Factors contributing for higher energy
consumptio
n
Most of the energy consumed in fertiliser
industry is in the production of nitrogenous fertiliser and that too in the
production of ammonia. In the past the energy consumption per unit production
has been high. The new plants have been performing much better and the energy
consumption is comparable to the best in the world. The old plants have also
improved their performance but have the limitation of old technology and
inefficient feedstock.
The energy consumption for the production of
ammonia in a modern steam reforming plant is 50-60% above the thermodynamic
minimum. More than half the excess consumption is due to compression losses and
release of low-level energy that is not economical to recover. The practical
minimum consumption is assumed to be about 140% of the theoretical minimum.
The record of Indian fertiliser industry on
energy front in the 70’s and 80’s was not been very good. There have been many
reasons for the high-energy consumption. These have been analyzed as under.
4.7.1 Low
capacity utilization
The ammonia process is a continuous
operation, consisting of many sub-processes, leading to the final production of
ammonia. During startup lot of energy is consumed to bring the operation
parameters of all the sub-processes to those levels required for operational
performance & stability. Since a large ammonia plant handles large
quantities of inflammable fluids, a number of safety features are built in to
the processes to trip the plant and bring in to safe condition in case of a
disturbance endangering the plant. This is also necessary to avoid any major
accident. If for any reason any one sub-process in the ammonia production gets
disturbed and the plant process goes in to dangerous operational zone, the
safety system automatically actuates and the plant gets shut down. The material in the process gets discharged
in to atmosphere and burnt. Frequent shutdowns thus result in to wastage of
energy. Unfortunately, the plant outages/trips have been very frequent in the
70’s, 80’s and in some plants even in the 90’s. This is indicated by the low
capacity utilization of nitrogenous plants (Table 4.7.1).
Table
4.7.1 Capacity Utilization (CU) of Nitrogenous
Fertiliser Plants
|
Year
|
82-83
|
84-85
|
86-87
|
88-89
|
90-91
|
92-93
|
94-95
|
96-97
|
98-99
|
|
CU
(%)
|
67
|
74
|
79
|
85.2
|
85.7
|
88.1
|
91
|
93.2
|
99.2
|
Main factors for low capacity utilization
are as follows:
4.7.1.1
Power Supply
The power supply from utilities was not
stable causing the plant to trip due to frequent interruptions in power supply
and fluctuations in voltage. Due to sensitive nature of plants trip systems are
in-built to take the plant to safe condition after tripping and venting all the
gases in process and burning them off. Because of this perennial problem faced
by most of the plants the Government allowed each fertiliser plant to have its
own captive power plants.
4.7.1.2
Steam Supply
The plants of 70’s and 80’s vintage had
their steam supply from steam generation plants using coal. The quality of coal
supplied to these plants has been of poor quality with very high ash content
resulting in to extensive wear & tear in boilers, breakdowns and
interruptions in steam supply. There were quite a few interruptions for
non-availability of coal at pithead or non-availability of railway wagons to
transport the coal. It also increased energy consumption.
4.7.1.3 Indigenisation of Spares
Due to non-availability of foreign exchange
attempts were made to utilize spares from indigenous sources that were not
proven in quality. Further because most of the plants were in Public Sector,
the purchases were made from the lowest cost suppliers rather than suppliers of
proven performance.
4.7.1.4
Unreliable Instrumentation
Internationally the capacity utilizations
were low as manufacturers were yet developing very high reliability machinery
and process control instruments that relied largely on human factors. It is
only in late 80’s that electronically controlled instruments for better/auto
control and analysis was installed. With mechanical instruments many trips were
caused by the mal-functioning of the instruments themselves. Besides after the
plant tripped, there were no clues as to what caused it. Restart without
diagnosis and corrective action would interrupt the process again with
consequent lot of energy waste.
Adverse industrial relation scenario was
also contributed to bad performance. The labour unions were very strong and
non-cooperative during the period. Besides their level of skills was low.
Despite training centers attached with each fertiliser plant the quality of
manpower could not be developed fast enough as the management’s did not see the
need to revise the curriculum to meet the current and future needs.
4.7.2 Selection Of Equipment / Available Technology
The technology selection and equipment
selection for the plants being set up in 70’s was not up to the mark. Besides
the Indian design and consultancy organizations involved were on the learning
curve. The process suppliers did not part with the best technology, sent raw
hands to our detailed engineering consultants and recommended purchase of
spares with original equipment that were really not needed.
Foreign exchange availability was a major
limitation during the 70’s and 80’s with the result that the country had to
select the process supplier who would also provide project loans. The process
suppliers were further tied up with equipment manufacturers for supply of
equipment with deferred payment terms. In the deal they would sell the
equipment that was not proven. A number of critical equipments were supplied
that resulted in to major plant limitations. The boiler feed water pumps and
untried centrifugal compressors are only few examples.
4.7.3 Feedstock
The best feedstock for nitrogenous fertiliser
is NG. During the period there was urgent need to produce indigenous fertiliser
with the available feedstock. The naphtha based and fuel oil based had to be
put up though they were not the best feedstock with inherent high energy
consumption.
