Ammonia Recovery from Coke Oven Gas
Ammonia Recovery from Coke Oven Gas
The coke oven by-product plant is an integral part of the by-product coke making process. In the process of converting coal into coke using the by-product coke oven, the volatile matter in the coal is vapuorized and driven off. This volatile matter leaves the coke oven chambers as hot, raw coke oven gas is a mixture of gases. Raw coke oven gas contains a large variety of gases which vary depending on the process and coal source. However, concentrations tend to be typically around by volume hydrogen (H2) – 48 % to 55 %, methane (CH4) – 28 % to 30 %, carbon monoxide (CO) – 5 % to 7.5 %, carbon di-oxide (CO2) – 1.5 % to 2.5 %, nitrogen (N2) – 1 % to 3 %, high paraffins and unsaturated hydrocarbons – 2.5 % to 4 %, and oxygen – less than 0.5 %. Minor components include ammonia (NH3), hydrogen sulphide (H2S), hydrogen cyanide (HCN), ammonium chloride (NH4Cl), benzene (C6H6), toluene (C7H8), xylene (C8H10), and naphthalene (C10H8) and other aromatics, tar components, tar acid gases (phenolic gases), tar base gases (pyridine bases), and carbon di-sulphide (CS2).
After leaving the coke oven chambers, the raw coke oven gas is cooled which results in a liquid condensate stream and a gas stream. The functions of the by-product plant are (i) to take these two streams from the coke ovens, (ii) to process them to recover by-product coal chemicals, and (iii) to condition the gas so that it can be used as a fuel gas. Raw coke oven gas contains various contaminants, which give coke oven gas its unique characteristics. These consist of tar vapours, light oil vapours (aromatics) consisting mainly of benzene, toluene, and xylene (BTX), naphthalene vapour, ammonia gas, hydrogen sulphide gas, and hydrogen cyanide gas.
In order to make raw coke oven gas suitable for use as a fuel gas at the coke oven battery and elsewhere in the steel plant, it is necessary (i) to cool the coke oven gas (ii) to condense out water vapour and contaminants, (iii) to remove tar aerosols to prevent gas line / equipment fouling, (iv) to remove ammonia to prevent gas line corrosion, (v) to remove naphthalene to prevent gas line fouling by condensation, (vi) to remove light oil for recovery and sale of benzene, toluene and xylene, and (vii) to remove hydrogen sulphide to meet local emissions regulations governing the combustion of coke oven gas.
Historically, the coke oven gas by-product chemicals were of high value, frequently more so than the coke itself, particularly in agriculture and in the chemical industry, and the profits made from their sale were frequently of high importance than the coke produced. Nowadays however, majority of these same products can be more economically produced using other technologies such as those of the oil industry. Hence, with some exceptions depending on local economics, the main emphasis of a modern coke by-product plant to purify coke oven gas became more about the treatment of the gas to produce environmentally clean fuel and less about the recovery of by-products. The purpose is to treat the coke oven gas sufficiently so that it can be used as a clean, environmentally friendly fuel.
The general principle observed in the sequence of operation in the by-product plant is to follow the tar / liquor separation from the gas by sequential removal of components. The operation is normally carried out at positive pressures of 7 kilo pascal (kPa) to 15 kPa to avoid air in-leaks. A normal sequence for a plant hence is (i) tar and liquor separation, (ii) primary gas cooling, (iii) compression in exhausters, (iv) electrostatic tar droplet removal, (v) secondary / final gas cooling (frequently combined with ammonia removal), (vi) ammonia removal, (vii) benzol removal, (viii) naphthalene removal (if necessary), and (ix) hydrogen sulphide removal. The sequence can be varied if, for example, two processes are inter-dependent. As an example, in some process streams, the ammonia and hydrogen sulphide removal processes are inter-dependent.
As mentioned above, coke oven gas is a complex gas with several constituents, some of which are useful for several applications. As an example, hydrogen, methane, and carbon monoxide are retained in the coke oven gas so that it can be used as a fuel in the steel plant. Paraffinic and unsaturated gases being useful components are also retained in the coke oven gas meant for further use. Small traces of harmless, inert gases such as oxygen, nitrogen, and carbon di-oxide are also allowed to remain in the final gas as inert but harmless components in the final gas. The remainder of the components are removed in the by-product plant as far as is practical.
Ammonia is present as a vapour in the coke oven gas, readily dissolving in water to form a weakly alkaline solution. Since the ammonia is associated with the problems of corrosion when hydrogen sulphide and ammonia are present, it is normally removed at the earliest possible stage, immediately following the tar precipitation. There are few economically favourable outlets for the ammonia or its associated compounds. Processes for ammonia removal make use of its properties of a weakly alkaline compound to react with acidic reagents of one type or another.
Ammonia removal from both the coke oven gas and the flushing liquor is now a universal requirement of coke-ovens as it is corrosive to carbon steel plant pipework and oxidation forms nitrogen oxides (NOx), which are detrimental to the environment because of the over-nitrification of ground waters and soils besides the formation of NOx noxious gases.
