Technologies and Measures for Energy Efficiency Improvement in Fertilizer Industries
The Indian fertilizer industry made a very humble beginning in 1906, when the first manufacturing unit of Single Super Phosphate (SSP) was set up in Ranipet near Chennai with an annual capacity of 6000 Tonnes of Rock Phosphate (P2O5). The Fertilizer & Chemicals Travancore of India Ltd. (FACT) at Cochin in Kerala and the Fertilizers Corporation of India (FCI) in Sindri in Bihar (now Jharkhand) were the first large sized fertilizer plants set up in the forties and fifties with a view to establish an industrial base to achieve self-sufficiency in food grains. Subsequently, the Green Revolution in the late sixties gave an impetus to the growth of fertilizer industry in India and the seventies and eighties witnessed a significant addition to the fertilizer production capacity. However, there has not
been any substantive addition to fertilizer production capacity during the last 15 years.
Production of nitrogenous fertilizers is highly energy intensive.Ammonia is used as the basic chemical in the production of nitrogenous fertilizer. Production of ammonia itself involves almost 80% of the energy consumption in the manufacturing processes of a variety of final fertiliser products. Therefore, ammonia is considered a key intermediate for determining the overall energy efficiency of fertiliser production. Besides air as the source of nitrogen, the ammonia-manufacturing process have choice of using raw materials such as water, natural gas, naphtha, fuel oil, coal, coke oven gas. Natural gas is the best feedstock for ammonia production. However, the use of natural gas in India for urea production is constrained due to its scarce availability.
Better feedstock and process technologies, together with improved operation and maintenance practices, retrofitting, and so on have resulted in significant amount of energy savings during ammonia production. The average specific energy consumption for ammonia production in India has improved significantly from 57.35 Giga Joules (GJ)/tonnein1985-86to37.53GJ/tonnein2007-08.The average energy consumption of 25%ofthemostefficientIndianammoniaplantsis32.7GJ/tonnein2007-08.
The fertilizer industry is one of the major consumers of hydrocarbons. The fertilizer sector accounts for 8% of total fuels consumed in the manufacturing sector. Energy costs account for nearly 60 to 80% of the overall manufacturing cost. The absolute energy consumption by this sector has been estimated at 628 million GJ annually. The specific energy consumption per ton of urea varies between 21.59 GJ for the most efficiently operating plant to 52.38 GJ for the most inefficient plant during 2007-08. Energy intensity in India's fertilizer plants has decreased over time. This decrease is due to advances in process technology, better stream sizes of urea plants and increased capacity utilization.
Energy is consumed in the form of natural gas, associated gas, Naphtha, fuel oil, low sulfur heavy stock and coal for process. LDO, LSHS, HFO and HSD are also used in diesel generators. Large fertilizer plants generate part of their own power through cogeneration mode in Turbo Generator (TG) sets, while smaller plants depend exclusively on purchased power or power from DG sets. With the ever increasing fuel prices and power tariffs, energy conservation is strongly pursued as one of the attractive options for improving the profitability in the Indian fertilizer industry.
Specific Energy Consumption (SEC)
Ammonia is the intermediate product in Urea production. Out of total energy consumed for the production of Urea, 80% is consumed in Ammonia production. Hence, efficient production of Ammonia has greatest impact on Specific Energy Consumption. Hence, efficient production of Ammonia has greatest impact on Specific Energy Consumption.
Potential for Energy Efficiency Improvement
The biggest drawback of the Indian fertilizer industry is its reliance on non-natural gas-based plants. If we consider only the natural gas based plants, Indian plants compare favorably with international practices (Table 10.5a). The figures in brackets are the improvement potentials if plants were to reach best practices available in India. The highest energy saving potential is observed with fuel oil based plants.
The best practice energy intensity worldwide is 28 GJ/Tonne of ammonia, and is a result of auto-thermal reforming technology process. Auto thermal reforming process is a mixture of partial oxidation and steam reforming technology. According to the European Fertilizer Manufacturing Association (EFMA), two plants of this kind are in operation and others are at the pilot stage.
