ENERGY_PERFORMANCE_ASSESSMENT_FOR_EQUIPMENT_AND_UTILITY_SYSTEMS_(CHAPTER-17:ENERGY PERFORMANCE ASSESSMENT OF FERTILISER INDUSTRY)

 ENERGY PERFORMANCE ASSESSMENT FOR EQUIPMENT AND UTILITY SYSTEMS (CHAPTER-17:ENERGY PERFORMANCE ASSESSMENT OF FERTILISER INDUSTRY)

Introduction

The inorganic, organic, natural or synthetic chemical elements that provide nutrient for the growth of plant are generally considered as fertiliser. The nutrients are categorized further as Primary, Secondary and Micronutrient based on their requirement by soil. The primary nutrients are: Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorus and Potasstum. Of the primary nutrients, Carbon, Hydrogen and Oxygen are supplied through air and water while Nitrogen (N), Phosphorus (P,O,) and Potassium (K) are supplied through chemical or mineral fertilisers. There are three secondary and more than 13 micro nutrients that also play an important role in plant growth. Indian Fertiliser Industry is the third largest producer of fertiliser in the world. There are 152 plants in operation. This is comprised of 30 urea, 19 DAP and NP/NPK complex, 91 single super phosphate, 10 ammonium sulphate, one calcium ammonium nitrate and one ammonium chloride fertiliser plants.

India produced 38 milllion tonnes of fertiliser products during 2013-14. Urea accounts for 60% of total production of fertilisers. Production of urea is highly energy intensive and accounts for almost 85% of the energy requirement of the fertiliser sector. Contribution of other fertilisers is very small compared to urea production. For example, the weighted average energy requirement for production of urea is around 6.2 Million kcal/tonne of urea compared to 0.25 Million kcal/tonne for a Complex fertiliser. Further, for production of urea, 80% of the energy is consumed in production of ammonia. Therefore, the scope of energy conservation is maximum in ammonia plants. Hence, efforts for conservation of energy are focused on ammonia plants.

During sixties and mid-seventies, emphasis was on establishing fertiliser production capacity by putting up new plants. These plants were based on raw materials readily available at that time. In 1970’s due to addition of petroleum refineries, naphtha became the preferred feed stock for fertiliser plants. Again due to surplus availability of fuel oil, six plants were constructed based on this feedstock. During this period, fertiliser industry being in early stage of development in India, focus was on reliability of plants and focus was not on energy efficiency. However, with passage of time, energy became scarce and expensive. This necessitated revamp of older plants, by changing over to Natural Gas (NG) as feedstock and upgrading technology. During eighties, large quantity of natural gas was available from Bombay High. This provided more efficient and cheaper feedstock. By this time, the fertiliser technology had made significant advancement. This was the period during which the fertiliser industry grew very rapidly. During nineties, the technological developments in manufacturing of ammonia/ urea were at their best. Significant fertiliser production capacity was added with more efficient plants. However, due to unabated increase in price of hydrocarbon products, reduction in energy cost became the ultimate focal point of the industry.

As a result, a number of retrofits were implemented by old plants like change in ammonia converter basket, better heat recovery from reformer furnace exhaust, gas turbines coupled with HRU etc. These measures brought down energy consumption significantly. A number of old vintage plants not operating at reasonable levels due to obsolete technology and equipments became unviable and were shut down in 1990s.

Fertiliser Manufacturing Process

Ammonia is the building block of all nitrogen containing fertilisers. Total installed ammonia capacity in India is 13.70 tonnes. Almost 95% of ammonia produced is used for making urea and the balance is used for manufacture of complex fertilisers. Urea is the major nitrogenous fertiliser and accounts for 83 % of the total nitrogen production. Other nitrogenous fertilisers are ammonium sulphate (AS) and calcium ammonium nitrate (CAN), which are produced in smaller quantities. Among complex fertilisers, di-ammonium phosphate (DAP) and various grades of NP/NPK are produced. Apart from these, single super phosphate (SSP), also contributes to the phosphatic production.