Historical aspects of ammonia removal from coke oven gas
Since the 1860s, ammonia obtained from the destructive distillation of coal was used as a source of nitrogen for fertilizer purposes. Ammonia liquor was obtained by sulphuric gas absorption which was employed after scrubbing coal gas with water. Ammonium sulphate, obtained from the process, was then used as fertilizer. It was in 1889 that Ludwig Mond, a German-British chemist, discovered, during his search of a process to produce ammonium sulphate, that coal combustion could produce ammonia when the reaction takes place with air and steam. The produced gas, in combination with other species, was named Mond gas. His findings suggested that low-quality coal could react with superheated steam producing high valuable gas. Dilute sulphuric acid sprays were then employed to remove the ammonia, forming ammonium sulphate in the process.
Different from other processes, Mond’s modification was based on the restriction of the supplied air, which was then filled with steam to generate low-temperature atmospheres. Since these low temperatures were lower than the temperatures needed to dissociate ammonia, the recovery of the latter was maximized. Mond realized at the time the great potential of ammonia to satisfy the fertilizer market, although his initial interest was to ensure the supply of ammonia to his alkali factory.
Mond’s process was utilized at Brunner from 1902, which was followed by implementation at various other sites in Britain, Argentina, Spain, and the USA by 1903. These plants needed massive capital investment to be economically viable. Over 182 tons of coal per week were employed to produce ammonia profitably with efficiencies as high as 80 %.
Further technological advances took place across Europe driven by companies such as Semet and Evence Coppee which developed new techniques to increase the ammonia recovery from coke ovens. Amongst other leaders that worked on this subject, Heinrich Koppers introduced the coke oven process into the US industry in 1907. Establishing Heinric Koppers AG in 1904, he created the so-called ‘half direct’ process for recovery tar and ammonia from coke oven gas. By 1923, 90 % of all coke oven plants in the Ruhr district used this process.
Before World War II, significant quantities of ammonia were produced from derived coke producing processes through coal conversion. Coal-based ammonia production dominated the industry until then. It is estimated that around 2.7 million tons of ammonia per year were produced in this manner. However, the situation would change by 1960, where most of the ammonia (by this time, 16 million tons per year) was obtained using natural gas from which hydrogen was obtained.
Historically the removal of ammonia from coke oven gas has yielded one of the more profitable by-products, that of ammonium sulphate. Nowadays the cost to produce ammonium sulphate frequently outweighs the revenue from the product, however there are still several coke oven and by-product plants around the world producing ammonium sulphate. More modern processes for ammonia removal include the water wash process in which the coke oven gas is scrubbed by water, which dissolves the ammonia, along with some hydrogen sulphide and hydrogen cyanide. The resulting scrubbing solution is pumped to an ammonia still where steam is used to strip out the ammonia. The ammonia vapours from the still can be processed to form ammonium sulphate, condensed to form a strong ammonia solution, incinerated or catalytically converted to nitrogen and hydrogen which are then recycled back into the coke oven gas. The incineration of the ammonia vapours is normally not an option in areas where environmental laws restrict the emission of NOx and sulphur oxides (SOx).
Ammonia from steel plants used to be the primary source for the production of fertilizers before the end of the first half of the twentieth century. However, because of the increasing introduction of natural gas and its cleaner processing, ammonia from the steel plants has been relegated, now sharing less than 12 % of the overall production of ammonia worldwide. Although some developing economies still use this concept, i.e., producing ammonia mostly in the form of ammonium sulphate from the coke oven gas, costing and environmental regulations have led to a decline of ammonia production through this method.
However, since steel is a significant component of today’s society, the use of ammonia from the generation of coke oven gas or steel plants can still be a potential source for chemical storage which can be implemented for backup applications in power and heat supply auxiliary processes. Present studies and large-scale projects show that ammonia, recovered from waste streams found in the entire steel production chain, can have considerable advantages for energy support and chemical production.
Source and composition of the ammonia liquor
The water in the excess flushing liquor originates from (i) free moisture in the coal charge, (ii) bound, or combined water in the coal, and (iii) additional water introduced to the plant, for example by steaming during oven charging or for hosing down the plant. On a typical coke oven and by-product plant, the liquor production rate is around 120 litres/ton (l/t) to 130 l/t of coal carbonized. The flushing liquor supply to the battery is around 4,000 l/t of coal carbonized, depending upon the battery and collecting main arrangement. The liquor contains dissolved ammonia and a wide variety of compounds. These are normally grouped as ‘fixed’ or ‘free’ compounds, Fixed compounds are relatively stable while the ‘free’ compounds can be decomposed solely by heating to around 100 deg C. The ‘free’ compounds include the hydroxide, carbonate, bi-carbonate, sulphide and cyanide while the ‘fixed’ compounds include the chloride, thiocyanate, thiosulphate and sulphate.
In addition, the ammoniacal liquor contains smaller but important concentrations of dissolved neutral oils, phenols, and tar bases (pyridine etc.). The ammonium chloride content of the ammonia liquor is important, since this compound represents almost the entire chlorine present in the coal charge and the ammonium chloride is contained in the condensates arising in the gas collecting main and the primary gas cooling. In view of the particularly corrosive nature of chlorides, and the fact that they do not vapourize readily at normal process temperatures, their concentration has to be avoided in any system for ammonia liquor treatment or disposal. For example, use of this liquor or derivative as a make-up source to coke quenching systems has frequently aggravated the corrosion problems in these systems. Because of this, particularly when new plant processes are being considered, it is necessary to make a rough chloride balance to confirm that high concentrations are prevented.