Tata Chemicals owns and operates one of the more energy-efficient plants for the production of ammonia and urea in India with an energy intensity of 30.3 GJ/Tonne of ammonia and 22.5 GJ/Tonne of urea. These energy intensity values are among the lowest recorded internationally. Manufacturing facilities at Babrala comprise an ammonia plant of 1520 TPD and a urea plant of 2864 TPD capacity which were implemented and commissioned in December 1994. Even though the plant currently uses natural gas, it has been designed for full flexibility in the use of natural gas and naphtha as a feedstock and fuel.
When only natural gas-based plants are considered, India appears to maintain very competitive plants compared to the world average (Table 10.5a). However with latest changeover of number of plants from naphtha to natural gas, India has now 80% ammonia capacity based on natural gas as of 2007.
India's national average figures of specific energy consumption for ammonia plants are close to the world average but there is wide variation in energy consumption of various plants. It varies from 32 GJ/Tonne to 63 GJ/Tonne with a weighted average of 37.55 GJ/Tonne. This wide variation is mainly because of the operation of Naphtha & fuel based plants, which have higher energy consumption than gas based plants. In a competitive environment, with energy cost representing between 60% to 80% of total production cost depending on the type of plant, companies will be compelled to gradually switch over to natural gas in order to have an energy consumption per ton of output closer to world average and become more competitive in the international market.
Categories of Energy Efficiency Improvement
Over the past 30 years, induced by major technological improvements and by a better energy management, the energy used to produce each ton of ammonia has declined by 30 to 50%. Technology-wise, three different process stages can be distinguished where energy improvements are possible:
Steam reforming phase: This is the most energy intensive operation, with the highest energy losses. Different methods are available to reduce losses that occur in the primary reformer, viz., installing a pre-reformer, shifting part of the primary reformer load to the secondary with installation of a purge gas recovery unit, and upgrading the catalyst to reduce the steam/carbon ratio. It is possible to reduce energy losses by 3-5 GJ/Tonne of NH3.
CO2 removal phase: The removal of CO2 from the synthesis gas stream is normally based on scrubbing with a solvent. A reduction of the energy requirement for recycling and regeneration of the solvent can be achieved by using advanced solvents, pressure swing absorption or membranes. Energy savings are in the order of 1 GJ/Tonne of NH3.
Ammonia synthesis phase: A lower ammonia synthesis pressure reduces the requirement for compression power, and also reduces production yield. Less ammonia can be cooled out using cooling water so more refrigeration power is required.Also the recycling power increases, because larger gas volumes have to be handled. The overall energy demand reduction depends on the situation and varies from 0-0.5 GJ/Tonne of NH3. Another type of catalyst is required to achieve the lower synthesis pressure. Furthermore, adjustments have to be made to the power system and the recycle loop.
Additionally, energy price escalation and growing concerns regarding pollution have intensified the attention on energy conservation at all levels. Improving energy efficiency does not necessarily require investment and can result from a better balancing of energy flow along the process. The optimization of operations and maintenance practices, by reducing waste heat and capturing excess heat to channel it back into the system, allows a better energy distribution and constitutes major energy efficiency improvements.
Some plants in India have realized considerable energy savings by increasing awareness at all levels in the plant, monitoring energy consumption during production, and identifying potential energy-savings opportunities Some Technologies that can be adopted by fertilizer plants for energy efficiency improvement are briefly described below:
Technologies & Measures for Energy Efficiency Improvements
1. Haldor Topsoe Exchange Primary Reformer (HTER-p) (Ammonia Production)
HTER-p is introduced reforming section in ammonia plant to reduce size of the primary reformer and at the same time reduce the HP steam production. HTER-p is a new feature, initially developed for use in synthesis gas plants. In ammonia plants this is operated in parallel with the primary reformer, and that is why the name is HTER-p. The exit gas from the secondary reformer heats the HTER-p, and thereby the waste heat normally used for HP steam production can be used for the reforming process down to typically 750-850 C, depending upon actual requirements. The technology was implemented in a synthesis gas plant in South Africa in the year 2003.
