INGENIEUR
b. Excessive steam consumption in sterilisation
and ineffective evacuation of residual air in
steriliser vessels
Systems thinking helped to trace another
weakness of the present process design at the
mill to the ineffective evacuation of residual air
in the steriliser vessel before the start of the
sterilisation process. The present multiple peak
preconditioning cycle cannot adequately remove
residual air trapped inside fruit bunches and this
leads to the inefficient use of steam sterilisation
medium both in terms of use of excessively high
pressure and consumption of process steam.
Identifying the problem area helped to
focus on coming up with a new method to bring
about efficient air removal at the onset of the
sterilisation process in conventional horizontal
sterilisers whereby flowing steam flushes out
air from within the fruit bunches. This in turn
enables the sterilisation of fruit bunches in a
single peak preconditioning cycle using process
steam at a lower pressure and temperature,
resulting in lowered steam demand, and reduced
sterilisation time, all of which translate into drastic
improvements in the overall steam efficiency of
the palm oil mill.
An important fact that underpins the proposed
new method is that in reality crude palm oil
extraction process requires temperatures that are
relatively low of not more than 100°C.
A patent has already been granted for the said
new method.
c. Loss of heat-to-power conversion potential of
in-house CHP - a missed opportunity
A spin-off of reduced steam pressure for
satisfactory sterilisation is that it paves the way
to reduce the process steam pressure for the
remaining mill processes across the mill. This
allows lowering the back-pressure exhaust steam
(process steam) pressure discharged by the CHP
plant.
Once a lower back-pressure is allowed, specific
steam consumption and power-to-heat ratio of the
CHP plant will be improved. A positive outcome of
the reduction in exhaust steam pressure of the
CHP plant is that it will enable the plant to meet the
mill power demand with a reduced process steam
flow demand brought about by the elimination of
steam losses.
The large heat demand in the form of processed
steam and the low process temperatures
(< 110°C) at the mills present an opportunity
for large-capacity electricity power generation at
exceptionally high efficiencies, up to more than
80% on gross calorific value (GCV) by way of CHP.
d. Boiler inefficiency due to fluctuating demand
and peak demands
It is a widely-known fact that the unique feature
of palm oil mill boiler operation is the imposed
fluctuating steam demand and steam demand
peaks, which frequently exceed boiler capacity.
Reducing and stabilising the steam demand
from the co-generation plant will allow the existing
co-generation plant, including the boiler to operate
efficiently at steady state, thereby improving
combustion efficiency to further save biomass
currently burnt in the boilers. These contribute to
an energy-efficient palm oil mill. Another spin-off
is boiler control then becomes amenable to full
automation.
e. Inefficient use of biogas in gas engines
The in-house CHP plant reconfigured for additional
power generation at the mill would provide, in
addition, an opportunity to co-fire biogas from the
POME treatment as a partial substitute fuel in
its steam boiler instead of using a dedicated gas
engine-generator as at present. Economic benefits
of biogas fuelling the reconfigured CHP plant in the
mill include:
• Generating power at exceptionally high
efficiencies, which will more than double
the amount of electricity generated from
the biogas compared to gas enginegenerator
sets;
• Providing operational flexibility to the CHP
plant;
• Avoiding capital and maintenance costs for
a dedicated gas engine-generator set; and
• The solid biomass retrieved from being
used as fuel in the steam boiler can now be
conveniently delivered for CHP generation
elsewhere.
f. Boiler emissions control
Air pollution control system (APCS) is being
installed for compliance of Environment Quality
(Clean Air) Regulations. However, most air
74 VOL 82 APRIL-JUNE 2020