Ammonia removal processes
Ammonia is a by-product of the coking of coal. It is a constituent of the coke oven gas leaving the coke ovens, with a typical concentration in raw coke oven gas of 6 grams per normal cubic meter (g/N cum). The solubility of ammonia in water leads to its presence in the coke oven battery flushing liquor with a typical concentration of 5 grams per litre (g/l) to 6 g/l total ammonia. As a result, because of the net production of flushing liquor in the coke oven and by-product plant, referred to as excess flushing liquor but also known as coal water, virgin liquor, or weak ammonia liquor, there arises a liquid stream as well as a gas stream from which ammonia is to be removed. The quantity of excess liquor is around 12 % by weight of the dry coal throughput, depending on the coal moisture content.
Ammonia removal from gas streams remains a universal feature of coke oven and by-product plants. The reason for this is that ammonia, in the presence of the other coke oven gas contaminants such as hydrogen cyanide, hydrogen sulphide, oxygen, and water, is extremely corrosive to carbon steel. Earlier, an added incentive was the profitable sale of by-products such as ammonium sulphate, anhydrous ammonia, and concentrated ammonia solutions. With the possible exception of anhydrous ammonia, the reduced market value of these by-products, especially in the industrialized nations, no longer makes their production profitable.
Ammonia removal from liquid streams is performed mainly for environmental reasons. The primary ammonia handling equipment in the by-product plant deals with the removal and disposal of the ammonia present in the coke oven gas. However, these systems frequently include facilities to handle the ammonia arising in the excess flushing liquor. To help understand how such facilities are incorporated into the overall ammonia handling system, the ammonia removal processes are described first with the treatment of ammonia in the excess flushing liquor. The main processes for removal of ammonia from coke oven gas are then described. As these processes generate in several cases a concentrated ammonia vapour stream, and finally the alternatives available for treatment of this stream are described.
Treatment of excess flushing liquor
Available options – In some of the locations, excess flushing liquor can be disposed of without prior treatment, using deep well injection. Conventionally, excess flushing liquor was normally used to quench the hot coke. However, this is no longer an acceptable practice for environmental reasons. In the absence of such simple disposal methods, the remaining alternatives are the removal of the majority of ammonia from the liquor by distillation, normally followed by final treatment in a biological effluent treatment plant (BETP). The biological effluent treatment plant can be on site or it can be operated by the local authority in which the coke oven and by-product plant is located. It is possible to use biological effluent treatment alone to remove ammonia from excess flushing liquor, however, the size and operating cost of a biological effluent treatment plant is considerably reduced when preliminary removal of ammonia by distillation is performed.
The flushing liquor distillation followed by further treatment in a biological effluent treatment plant (BETP) would remove its ammonia content. The distillation stage reduces operational costs and involves feeding the flushing liquor to a distillation column and feeding a counter-current flow of stripping stream beneath it, which causes the ammonia to vapourize out of the overhead vapours before further treatment. The treated liquor is then cooled and passed onto the biological effluent treatment plant for further ammonia removal by decomposing, for example, ammonium chloride and ammonium sulphate, upon alkali addition.
Distillation of excess flushing liquor – The distillation of excess flushing liquor involves feeding the liquor to the top of a trayed distillation column, normally called an ammonia still, and feeding a counter current flow of stripping steam at the bottom. The stripping steam distils off the ammonia which leaves with the overhead vapours and passes on for further treatment. The stripped liquor is pumped from the bottom of the still and cooled before discharge to the local sewer or on-site biological effluent treatment plant. Typical levels of total ammonia in stripped liquor range from less than 50 parts per million (ppm) to 150 ppm.
Not all the dissolved ammonia present in excess flushing liquor is readily steam strippable. Several chemical species present in the flushing liquor, lead to the formation of different ammonium salts in solution. These include ammonium carbonate, ammonium chloride, and ammonium sulphate among others. Salts such as ammonium carbonate are easily decomposed by heat in the still to yield free molecules of ammonia. However, other salts such as ammonium chloride and ammonium sulphate are not decomposed and retain the ammonia in a ‘fixed’ form. The fraction of fixed ammonia to total ammonia in excess flushing liquor is typically 20 % to 50 %. To allow the distillation of the fixed ammonia, the excess flushing liquor is to be made alkaline. The typical reaction which then takes place, liberating free molecules of ammonia is NH4+ + OH = ΝΗ3 (g) + Η2O.
In addition, alkali is determined by mass balance, based on chemical analysis of the excess liquor to give the concentration of the fixed ammonia present. The form of alkali used in the distillation of excess flushing liquor has changed over the years. For several years, a suspension of calcium hydroxide (lime) was used. This material had the advantage of low cost and ready availability, but the formation of insoluble calcium salts such as calcium carbonate created a major issue with fouling. The ammonia stills need a considerable marking of overdesign to allow continued operation while partially fouled, but even so they had to be taken out of service regularly for cleaning.
For avoiding the issue of fouling, sodium carbonate (soda ash) has also been used. This has the advantage that insoluble salts are not formed, but impurities can be present to cause fouling and on-site storage and mixing equipment is needed. An environmental factor when using reagents such as lime and soda ash is that any insoluble deposit formed can create a solid waste disposal problem. The generally ready availability and ease of handling of sodium hydroxide (caustic soda) solution has made this the alkali of choice for present day design of ammonia stills. Caustic soda is the most expensive of the alkalis traditionally used, but its consumption can be closely controlled which is of great benefit where limitations are imposed upon still effluent pH.
