1500
-
5
VOC inlet concentration (mg/m
3
)
VOC inlet concentration (mg/m
3
)
Fig. 5.12
Life-cycle CO
2
and VOC emissions for VOC
abatement technologies [3]: Flow rate
=
5000 m
3
h
-
1
; outlet
VOC
=
50 mg m
-
3
; ACA-SR
=
activated carbon adsorption with
steam regeneration; CO
=
catalytic oxidation,
T
oxidation
=
350°C;
CR
=
cryogenic recovery,
T
cooling
=-
50°C; BO
=
biological
oxidation.
sions. Thus, depending on the operating conditions
of the system, the cryogenic recovery process ranges
from being the worst option to approaching the
BPEO/BAT. The ACA-SR process was found to be the
BPEO/BAT over all flow rates and concentrations
investigated. Furthermore, the VOC and CO
2
emis-
sions were negative due to crediting the system for
the recovery of xylene and the associated avoided
burdens, which otherwise would arise from the
energy-intensive primary production of this solvent
(Fig. 5.12). Thus, contrary to the findings for cryo-
genic recovery, it is more beneficial environmentally
to recover the pollutant in the ACA-SR process
rather than to destroy it.
This case study therefore highlights the value of
LCA for process selection, in that it challenges the
widely accepted 'truths' on waste management hier-
archy and shows the importance of considering the
operating conditions of the system throughout the
whole life-cycle for identifying more sustainable
options.
microorganisms to oxidise organic material in the
liquid phase.
The study found that removal of the VOC by these
processes generated the additional emissions of
VOCs and other pollutants elsewhere in the life-cycle
[72]. The worst process in these terms was the
cryogenic recovery at low VOC inlet concentrations,
which for each kilogram of VOC removed generated
on additional 0.06 kg of VOC and 22 kg of CO
2
(see
Fig. 5.12). The main reason for this is a high energy
requirement for the production of liquid nitrogen,
which exceeds the benefits of VOC recovery. These
findings lead to the conclusion that a pollutant
recovery may not always be a better option envi-
ronmentally than destroying it, as stimulated by the
waste management hierarchy set out by the Euro-
pean Commission [73] and the Environment Agency
[74] in the UK. However, as the amount of VOC
recovered increases, the cryogenic process becomes
more competitive and for inlet VOC concentrations
above 600 mg m
-3
it becomes the second most
favourable option to ACA-SR in terms of CO
2
emis-
Life-cycle assessment and process optimisation
Another process-oriented application of LCA is for
process optimisation. As noted earlier, one of the
objectives of LCA is to identify options for environ-
mental improvements of a system in which complete
supply chains are considered. The main problem