Ammonia

The basic process of ammonia synthesis is Haber process where ammonia is synthesized by catalytic reaction of nitrogen and hydrogen at high pressure. Nitrogen is derived from atmosphere either by air

separation or using air for combustion of hydrocarbons generating a mixture of nitrogen, hydrogen and carbon oxides. Hydrogen can be derived from a variety of raw materials including water (electrolysis), heavy hydrocarbons (partial oxidation), coal/coke (gasification), light hydrocarbons (steam reforming). In all cases of fossil fuels, part of hydrogen produced is derived from water used as steam.

Distinct process steps for manufacturing ammonia using hydrocarbon reforming process are described

below (Figure).

Feedstock Desulphurization

Hydrodesulfurization (HDS) is a catalytic process used to remove sulphur (S) from natural gas or naphtha feedstock. The important reason for removing sulfur is that sulfur, even in extremely low concentrations, poisons the downstream catalysts in steam reforming process. The hydrodesulfurization unit consists of a fixed-bed reactor operating at 300 to 400 °C, typicaly in the presence of a catalyst consisting of alumina impregnated with a combination of nickel and molybdenum (NiMo) or cobalt and molybdenum (CoMo) catalyst, for specific feed stocks. Sulphur compounds are hydrogenated to H2S

Primary reformer (Tubular reformer)

The purpose of the primary reforming is to convert bulk of the hydrocarbons feed to H, and CO. The overall reaction is endothermic and requires a large amount of external heat, which is provided by burning NG/naphtha fuel in the radiant chamber in which catalyst tubes are installed. The tubes are of nickel alloy, centrifugally cast and packed with catalyst. The catalyst is nickel based with promoters such as potassium and supported on calcium aluminate base. Temperature of the tubes wall is maintained around 900 °C. There are many designs of reformer furnaces. All these aim at efficient heat transfer.

The fuel burners may be placed at top, side or bottom of the furnace. The feed gas from the desulphuriser is mixed with process steam so as to maintain a steam to carbon molar ratio (S/C ratio), at around 3.0 —3.5. Following endothermic reforming reaction takes place inside the tubes filled with catalyst.

The flue-gas leaving the radiant box has temperatures in excess of 900 °C, after supplying the necessary

high level heat to the reforming process. The heat content (waste heat) of the flue-gas is used in the

reformer convection section, for heating of various process streams. Fuel energy requirement in the

conventional reforming process is 40-50% of the process feed-gas energy. The flue-gas leaving the

convection section at 120-200 °C is one of the main sources of emissions from the plant. These

emissions are mainly CO2, NOx, with small amounts of SO2and CO.

Secondary reformer

About 10-12% of the hydrocarbon feed remains unconverted in the primary reformer because of the chemical equilibria at the actual operating conditions. In the conventional reforming process the degree of primary reforming is adjusted so that the air supplied to the secondary reformer meets both the heat balance and the stoichiometric synthesis gas requirement.

The process air is compressed to the reforming pressure and heated further in the primary reformer convection section to around 600 °C. The process gas from primary reformer is mixed with the air in a burner and then passed over a nickel-containing secondary reformer catalyst. The reformer outlet temperature is around 1,000 °C, and up to 99% of the hydrocarbon feed (to the primary reformer) is converted, giving a residual methane content of 0.2-0.3% (dry gas base) in the process gas leaving the secondary reformer. The process gas is cooled to 350-400 °C in a waste heat steam boiler or boiler/ superheater downstream from the secondary reformer.

The catalyst used for secondary reforming is similar to that of primary reforming, except it has lower nickel content (4-8 wt%) and is designed to withstand the high temperature. Rings and multi-hole pellets are the usual shapes. Either a refractory alumina or magnesium aluminate carrier is used. The catalyst is rugged with high crush strength and high temperature resistance.

Alumina balls are placed in several layers on top of the catalyst bed to prevent impinging hot gases from damaging the catalyst. Also they protect the catalyst from prolonged high temperature damage.