The non-fouling characteristics of caustic soda allows the use of more economical still designs, incorporating valve trays in place of the traditional bubble cap trays. In practice, the caustic soda is injected into the ammonia still near to, but not at, the top tray. This allows dissolved acid gases such as hydrogen cyanide and hydrogen sulphide to be stripped from the liquor first, before they can react with the caustic soda to form fixed salts.
Materials of construction for ammonia stills are chosen for their corrosion resistance. Cast iron has traditionally been used, with generous allowances for corrosion, although it is now frequently more economical to use materials such as Hastelloy, a nickel-based alloy, titanium, and 316 grade stainless steel. The upper sections of ammonia stills where both ammonia and acid gases are present normally need the use of highly corrosion resistant materials.
A common operating issue in the distillation of excess flushing liquor is the presence of tar carryover which can lead to serious fouling in the still. The normal solution to this problem is to install sand or gravel bed filters in the excess flushing liquor supply line. These are manually or automatically operated. The removed tar is back-flushed to the tar and liquor decanters.
Ammonia removal from coke oven gas
There are three methods used for this recovery which are direct, indirect, and semi-direct. These are described in Tab 1.
|Tab 1 Ammonium sulphate scrubbing methods|
|Direct||Tar is removed by cooling down the inlet gas. The gas is then passed through a saturator and washed with sulphuric acid. Ammonium sulphate is produced, centrifuged, washed and dried.||High recovery of effluents with low investment and reduced operating expenses.||Products could be contaminated (i.e. tar, pyridines and chloride). If the reactor is used as scrubber and crystallizer, the pH is difficult to handle to reduce impurities. Chloride from fuel or water could also react with the mixture and generate corrosion problem.|
|Indirect||The gas is cooled and liquefied. Liquors are passed through a bubble-cap still. ‘Free’ ammonia from the salts is released when the liquors get in contact with steam, while posterior treatment through lime decomposes the ‘fixed’ ammonium salts. Ammonia can then be stripped with water or combined with sulphuric acid to form ammonium sulphate.||The method has considerable flexibility, with a product (i.e., ammonium sulphate) free of impurities.||The effluent disposal is highly problematic, with high ammonia losses because of reduced reaction and absorption. Due to the higher complexity, the operation is more expensive than with the direct method.|
|Semi-direct||This method offers a solution that incorporates concepts from both the indirect and direct processes. The gas is cooled, and traces of tar are removed. Aqueous condensates are sent to ammonia spill. The mainstream and the released ammonia are combined and heated up to 70 deg C. The gas is scrubbed with a nearly saturated ammonium sulphate solution comprised of 5% to 6% sulphuric acid. This last part of the process takes place at temperatures between 50 deg C and 70 deg C.||The method is the preferred option for extensive facilities, with more significant ammonia recovery and salts free from impurities.||Although superior to the direct and indirect methods, the water balance during the saturation operation needs to be carefully controlled to enable the proper reaction of species. Moreover, salt incrustation could lead to maintenance problems.|
The presently used methods for removal of ammonia from coke oven gas are variations of three basic processes namely (i) the ammonium sulphate process, (ii) the Phosam process, and (iii) the water wash process.
The ammonium sulphate process – Because of the corrosive nature of ammonia, its removal is a priority in coke oven by-product plants. The ammonium sulphate process can take various forms but all basically involve contacting the coke oven gas with a solution of sulphuric acid. Variations include the use of an absorber, in which the sulphuric acid solution is sprayed into the gas, or the use of a saturator in which the gas is bubbled through a bath of sulphuric acid solution. The sulphuric acid reacts readily with the ammonia in the coke oven gas to form ammonium sulphate. This is then crystallized out, removed from the solution and dried for sale typically as a fertilizer.
The ammonium sulphate production process (Fig 1) involves the retention of ammonia in solution by the addition of sulphuric acid (H2SO4) generating ammonium sulphate ((NH4)2SO4) as per the reactions NH3 + H2SO4 = (NH4)HSO4, and (NH4)HSO4 + NH3 = (NH4)2SO4. The (NH4)2SO4 is recovered by crystallization before the formed crystals are centrifuged, washed, and then dried. This method of NH3 recovery differs in practice depending on the type of gas / liquor contacting device and the equipment used for crystallization. There is, however, an economic disadvantage with the ammonium sulphate process since the price of H2SO4 needed to form (NH4)2SO4 can cost higher than the value of the produced ammonium sulphate crystals. An estimated 2.5 kilograms (kg) to 3 kg of ammonia are produced per ton of dry coal used for the production of coke due to the high temperatures of the process.
Fig 1 Ammonium sulphate production process
The ammonium sulphate process removes ammonia from the coke oven gas by absorption in a solution of ammonium sulphate and sulphuric acid. The ammonium sulphate produced by the reaction of ammonia with sulphuric acid is recovered by crystallization. The crystals are then centrifuged, washed, and dried. The different ammonium sulphate systems in operation differ in the type of gas / liquor contacting device and the type of crystallization equipment used. An early and still very commonly used system employs a dip tube extending below the surface of the acid / ammonium sulphate solution in a vessel referred to as a saturator (Fig 2).