Operating conditions in the HTER-p are adjusted independently of the reformer in order to get the optimum performance of the primary overall reforming unit. In this way, up to around 20% of the natural gas feed can by-pass the primary reformer.
2. Uhde Dual PressureAmmonia Technology (Ammonia Production)
At present, reducing the cost of plant by increasing the plant capacity is a major thrust in conventional ammonia process. To overcome the constraints in increasing the plant capacity beyond 2000 metric tons per day, Uhde has developed Dual Pressure technology. Dual Pressure process focuses on the de-bottle necking of the conventional synthesis loop. A synthesis reactor has been introduced at an intermediate pressure level in the synthesis gas loop, which makes synthesis, and separation of ammonia possible in between compressor casing and the synthesis gas volume flow to the high-pressure loop is significantly reduced.
The production can be raised by about 65%. Gives a superior hydrogen yield. Energy consumption is reduced by up to 4%. Cost of production is reduced by 10% to15%.
3. Megammonia (Ammonia Production)
The Megammonia technology is designed in the year 2003 jointly by M/s Lurgi and M/s Ammonia Casale for large scale production capacity of 4000 TPD Ammonia. Using natural gas, steam and air as feedstock, following five principal steps as below produces ammonia:
i. Air separation: 95% oxygen and 99.99% pure nitrogen is produced from air.
ii. Catalytic Partial Oxidation: Desulphurised natural gas, after addition of steam is first preheated in a fired heater and then reformed over a nickel oxide catalyst to CO, H2 and CO2 following partial oxidation.
iii. CO-Shift: Reformed gas is passed through two beds of conventional HT shift catalyst (Copper promoted Iron / chromia based) in series to convert remaining CO to H and CO2.
iv. Gas Purification: CO2 is removed by absorption in cold methanol and other impurities like CO, CH4 and Ar are removed by washing the gas with liquid nitrogen.
v. Ammonia Synthesis: The extremely high purity of ammonia synthesis gas results in higher conversion of gas per pass, lower circulator duty and lower refrigeration duty.
Reduction in the capital cost by 18-20%. Operating cost is expected to be lower around 12 -15% over the most advanced conventional technology. CO2 emission is expected to reduce by around 30% as compared to other conventional technologies.
4. HydroMax Technology (Ammonia Production)
Alchemix Corporation U.S.A developed the Hydromax technology. The technology is used for production of hydrogen using either relatively cheaper coal or using inexpensive fuels like municipal waste, biomass and petroleum coke etc. in presence of metal like iron. The technology involves a two-step process. In the first step, steam reacts with molten iron to form iron oxide and hydrogen and in the second step, iron oxide is reduced back to pure metal by adding carbon. Iron simply acts as a carrier for oxygen. In both steps, hydrogen production and reduction of iron oxide back into iron occur in the same reactor at the same temperature of 1250°C.
Carbon dioxide and hydrogen are produced in separate compartments and do not require CO2 removal system. Cost of production is almost four times less than Steam Methane Reforming (SMR) production cost. Emission of greenhouse gases is 34% less than SMR process.
5. Feedstock conversion from Naphtha to Regassified Liquified Natural Gas (R-LNG) in Ammonia-Urea plants
The type of feedstock has a major influence on energy consumption in an Ammonia-Urea plant. Hydrogen to carbon ratio increases as we move from liquid hydrocarbons (Naphtha, FO, LSHS, etc.) to gaseous hydrocarbons (Natural Gas). Besides, associated impurities namely sulphur, etc. are present only in traces in the case of gas. With the steep rise in the cost of liquid hydrocarbons in the last five-to six years,Ammonia- -Urea production from liquid hydrocarbons plants has become very costly. Most significant difference between Naphtha and Natural Gas based Ammonia plants are in the Desulphurization Section. Since gas does not contain much sulphur unlike in Naphtha, hence pre-desulphurization section need not be operated. Other important aspect is in the hydrogen to carbon ratio, which is high in case of gas.As a result, less steam is consumed in the reforming section and less CO2 is generated. After the reforming section, plants operating on Naphtha or gas are identical except in the quantum of generation of CO2.