CO Shift Conversion — High Temperature (HTS)

The process gas from the secondary reformer contains 12-15% CO (dry gas base) and most of the CO

is converted to CO, in the shift section according to the reaction:

The reaction is carried out in two stages due to equilibrium considerations. In the first stage, gas is passed through a bed of iron oxide/chromium oxide catalyst at around 400 °C, where the CO content is reduced to about 3% (dry gas base), limited by the shift equilibrium at the actual operating temperature.

This step is called high temperature (HT) reaction. Therefore, higher CO slip from this section has penalty for energy consumption. The catalyst is an iron oxide (magnetite) catalyst structurally promoted by chromium oxide.

CO Shift Conversion — Low Temperature (LTS)

The gas from the HT Shift converter is cooled and passed through the Low Temperature Shift (LTS) converter. This LTS converter is filled with a copper oxide/zinc oxide-based catalyst and operates at about 200- 220 °C. The residual CO content in the converted gas is about 0.2-0.4% (dry gas base). A low residual CO content is important for the efficiency of the process. Oxides of carbon are poison to synthesis catalyst. Therefore, the residual CO and CO, are converted back to methane which is inert to the synthesis catalyst. The hydrogen consumed in the methanation reaction reduces the ammonia production to that extent.

LTS catalysts are high-copper catalysts based on oxides of copper, zinc and aluminum. The copper oxide is very finely dispersed resulting in high activity. Copper/zinc/aluminium LTS catalysts are seriously poisoned even by trace amounts of chlorides. Some suppliers also recommend LT catalyst guard to protect the catalyst against poisoning.

CO2, Removal Section

After leaving the low temperature shift conversion section, the process gas contains apart from synthesis gas, around 18% CO2. Other constituents i.e. CO, CH4, inerts are present in very small quantities. The process gas stream also contains excess process steam. Separation of CO2, from reformed gases is carried out by selective absorption of CO2, in a solution and subsequently releasing it by depressurization and heating of the solution.

The process gas is cooled from 220°C to 50°C, in heat recovery heat exchangers and passed through CO2, absorber operating at pressure maintained at around 30 kg/cm2. The solution contains an activator to enhance CO2, absorption, corrosion inhibitor and is free from suspended matter. The process gas from CO2, absorber contains residual CO2, around 300-2000 ppm along with other residual impurities, which are removed in the downstream gas purification section.

The rich solution leaving the absorber bottom is loaded with CO2, and is depressurized through a pressure control valve / hydraulic turbine. It is then heated through a number of heat recovery heat exchangers and then sent to the regenerator, operating at slightly higher than atmospheric pressure. CO2, gets released, which is then cooled in heat recovery exchangers and sent to urea plant.

Process condensate is stripped of all these chemicals by means of steam and the stripped effluent water is recycled in the process. In the CO, absorption solution, activators viz DEA, glycine etc; are added to enhance CO, absorption. Further, the CO, rich solution at boiling temperature is highly corrosive. Therefore, carbon steel equipment are provided with passivation layers (oxidation layers) by adding corrosion inhibitor i.e. titanium oxide. Side stream filters are provided to remove suspended matter from the solution.

Methanation

The small amounts of CO and CO,, remaining in the synthesis gas, are poisonous for the ammonia synthesis catalyst and must be removed by conversion to CH, in the methanator. The reactions take place at around 300 °C in a reactor filled with a nickel containing catalyst. Following reactions take place.

Residual CO and CO, are converted to methane, which acts as inert to the downstream ammonia synthesis catalyst. The catalyst is nickel based, dispersed on silica, calcia and magnesia. Nickel content

is about 20-34%. It comes in various physical shapes i.e. spheres, tablets, extrusions, rings etc.

Gas purifier

An alternative to methanation for removal of CO and CO2, from process gas is cryogenic purification. Cryogenic purification also removes residual methane and argon in the synthesis gas. The process involves washing of gas with liquid nitrogen. Cryogenic system operates at temperature of minus 170°C to minus 200°C. The off-gas is used as fuel in reformer furnace.