Fig 2 Ammonium sulphate process
The solution strength is maintained around 4 % acid. Coke oven gas is fed through the dip tube and gas / liquid contact is affected as the gas bubbles move up through the solution in the saturator. Acid is continuously added to the saturator. The heat of reaction between ammonia and sulphuric acid causes the evaporation of water into the coke oven gas. The concentration of ammonium sulphate reaches saturation, causing crystals to form directly in the saturator where they are allowed to grow until they are removed from the system. By means of agitation and circulation of the solution, the fine crystals are retained within the process solution of ‘mother liquor’ as it is known. The traditional material of construction for the saturator and all wetted surfaces is lead lined carbon steel. Alloys such as Monel and 316 grade stainless steel are also used. Brick lining is used to protect the lead lining, which suffers from ‘creep’ and damage by erosion.
The availability of acid resistant materials such as 316 grade stainless steel has allowed the development of the modern ammonia absorber systems. In these systems, a circulating stream of ammonium sulphate / sulphuric acid solution is sprayed counter currently to the coke oven gas flow in an absorber vessel. Absorption of ammonia from the gas takes place on the spray droplet surfaces. A portion of the circulating liquor is continuously withdrawn and fed to a separate continuous crystallizer. Here, the liquor is concentrated using heat and negative pressure to evaporate the water and so promote crystallization.
The crystals are removed and a stream of mother liquor is continuously fed back to the absorber circuit. The operation of ammonium sulphate systems results in an increase in the heat content of the coke oven gas leaving the absorber or saturator. The reason for this is that to maintain the water balance in the system, especially in the case of saturators, water is required to be evaporated into the gas stream. In addition to the water added to the system with the acid, regular desaturations are necessary in which water is added to the mother liquor to dissolve crystal deposits and reduce fouling.
In some installations, gas heaters are provided upstream of the ammonia absorbers / saturators. The evaporation of water into the coke oven gas results in an outlet gas with a higher dew point than at the inlet. In order for downstream gas cleaning processes such as naphthalene, benzol and hydrogen sulphide removal to be operated effectively, the gas is to be cooled in ‘final’ or ‘secondary’ gas coolers. The ammonium sulphate processes accommodate ammonia from excess flushing liquor to be feeding the overhead vapours from flushing liquor distillation into the coke oven gas main upstream of the ammonia saturator / absorber. The major economic disadvantage with ammonium sulphate processes is the price relationship between sulphuric acid and ammonium sulphate. The sulphuric acid needed to make ammonium sulphate can cost up to two times the value of the ammonium sulphate product.
In case of spray saturator (Fig 3), the conversion of ammonia from the coke oven gas with sulphuric acid to ammonium sulphate is carried out in a so-called spray type or bubble type saturator. The formation of ammonium sulphate occurs by means of neutralization of sulphuric acid with ammonia. In the spray type saturator process, the coke oven gas is directed into the spray type saturator. The ammonium sulphate lye inside the saturator is continuously sprayed into the coke oven gas in order to wash out the ammonia. The water saturated coke oven gas containing hydrogen sulphide / hydrogen cyanide is leaving the saturator with a temperature of around 95 deg C at the top. After a downstream arranged vapour cooler with inside drop separators, the vapours are cooled down to a temperature of around 60 deg C and flow to the Stretford unit for the removal of hydrogen sulphide from the coke oven gas. The circulating ammonium sulphate lye inside the saturator is directed through overflow pots and circulating pumps back to the saturator to protect the lye inside the saturator against clogging. Discontinuously, a stream from the bottom of the saturator is taken out by means of an ejector and flows through the slurry cone to the centrifuge. Inside the centrifuge the sulphate grains are cleaned from the diluted sulphuric acid, dewatered and dried in a steam heated dryer and stored in a salt store.
Fig 3 Ammonium sulphate production with spray saturator
The Phosam process – Another process for ammonia removal from coke oven gas is the Phosam process. This process absorbs the ammonia from the coke oven gas using a solution of mono-ammonium phosphate (NH4H2PO4). The process produces saleable anhydrous ammonia. The Phosam method for ammonia recovery was developed by United States Steel to produce pure anhydrous ammonia. The high-value product means that this process is much more economically favourable than the ammonium sulphate process. The Phosam process is a means of producing a saleable, commercially pure anhydrous ammonia product from the ammonia present in raw coke oven gas. This high value product makes the process much more economically viable than the ammonium sulphate processes. In the Phosam process, ammonia is selectively absorbed from the coke oven gas by direct contact with an aqueous solution of ammonium phosphate in a two-stage spray absorption vessel.
The Phosam process selectively absorbs ammonia from the coke oven gas through direct contact with an aqueous solution of phosphate, which is added only in minimal quantities. The absorption solution actually contains a mixture of (i) phosphoric acid (H3PO4), (ii) mono ammonium phosphate, (iii) di-ammonium phosphate [(NH4)2HPO4], and (iv) tri-ammonium phosphate [(NH4)3PO4]. The reversible absorption reactions which take place are (i) H3PO4 + NH3 = NH4H2PO4, (ii) NH4H2PO4 + NH3 = (ΝΗ4)2HPO4, and (iii) (NH4)2HPO4 + NH3 = (ΝΗ4)3PO4.