Natural gas is ideal feedstock for ammonia production. It has several advantages besides being cheaper and easy to handle. It allows easy and shorter start up of the plant, thereby lesser unproductive consumption. The burners choking phenomena is completely solved and CO2 emission from furnace has reduced. Plant also runs trouble free and the catalyst life is also increased
6. Carbon Dioxide Recovery (CDR) Plant
With the steep rise in the cost of liquid hydrocarbons, Ammonia -Urea production from liquid hydrocarbons plants has become very costly. As major disadvantage of RLNG conversion is lesser CO2 production due to lower C/H ratio in RLNG as compared to Naphtha. CO2 generated with lean RLNG is not adequate to convert total Ammonia produced to Urea. One of the possible options to overcome this problem is the recovery of CO2 from flue gas from various furnaces. CDR plant is basically a low pressure CO2 removal section in which CO2 present in flue gases is absorbed & then regenerated to produce CO2 having 99.93 % purity. CO2 recovery from flue gases is a new concept in fertilizer industries.
Basic steps involved in CDR plant are:
a) Flue gas Pretreatment
b) Low pressure CO2 absorption in special solution KS-1
c) CO2 regeneration
d) CO2 compression to desired level
Though regeneration energy is very high in comparison to that of any normal CO2 removal section of ammonia plant, the cost effectiveness of the plant is very attractive because of the use of costlier Naphtha (as feed to balance the CO2 for Urea production) shall be stopped completely. There is substantial reduction in CO2 Emission as well.
7. Parallel S-50 Converter
The S-50 converter is a single bed radial flow converter, which is added downstream of the main converter to increase the ammonia conversion and at the same time improve the steam generation.
The converter allows ammonia synthesis loop to operate at lower pressure with increased conversion per pass.
8. Conversion of Single Stage GV System to 2-Stage GV System for CO2
Ammonia is manufactured by steam reforming of natural gas. During the process, CO2 is formed in the gaseous mixture and the same is removed from the gaseous mixture in the CO2 Removal Section designed by M/s. Giammarco Vetrocoke (GV) of Italy. The process gas containing CO2 enters the CO2 absorber where major amount of CO2 is absorbed in the lower portion of Absorber in semi-lean GV solution. Rest of the CO2 is absorbed in top portion ofAbsorber in lean GV solution.
The process gas, with around 300 ppm of CO2, leaves theAbsorber from top. The main feature of original single stage GV system are (1)Absorption by only lean GV solution and (2) Stripping only in one Regenerator. The heat of regeneration is provided by vapours generated in GV Reboilers heated by Process gas and live LP steam. Full quantity of GV solution is sent to flash tank after GV Reboilers to remove maximum amount of vapour and CO2. This lean GV solution goes to GV Absorber in two parts, the hot solution to the middle to absorb major amount of CO2 and cold GV solution to the top of GVAbsorber to absorb the residual CO2.
The main features of the modified 2-stage GV process are (1)Absorption by lean & semi lean solutions in GV absorber (2) High pressure & low-pressure stripping in HP Regenerator and LP Regenerator. The heat of Regeneration is provided by vapours generated in GV Reboilers heated by Process gas, steam generated in LP Steam Boiler heated by process gas and live LP steam. Partially regenerated GV solution (Semilean Solution) from Regenerators goes to GVAbsorber to the middle to absorb major amount of CO2 and strongly regenerated cold GV solution (Lean Solution) to the top of GVAbsorber to absorb the residual CO2.
The features result in better absorption of CO2 in Absorber and lower energy consumption for regeneration of the solution in Regenerators. Major benefits of the modification are:
Reduction of CO2 slip through Absorber by around 600 ppm, which has resulted in:
Higher availability of CO2 for urea production.
Decrease in hydrogen consumption in Methanation Section.
Decrease in LP steam consumption in CO2 Removal System from 38 T/hr to15 T/hr.