Ammonia synthesis

The purified process gas is called “make up synthesis gas”. It contains hydrogen and nitrogen in the mole ratio of 3:1. It also contains some inerts i.e. methane and argon. The gas is compressed in  Synthesis gas compressor to synthesis loop pressure of 120 — 200 Kg/cm’. The gas entering the ammonia converter consists mainly of unconverted gas recirculated in the loop along with “Make up” gas . The gas entering the converter contains H, and N, in the stoichiometric ratio of 3:1 plus 10-14% inerts and about 2% ammonia. The inerts consist mainly of methane, argon and sometimes helium. The synthesis of ammonia takes place by following reversible reaction

Operating temperature is 400-450 °C and the catalytic reaction is exothermic. The metallic iron catalyst is primarily made from magnetite, Fe3O4 that has been promoted using alkali in the form of potash and metals such as aluminium, calcium and magnesium. The catalyst is susceptible to permanent

poisoning by sulphur, arsenic, phosphorus, chlorine and heavy hydrocarbons; while oxygen bearing compounds will cause temporary poisoning. Ammonia synthesis is affected by pressure, temperature, inlet gas composition, space velocity and catalyst particle size etc. LeChatelier’s principle determines the reaction rate and equilibrium ammonia concentration. Thus, raising the pressure increases the equilibrium percentage of ammonia and accelerates the reaction rate. Reaction being exothermic, higher temperatures increase reaction rate but equilibrium percentage of ammonia decreases. Reaction rate is also affected by other factors like ammonia concentration in the incoming feed gas, presence of inerts, make up gas composition, catalyst activity and its particles size, etc.

The gas leaving the converter contains 12-18% ammonia. By continual recycling of the un-reacted nitrogen and hydrogen, the overall conversion is about 98% of make-up gas. The heat contained in the gas leaving ammonia converter at around 400 °C is utilized in a series of successive heat exchangers. The higher level of heat is recovered by raising steam at 105 kg/cm2 and then heating boiler feed water. It is then used to heat incoming synthesis gas feed to converter. After effective heat recovery, the gas is cooled with cooling water and further cooled by refrigeration, to condense most of ammonia as liquid. Liquid ammonia is separated and the unconverted gas is recycled back to the converter through recycle compression stage of synthesis gas compressor. The ammonia vapours generated, are compressed and liquefied in the refrigeration section.

The inerts (methane and argon) do not dissolve in the liquid ammonia. Their accumulation in the synthesis loop retards reaction rate. Thus the major portion of these inert gases is removed by taking out a purge stream from the loop. The purge gas has a typical composition of 60% H2, 20% N2, 13% CH4, 4% Ar and 2% NH3,

In the older plants, purge gas was used as fuel in the reformer furnace. Over the period, purge gas is re-processes to recover hydrogen by one of the processes viz. (i) cryogenic process (ii) Membrane separation (iii) Pressure Swing Adsorption (PSA).

Ammonia Storage

In the older plants of lower capacity, ammonia is stored in pressurized systems such as bullets and in Horton spheres. With the large scale industrial production of ammonia, it has become common to store ammonia at atmospheric pressures at -33°C. Atmospheric ammonia storage requires lesser capital per unit volume and is safer than sphere storage using pressures higher than atmospheric.

Urea

Urea is manufactured by reacting liquid ammonia with CO, gas at high pressure (160 — 250 ata) and temperature (190 °C). The reaction is non-catalytic. Distinct process steps for manufacturing urea are described below (Figure 17.2).

Urea synthesis

Urea is produced by synthesis from liquid ammonia and gaseous carbon dioxide. These two react toform ammonium carbamate, a portion of which dehydrates to urea and water. The reactions are as follows:

Hence, overall reaction is,

In synthesis condition (T=190°C, P = 160 kg/cm7), first reaction occurs rapidly and is completed, second reaction occurs slowly and determines reactor volume. The Mole ratio of ammonia to carbon

dioxide is 3.6 to 1. Asmall amount of air is added at compressor’s suction for passivation of Stainless

Steel surfaces.