The ammonia absorbed is recovered by steam stripping. This regenerates the absorption solution which is returned to the spray absorber. The steam stripping is performed at high pressure of around 1.3 MPa. The reason for this is that the reversible reactions which liberate the ammonia from solution are favoured by higher temperatures. Hence, by operating at high pressure (and hence higher temperature), the consumption of stripping steam is minimized. The overhead vapours from the stripper are virtually only water vapour and ammonia. These vapours are condensed and then fed to a fractionating column where anhydrous ammonia is recovered as the condensed overhead product. The fractionator bottoms product, mainly water, leaves the system as effluent.
The Phosam process can become contaminated by tar and by absorption of acid gases (hydrogen cyanide, hydrogen sulphide, and carbon di-oxide) in the recirculated solution. To remove the tar, a froth flotation device is installed in the solution circuit between the absorber and the stripper. Acid gases are removed by preheating the ammonia rich solution and feeding it into a vessel referred to as contactor. In this vessel, the preheat causes vapourization of water and acid gases from the solution. These vapours are vented back to the coke oven gas main and the remaining rich solution is fed to the stripper. A subsequent step to deal with any remaining acid gases and prevent them from contaminating the anhydrous ammonia, is to add sodium hydroxide to the fractionator feed. The sodium hydroxide fixes the acid gas compounds as non-volatile sodium salts which remain in the fractionator bottoms effluent stream. Fig 4 shows the schematic diagram of Phosam process.
Fig 4 Schematic diagram of Phosam process
An important operational feature is the control of the water balance in the system. Substantial quantities of steam are condensed in the solution stripper, and this condensate is to be re-evaporated from the circulating solution into the coke oven gas stream. The temperature of the solution returning to the absorber is around 60 deg C, and hence the coke oven gas becomes heated as it flows through the absorber. The increased gas temperature normally makes it necessary to install a final gas cooler after the Phosam absorber. The addition of phosphoric acid to the absorption solution is needed only to account for operating losses such as spillage. It is added at weekly intervals at a rate equivalent to 7.5 grams phosphoric acid (H3PO4) per kilogram ammonia produced.
The Phosam process is very efficient, capable of achieving higher than 99 % recovery of ammonia from coke oven gas. Other plant configurations are possible in which, for example, aqueous ammonia solution is produced instead of the anhydrous ammonia. Materials of construction are stainless steel for all areas in contact with phosphate solution of aqueous ammonia, and carbon steel for other areas. As in the sulphate process, ammonia present in excess flushing liquor is handled first by distillation, with the vapours being fed to the coke oven gas upstream of the Phosam absorber. Anhydrous ammonia can be used as a fertilizer by injecting it directly into the ground, and as an industrial refrigerant. Under license control, the Phosam process is also used in the production of methamphetamines.
Water wash process – The water-wash method involves the use of water to strip contaminants from the coke oven gas. One of the simplest and most frequently used methods of removing ammonia from coke oven gas is to absorb it in water. Aqueous absorption liquor is fed into ammonia washer vessel in a counter-current flow of the coke oven gas leading to ammonia solution of high concentration.
The rich ammonia solution formed, with a typical concentration of 5 g/l to 8 g/l, is then fed to a distillation column where the ammonia is stripped from the aqueous liquor using steam. The ammonia and water vapours leaving the top of the stripping column are passed on for subsequent treatment in a variety of ways which are described earlier. After stripping, the absorption liquor is cooled and returned to the washer. There is a continuous blowdown of stripped liquor from the circuit which is equivalent to the volume of steam condensed in the stripper column. This blowdown is plant effluent and needs biological effluent treatment to fully remove the residual ammonia. As no chemical reactions are involved, other than the dissolving of ammonia in water, the water wash process is temperature dependent and is most efficient at low coke oven gas temperatures (20 deg C to 30 deg C).
The ammonia washer vessel is normally placed immediately after the tar precipitator in a typical coke oven by-product plant. At this point, the gas retains some superheat from the gas exhauster, if the by-product is operated at positive pressure. To promote ammonia removal efficiency, gas cooling is needed to remove this superheat and to cool the gas to the optimum temperature range. Since this part of the process is temperature dependent, it has been found that ammonia removal is most efficient at low temperatures. Indeed, the solution of gases such as ammonia depends on the liquid phase temperature according to Henry’s law of solubility. The gas cooling stage is frequently incorporated into the ammonia washer vessel itself. The ammonia washer is not to be operated at a lower temperature than the outlet temperature of the gas cooling stage, otherwise fouling by naphthalene can result.
The vessel can be designed as a spray type absorber with several liquor respray stages, or as a packed tower as is common in several German designed plants. The type of packing normally used is vertically arranged expended metal sheets which promote gas / liquor contact but resist playing and fouling.
Use of aqueous absorption liquor results in the simultaneous absorption of considerable quantities of acid gases (hydrogen cyanide, hydrogen sulphide, and carbon di-oxide) from the coke oven gas. Hence, the ammonia stripping column is nowadays being frequently constructed of corrosion resistant materials such as titanium and 316 grade stainless steel, although several plants continue to operate cast iron stills. The stripping columns is equipped with bubble cap trays or with more economical valve trays. Because of the lower liquor temperatures in the washer and hence the reduced rate of corrosion, this vessel can be constructed entirely in carbon steel. Fig 5 shows the schematic diagram of the water wash process.