Cooling water is one process material that
passes through a lot of equipment for cooling. This water needs to be treated
to control corrosion in the process equipment and needs proven technology and
material inputs to make it suitable. Due to non-availability of foreign
exchange a number of fertiliser plants experimented with un-proven technology
and chemicals and the equipment suffered internal corrosion resulting in to
frequent interruptions due to heat exchanger failures.
4.7.4 Policy Environment
While there were many and great advantages
in administered price system to provide cheap fertiliser to the farmers and
compensate the manufacturer with reasonable cost of production, the system did
not provide incentive to the manufacturer to upgrade the technology. Capital
expenditures for up-gradation were difficult to get reimbursed and any
efficiency gains after up-gradation were moped up under pricing mechanism.
4.7.5
Management Practices
Awareness towards the energy conservation
was low during the decade of 1980’s.
Management emphasized on increasing production by improving on-stream
factor.
The energy consumption levels on all India
level are much improved now due to better operation & maintenance practices
and innovation and modernization of old plants.
The energy savings already achieved by the industry at the current
production level is equivalent about a million tonnes of fuel oil for a year
(for the fertiliser industry as a whole) when compared with 2002-2003 energy
consumption (for the current production) and 87-88 levels of energy
consumptions. Presently, the Indian gas based plants compare well with the
American gas based plants.
4.8
Energy conservation efforts
There is always scope for energy improvement
in various process steps and unit operation practices, change of catalysts,
change of design of equipments, better material of construction and removal of
bottlenecks. All the options for energy savings being brought will not be
applicable in all the plants. The applicable options will have to properly fit
in the specific plant process and layout. The change may have to be planned as
a revamp of process involves lot of detailed engineering, risk studies and
equipment/material procurement specifically manufactured for a particular
situation.
4.8.1 Improvement in Plant Reliability
The energy consumption of even an otherwise
well-designed plant has a very strong relation with plant on-stream
reliability. As brought earlier a shutdown and re-startup of the plant results
in to consumption of energy till it becomes a stable operation. The reliability
of all the equipment and systems is one of the most important issues. For
example ten equipments with 99.9% reliability reduces the total plant
reliability to 99.5% while in reality a plant has a few hundred equipment in
series. 99.9% reliability therefore does not make a reliable plant as any one
equipment failing can disrupt the whole operation.
The frequency of unscheduled shutdown of a
plant due to equipment breakdown and duration of such shutdown is a measure of
plant performance with regard to efficiency in operation, maintenance,
inspection and material management functions in the plant. Results of various
surveys of unscheduled downtime due to equipment breakdown show that there has
been continuous reduction in downtime due to equipment failures in ammonia and
urea plants (Table 4.8.1a).
Table 4.8.1a Downtime in Ammonia Plants for Various Survey
Period due to Equipment Failures
|
Reasons
|
1984-87
|
1987-90
|
1990-93
|
1993-96
(DDPY)
|
1996-99 (DDPY)
|
1999-02
(DDPY)
|
|
Mechanical
|
29.8
|
24.4
|
19.3
|
24.1
|
14.7
|
14.7
|
Downtime data in urea plants show trends
similar to those observed in ammonia plants.
Controlling the down time and increasing on-stream efficiency is one
area that save energy and may involve providing better instrumentation,
reliable maintenance quality, root cause analysis and better maintenance skills
and procurement systems etc. Results of survey on on-stream efficiency (Table
4.8.1b) show that In Indian plants there is scope for increasing the on-stream
days.
Table 4.8..1b Average Operating Factor, Service Factor and
Reliability Factor of Ammonia Plants for the Three Year Period
|
§ P
Plants
|
Operating
Factor %
|
Service
Factor %
|
Reliability
Factor %
|
On-Stream
Days
|
|
Reforming plants (29)
Gasification Plants (5)
ll Plants
|
87.1
78.1
85.7
|
89.6
83.4
88.6
|
94.7
92.7
94.4
|
316
284
311
|
4.8.2 Conservation Schemes in Ammonia Plants
4.8.2.1
Gas Making
In preparation of synthesis gas for ammonia
production, primary reformer is the major energy consumer. The reformers in old
plants had about 85 percent thermal efficiency that has now improved to more
than 90 percent in recent reformers. The
increase in efficiency has been possible due to recovery of low level heat from
the flue gases going to stack in preheating of combustion air, reduction in
heat loss through insulation of reformer box and excess air control. It
understandable that recovery of low level heat involves additional capital
expenditure but in view of the increasing cost of energy it has low payback
period.
Several plants have installed additional
heat recovery surface area in convection zone of reformer furnace. The reformer
exit flue gas temperature was brought down from 170°C to 148°C by
KRIBHCO-Hazira plants by installing a feed pre-heat coil in the low temperature
convection section of reformer flue gas duct. Thus, the fired heater has been
eliminated resulting in energy saving of 0.04 Gcal/MT of ammonia.