Recovery of un-converted reactants

In the urea reactor, the reaction is complete by 60-70% and due to recycling of unconverted reactants,

urea concentration in products coming out of reactor is 30-35%.The unconverted reactants are separated out from urea solution by reducing pressure and supplying external heat by steam, usually accomplished in three stages viz synthesis pressure, 12-18 ata and 4.5 ata. The vapours so generated are condensed to obtain carbamate solution and the process is highly exothermic. Utilisation of heat of condensation of carbamate vapours is the genesis of improving thermal efficiency of urea process. In most of the cases, steam is generated at low pressure (4.5ata).

Urea purification and low pressure recovery

Residual carbamate is decomposed in Medium Pressure (12-18 ata) & Low Pressure (4.5 ata) decomposers by heating with steam / hot condensate. The carbamate vapours leaving the top of decomposer, are sent to the condenser where they are absorbed in recycled aqueous carbonate solution. The absorption and condensation heat is removed by cooling water. The inert gases which essentially contain ammonia vapour now directly into low pressure falling film absorber, where the ammonia is absorbed by a counter current water flow.

Urea concentration section

Solution leaving low pressure decomposer bottom with about 72% urea is concentrated in two vacuum stages operating at 0.3ata and 0.03ata. Urea solution is concentrated upto 99.5 %.

Urea prilling

The final finishing of urea is carried out in prilling tower which are tall concrete structures. These can be natural or forced/induced draught. Melted urea leaving second vacuum separator is sent to Prilling bucket at top. In natural draught droplets falling get cooled and solidify by the ambient air entering from the bottom of the prilling tower. Some of the old generation plants have prilling towers with height lower than natural drafts and operate with provision of induced/ forced draught fans for cooling of the prills. The solid prills falling to the bottom of prilling tower are sent to product handling section / storage through belt conveyor.

Other Nitrogenous Fertilisers

Other nitrogenous products i.e. ammonium sulphate (AS), calctum ammonium nitrate (CAN), ammonium chloride (ACI) are mainly produced as by-products of other chemical plants. Currently, there is only one operating plant producing CAN. Ammonium sulphate is obtained as by product of caprolactum plants and in small quantities from steel plants.

Energy flow

The fertiliser making process is highly energy intensive accounting to nearly 60-70% of the production cost. This provides ample opportunities for reducing energy consumption.

Thermal Energy

Ammonia is produced from a gaseous mixture of nitrogen and hydrogen. Nitrogen is derived from atmospheric air and hydrogen is obtained mainly from fossil fuels. In exceptional cases where electricity is cheaper, hydrogen can also be obtained by water (electrolysis).

Thus, in ammonia manufacturing process, fossil fuel energy is used as “Feedstock” as well as “Fuel”.

Feedstock energy

Main consumer of fossil fuel energy is primary reformer where natural gas / naphtha are consumed as feedstock for production of ammonia.

Fuel energy

Fuel energy is consumed in following systems:

(i) Reformer furnace for supplying endothermic heat of steam reforming reaction.

(ii) Captive power plant for power generation.

(iii) Boiler for raising steam.

(iii) In some plants, fuel fired furnaces are used for pre-heating of natural gas, process air or superheating steam.

Steam production

(i) Steam production in ammonia plant: Steam reforming ammonia plants have process gas at high temperature at outlet of (a) secondary reformer at 950-1000 °C (b) high temperature shift converter at 400-450 °C (c) synthesis converter at 400-450 °C. This heat is recovered by producing steam at 105-110 kg/cm2 and also superheating the same up to about 505 °C.

(ii) Gas turbine / Process air compressor / Heat recovery unit: In some plants installed since 1990s, gas turbine is provided to drive process air compressor and steam is produced at 105 kg/cm2 from exhaust of the gas turbine.

(iii) Auxiliary / Service boiler: A standalone auxiliary boiler is provided to meet additional steam requirement, specifically during startup of the plant.