Fig 5 Schematic diagram of water wash process
An advantage of the water wash process is that excess flushing liquor and other aqueous plant streams (such as benzol plant effluent) can be used to absorb ammonia in the washer. The advantage of doing this is that as the excess liquor is going to be steam stripped in any case, there is a net saving of stripping steam if the excess flushing liquor is also used to absorb ammonia. For other plant effluent streams, it frequently makes sense to perform steam stripping as a preliminary effluent treatment step. Combining this with the ammonia absorption process minimizes overall steam consumption for the by-product plant. The excess flushing liquor is added at a point in the washer where its free ammonia concentration most closely matches the free ammonia concentration of the absorption liquor.
The presence of fixed ammonia does not influence absorption of free ammonia from the coke oven gas. If excess flushing liquor is used in the water wash process, the flow rate of the blow down effluent stream is increased to maintain the circulating liquor inventory. The fixed ammonia can be removed in the stripping column by the addition of caustic soda. Alternatively, the blowdown stream can be fed to a separate fixed ammonia still. The use of a separate fixed ammonia still avoids the presence of alkali in the recirculating absorption liquor, which can be responsible for forming fixed compounds with acid gases such as hydrogen cyanide and leading to the presence of these compounds in the by-plant plant effluent stream.
Treatment alternatives for ammonia vapour streams
After the removal of the ammonia from the coke oven gas, a vapour stream containing ammonia, water, and acid gases (hydrogen cyanide, hydrogen sulphide, and carbon di-oxide) gets generated. The processes which have been used or proposed for the further treatment of these vapours are (i) incineration of the vapours, (ii) production of concentrated ammonia liquor, (iii) catalytic ammonia destruction, (iv) production of ammonium sulphate, (v) production of anhydrous ammonia, and (vi) membrane distillation. The latter two are modification of the ammonium sulphate process and the Phosam process. In each case, the ammonia vapour stream replaces the coke oven gas feed stream. Naturally the size of the equipment needed for absorption of the ammonia is reduced. In the case of production of ammonium sulphate, this plant arrangement is referred to as the ‘indirect’ process. The remaining three alternatives are described below.
Incineration – Incineration of ammonia vapours has been widely practiced from the 1960s. The process simply reacts the ammonia with air in specially designed burners. The process is exothermic and no support fuel is needed other than for a pilot flame to initiate the combustion. The incinerator is refractory lined for heat resistance. The products of combustion, mainly nitrogen and water vapour, are emitted to atmosphere through a high stack. Depending on the source of the vapours to be incinerated, they can contain considerable quantities of the acid gases (hydrogen sulphide and hydrogen cyanide). These components are also incinerated, with any sulphur compounds present producing sulphur di-oxide.
The single stage combustion of ammonia occurs at high flame temperatures in excess air. These circumstances promote the formation of NOx which leave with the stack gases. To combat this effect, ‘low NOx’ incinerator design uses a two-stage combustion with intermediate cooling (by water sprays) between the stages. The first stage is used to incinerate majority of the ammonia, with carefully controlled air flow rates so that excess air is avoided. In the second stage, excess air is allowed to complete the incineration but at relatively low flame temperatures. By these means the NOx content in the stack gases can be held to less than 100 ppm. The presence of oxides of both nitrogen and sulphur in the stack gases has led to a close review of the suitability of this process with regard to atmospheric pollution regulations. New installations of this process are frequently nowadays limited to standby or emergency applications as a short-term gap measure, for example, during a maintenance outage of a more environmentally friendly process.
Production of concentrated ammonia liquor – Concentrated ammonia liquor in different grades of purity can be prepared from the ammonia vapour stream. Normally, a preliminary condensation is performed to reduce the quantity of water vapour present. The condensate is returned to the ammonia distillation column. The remaining vapours are then condensed to form the product ammonia liquor. The vapour temperature after the preliminary condensation determines the remaining water content and hence determines the concentration of the ammonia liquor produced. With the presence of acid gases in the ammonia vapours, particularly carbon dioxide, the concentration of ammonia liquor produced by condensation of the vapours is limited to 15 % to 20 % ammonia. This is since ammonium carbonate is also formed and crystallizes on the condenser surfaces at higher concentrations.
The problems caused by acid gases can be eased to an extent by a prior removal of these compounds from the enriched wash liquor before it is fed to the ammonia still. This is done in a ‘de-acidification’ column, which is a steam stripping column. Relatively small flow rates of steam are used to strip the more volatile acid gases from the wash liquor, leaving the ammonia largely in solution. The acid gases can be returned to the coke oven gas main, and the de-acidified solution then passes to the ammonia distillation column. A common method of cooling and condensing ammonia vapours is by direct contact with recirculated cooled ammonia solution in a packed column. This method avoids localized sub-cooling of the condensate which can lead to crystallization and fouling.
Carbon steel can be used to handle concentrated ammonia liquor below 50 deg C. At higher temperatures more corrosion resistant materials such as 316 grade stainless steel, Hastelloy, and titanium can be necessary. The production of concentrated ammonia liquor is also used to provide a low cost standby in plants where the normal ammonia handling facilities are out of service for maintenance.