There are other options available to improve
primary reformer efficiency. For
example, lower steam–carbon ratio would result in reduction in pressure drop
and reduced firing. According to data
available, steam-carbon ratio in the gas-based plants was in the range of 2.7
to 3.5 in 2001. However lowering of steam carbon ratio below 3.0 has the risk
of damages in the boiler/super heater after secondary reformer and should be
carefully evaluated.
Use of superior tube metallurgy allows use
of thin walled reformer tubes and hence increased catalyst loading. This would result in higher reforming
capacity and reduction in specific energy consumption if there is no limitation
in any other section to process increased output from the reformer. Bigger
inside diameter of the reformer tubes can have more catalyst thus processing
higher quantity of feedstock. Several
plants have changed the reformer tubes of better metallurgy.
Use of gas turbine for process air
compressor along with heat recovery from exhaust gases can result in higher energy
use efficiency. In a conventional plant
the compressor are run by a steam turbine with part of the exhaust steam going
to condensation. The loss of heat to the condenser is wasted to the cooling
water and further energy is to be spent in cooling towers to get rid of this
energy. In case of a gas turbine driven compressor there is no such loss but
only loss in flue gases. Two ammonia plants commissioned in 1990’s are using
gas turbine for process air compressor drive with heat recovery steam generation
units.
In IFFCO-Aonla I unit, flue gas temperature
was around 190°C. It installed a natural gas heating coil to recover the heat
that was being lost to atmosphere. As a result of this modification, the flue
gas temperature was reduced to 160°C resulting in energy savings of 0.043
Gcal/MT of ammonia.
4.8.2.2
Gas Purification
The existing CO-Shift section can be
modified to provide extra catalyst volume in the case of high steam dry gas
ratio. The CO slip can be brought down
to 0.05 percent (dry) saving hydrogen in methanator. This would result in lower inert in make up
gas and increased amount of synthesis gas with the same amount of
feedstock. In one of the plants, LT
shift guard reduced CO slip and achieved an energy saving of 0.06 Gcal/MT
ammonia. Carbon monoxide present in the
LT outlet gas can be selectively oxidized to carbon dioxide by Selectoxo
process leaving only traces of CO going to the methanator for conversion to
methane. The scheme was implemented by
one of the ammonia plants.
The existing CO2 removal section
can be revamped with more efficient CO2 removal process like a
MDEA, glycene etc. This would result in lower heat requirements
for regeneration, higher capacity in absorption and regeneration, lower CO2
slip etc. A number of units have
switched to a MDEA CO2 removal system. For example IFFCO-Kalol
changed from MEA to modified MDEA solution.
Change of tower internals and packing can be carried out to improve the
absorption and regeneration efficiencies.
RCF-Thal replaced the tower packing in both ammonia plants and achieved
an energy saving of 0.015 Gcal/MT in each plant. Operating parameters like
ratio of semi-lean to lean solution, recalculation rates etc. can be optimized
to achieve the lowest energy consumption in this section.
4.8.2.3
Ammonia Synthesis
Reduction in synthesis pressure results in
savings of energy in compression. With
the development of low temperature and low-pressure catalyst, it is possible to
achieve relatively high conversion at low pressure. The pressure drop across the converter can be
reduced with radial gas distribution in catalyst bed. The improved ammonia synthesis converters
generally incorporate inter-bed heat exchanger instead of quench type converter
for better thermal efficiency. The units
were able to save energy of the order of 0.2-0.35 Gcal/MT with the
revamp/change of converters. A number of
old plants like MFL, SFC, NFL-Bhatinda, Panipat and Nangal changed from S-100
axial flow converter to radial flow S-200 converter arrangement. In almost all cases loop pressure came down
with substantial energy savings.
KRIBHCO-Hazira, RCF-Trombay and
IFFCO-Phulpur and Kalol plants have also changed the internals of their Kellogg
converters with Ammonia axial baskets.
The converter pressure drop came down from 4 to 3 kg/cm2 and energy
savings was more than 0.2 Gcal/MT. The new catalyst used in all the revamped
converters is highly active with size of 1.5-3.0 mm as compared to the 6-10 mm
size catalyst before revamp, resulting in higher conversion per pass.
In order to achieve optimum conversion in
synthesis converter, it is necessary to purge a certain quantity of gas from
synthesis loop so as to reduce inerts concentration in the loop. This purge gas contains inerts (methane +
argon), hydrogen and ammonia. This gas is some times used as fuel in primary
reformer, after recovery of ammonia in purge gas absorber. In order to utilize this gas in more useful
manner, a hydrogen recovery unit (also called purge gas recovery unit) is
installed. The hydrogen so recovered is sent back to the synthesis loop to save
energy and / or increase production.
Almost all the reformer-based ammonia plants installed purge gas
recovery unit (PGRU) saving energy in the range of 0.15 to 0.25 Gcal/MT
ammonia.
SOURCE Technology
Assessment Report- Fertilizer Sector
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