By this, an energy saving of around 1GJ/Tonne of ammonia can be achieved.
9. LTS Guard Reactor & BFW Preheater
The reformed gas from Reforming Section flows to HT Shift Convertor after cooling in HP Waste Heat Boiler from 988oC to 380oC. The carbon monoxide content of the process gas is reduced from 12.96% to 3.46% in HT Shift Converter through shift reaction, which takes place in the reactor in presence of Iron-chromia catalyst. Process gas temperature of around 444oC at the outlet of HT Shift Convector is reduced to around 210oC by heat recovery in a Waste Heat Boiler and Boiler Feed Water Preheater.
Installation of a new LT Shift Guard Reactor before LT Shift Converter reduces the CO slippage from the Shift Conversion Section. The CO slip gets considerably lowered with the LT Shift Guard in line. Lower CO slip in turn, results in additional Ammonia production due to reduction in the consumption of hydrogen in Methanator. Considerable energy saving can be achieved by installation of a BFW Preheater down stream of the new LT Shift Guard Reactor.
Reduction of CO slip through Shift Conversion Section by around 300 ppm. This gives higher availability of CO2 for urea production. Hydrogen consumption in Methanation Section can also be considerably decreased. Installation of the BFW Preheater results in considerable energy savings.
10. The Poolcondenser concept (Urea Production)
The Pool condenser concept is introduced to de-bottleneck very large capacities indeed. In case a stripping plant is considered in urea plants, the Pool condenser is installed with a parallel-operated stripper. Conventional urea plants are revamped by using this concept to change the plant into a stripping unit. In this way the plant capacity is increased and the utility consumption is decreased drastically. The Pool condenser is a horizontal high-pressure vessel in which reaction volume and condensing including retention time, which is needed to produce urea, is already in this Pool condenser. The technology is implemented at PIC in Kuwait.
Very large capacities are de-bottlenecked.
11. Modified trays in Urea reactor
Due to advancement in technology and current fertiliser scenario, it is necessary to upgrade the plant equipments to reduce energy consumption. One new development is new modified tray design for Reactor in place of conventional design. Installation of these modified trays have further improved plug flow and reduced back mixing in the reactor and hence conversion ofAmmonium Carbamate to Urea in the Reactor is enhanced.
Conversion efficiency in Reactor is increased with considerable saving of medium pressure steam per tonne of production. Materials of construction of new trays are more corrosion resistant and have more life as compared to material used for conventional trays.
12. Use of Advanced Process Control (APC) with Distributed Control System (DCS)
In control theory, Advanced process control (APC) is a broad term composed of different kinds of process control tools, often used for solving multivariable control problems or discrete control problem.APC are often used for solving multivariable control problems or discrete control problem. APC makes it possible to control multivariable control problems. Since these controllers contain the dynamic relationships between variables, it can predict in the future how variables will behave. Based on these predictions, actions can be taken now to maintain variables within their limits. APC is used when the models can be estimated and do not vary too much. Normally an APC system is connected to a distributed control system (DCS). The APC application will calculate moves that are send to regulatory controllers. Historically, the interfaces between DCS and APC systems were dedicated software interfaces. Nowadays the communication protocol between these system is managed via the industry standard Object Linking and Embedding (OLE) for process control (OPC) protocol.
The key advantages ofAPC with DCS are:
Safer plant operations
Avoiding unnecessary plant trips
Better plant performance and maximized production
13. Simulation of Absorption and Desorption Columns for CO2 Removal
Acomputer programme has been developed which simulates the performance of an absorption column for CO2 removal by using chemical solvents such as DEA promoted carbonate solution. The computer predictions have been validated by using industrial column data from fertilizer industries. In addition, another computer program has also been developed to simulate the performance of steam desorption of bicarbonate solution for solvent regeneration in the CO2 removal systems of fertilizer plants.
The modeling equations are rigorous as they take into account point to point variation of all important transport and physical parameters, heat effects, gas and liquid temperature profiles, enhancement in gas absorption due to mass transfer with chemical reaction, etc.