Steam utilization

Steam at high pressure and temperature (105 kg/cm? 505 °C), is utilized to drive turbine for synthesis gas compressor. Steam extraction at medium pressure (40 kg/cm2) is used mainly as reformer feed and for driving process air compressor, refrigeration compressor, CO, compressor. It is also used to drive high rating pumps for boiler feed water (BFW), cooling water (CW) etc.

From bigger turbines, steam is also extracted at different pressure levels to drive smaller turbines as well as heating the process fluids.

In modern design of ammonia plant flow sheet, emphasis is laid on optimization of steam production and consumption not only for ammonia plant battery limit, but for the entire fertiliser complex.

Electrical Energy

Almost all the Indian ammonia plants are having captive power plants so as to avoid disruptions in plant operation due to power interruptions which are more likely from grid power. However, these plants are also connected with power grid to meet contingencies and draw minimum power to be used in township and other non-plant use.

There are three models of generating captive power.

(i) Where natural gas is available in sufficient quantity, gas turbine is coupled with power generator and steam is produced by heat recovery from exhaust gases with supplementary firing. Rating of turbo generator is up to 25 MW at 11 kV under ISO conditions. Normally two sets are provided. In India, almost all the gas based ammonia plants installed since 1980s are having gas turbo-generators / Heat Recovery Units (HRU).

(ii) Where supply of natural gas is restricted or not available at all, steam is generated in boilers by firing alternative fuels like furnace oil, coal etc. Steam turbo generators are provided.

(iii) Plants are also having diesel generating sets of capacity around 5-7 K VA to be used as “Emergency power’, to run essential services, in case of tripping of main power supply.

Material and Energy Balance

A typical material and energy flow diagram of an ammonia/urea fertiliser complex is illustrated below in Figure

Calculate specific energy consumption for ammonia as Geal/Tonnes ammonia

Calculate specific energy consumption for urea as Gceal/Tonnes urea

Solution

(a) Material balance of natural gas

Ammonia / urea complex Steam Balance - Illustration

Brief description of steam network

In an ammonia / urea complex, steam is generated at high pressure (105 kg/cm2, 515 °C), mainly in non-fired heaters viz. Reformed gas boiler (RG boiler) and Ammonia synthesis boiler (Synthesis boiler) where vast quantity of heat is available from highly exothermic reactions. High pressure steam is also produced in direct fired auxiliary boilers located either in ammonia plant or Off-sites. Total steam produced is fed into a common HP steam header being maintained at 105 kg/cm2, 515 °C. HP steam is consumed in steam turbines coupled with gas compressors. These are turbines are of extraction type, condensing type or a combination of both, depending on steam requirement in the downstream facilities. In most of ammonia plants, whole of HP steam is fed into synthesis gas compressor turbine, from where steam is extracted at medium pressure (40 kg/cm2). A little quantity of HP steam is sent through by-pass throttling valve to keep the by-pass steam header hot and by-pass valve in floating condition.

Medium pressure (MP) steam is utilized mainly as reformer feed along with natural gas to produce reformed gas for production of ammonia. It is also used for running steam turbines of process air compressor, refrigeration compressor, cooling water pumps, boiler feed water pumps, ID/FD fans for

reformer and other pumps/fans of medium duty.

In the steam condensing turbines, exhaust steam is condensed in water cooled condenser and thus, a considerable quantity of heat is lost in cooling water sink. On the other hand, in steam extraction turbines, there is no heat loss to the sink but there must exist consumers of extracted steam.

Typical steam balance diagram

A typical steam balance diagram (Figure 17.4) in an ammonia/urea complex is given below. The diagram is self- explanatory. Impact of quench water added for de-superheating of the superheated steam obtained after expansion, is ignored.

From the steam balance diagram given above, calculate the following:

MOOD>

Energy balance in HP steam header.

Energy balance around ammonia synthesis turbine.

Energy balance in MP steam header.

Energy balance around process air compressor.

Total energy lost in heat sink of cooling water.

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Chapter 1