Factors contributing for higher energy consumption AMMONIA PLANT

Factors contributing for higher energy consumptio

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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