Catalytic ammonia destruction – The process was first developed and installed by the company ‘Firma Carl Still’ in 1968, this process has gained popularity because of its ability to dispose of the ammonia removed from coke oven gas without the raw material cost of the sulphate process and without the emission of pollutants. The process begins with the ammonia vapours coming from the ammonia distillation column. Normally, a partial condenser is used to reduce the content of water in the vapours. The condensate is returned to the column. The remaining vapours, including any acid gases, flow into a top mounted burner installed on a refractory lined ammonia destruction reactor. Coke oven gas support fuel and a stoichiometric amount of combustion air are also fed to the burner, at closely controlled flow rates. Within the reactor is a nickel catalyst bed. Fig 6 shows schematic diagram of the catalytic ammonia destruction.
Fig 6 Schematic diagram of catalytic ammonia destruction
The reaction kinetics favour the combustion of the coke oven gas with the air. This results in a reducing atmosphere consisting of the ammonia vapours and coke oven gas combustion products at a temperature of 1,150 deg C. Over the nickel catalyst, ammonia is cracked to nitrogen and hydrogen. Hydrogen cyanide reacts with the water vapour present to form nitrogen, hydrogen, and carbon monoxide. Hydrogen sulphide passes through the catalyst bed unreacted. The reactor gases then flow through a waste heat boiler where they are cooled to around 300 deg C, by raising high pressure steam.
The gases can then be passed directly to the raw coke oven gas main, closing the process loop, or they can first be further cooled by water quenching in a tail gas cooler vessel. With careful control of the coke oven gas and air flow rates, no oxides of nitrogen or sulphur are formed, and no excess air is returned to the coke oven gas main. The addition of the tail gas results in a small reduction in the calorific value of the coke oven gas. This is because of the volume of nitrogen which enters the process as combustion air. Materials of construction in the ammonia destruction plant are primarily carbon steel, with Hastelloy or titanium for the ammonia vapour feed lines. Refractory lining is used where necessary for heat protection.
Membrane distillation – Alternative new technologies such as membranes are easing even further the removal of ammonia from flushing liquors. Membrane distillation is being investigated worldwide as a highly efficient and affordable technology. Hydrophobic membranes (flat-sheet, hollow fibre, and spiral wound) are preferred for ammonia extraction due to their hydrophobic characteristics, excellent organic resistance, and chemical stability with acidic and alkaline solutions. The strong hydrophobicity of the membrane prevents liquid transportation through it, hence facilitating the separation of species with different vapour pressures. The partial pressure gradients across the membrane results in the transfer of the volatiles from the liquid phase to the vapour phase. As the temperature gradient is maintained across the membrane, the transport of water vapour occurs continuously. Meanwhile, other species remain on the other side of the membrane, hence separating water from the mixture. Conventional flat-sheet porous have been applied for membrane distillation with efficiencies varying between 70 % to 90 %.
Utilization of ammonia from steel plant
Ammonia from steel plant can be used in several ways. Similar to ammonia produced by any other method, the versatility of the chemical enables its use for fertilizing, heat production, and chemical process applications. However, differently from other processes, ammonia from steel plant can be used using gaseous waste streams only available on site to reduce cost or to generate extra heat / power for processes needed to produce steel. Here some of these specific processes are evaluated.
Ammonium sulphate can be used as a fertilizer. Although its use is relatively smaller when compared to urea, ammonium nitrate solutions, and anhydrous ammonia for this application, ammonium sulphate can still be used for this application if available. The manufacturing and delivery method vary depending on the customer’s specifications. As previously described, ammonia or ammonium sulphate scrubbed from the coke oven gas are still employed as fertilizer. Anhydrous ammonia can be used as a fertilizer by injecting it directly into the ground. Also, it is used as an industrial refrigerant. Under license control, it is also used in the production of methamphetamines.
Incineration of ammonia vapours product of the coke oven gas process has been a common practice since the 1960s. Ammonia, with a low heating value (LHV) of 18 mega joules per kilogram (MJ/kg), can be burned using a pilot flame without the need of other doping agents. Several organizations use these systems to reduce costs and lessen contaminants, several of which are intrinsically dedicated to the production of steel. However, the process carries out the production of emissions such as NOx and SOx, which are extremely detrimental to the environment.
Since ammonia contains nitrogen, its combustion at high temperatures generates NOx emissions higher than those established for environmental regulatory purposes, needing the implementation of novel combustion techniques to reduce temperatures while burning chemicals such as sulphur content species. Hence, the presence of these unwanted emissions in the stack has led to a close review of the process, with new installations now being used only for standby or emergency applications as short-term measures, i.e., for maintenance or unstable operation. Also, some organizations have looked at incineration as a possible solution to some of their heating requirements. Specialised steel production, which employs ammonia as part of their process, have attempted to utilize ammonia containing gases for additional heating applications.
Concentrated ammonia liquor can be produced from various ammonia vapour streams. A preliminary condensation is carried out to minimize water content. The condensate is returned to the distillation column, with the remaining vapours then condensed to produce ammonia liquor. Ammonia liquor produced by this method has a limited concentration of 15 % to 20 % ammonia, a result of the ammonium carbonate which is also formed on the condenser surface. This issue can be reduced by reducing acid gases such as carbon di-oxide by ‘de-acidification’ processes through steam stripping. A standard method to cool and condense ammonia vapours is by direct contact with recirculated cooled ammonia, hence avoiding localized sub-cooling which can lead to crystallization and fouling. The production of concentrated ammonia liquor is also used to provide a low-cost standby in plants where the regular ammonia handling facilities are out of service for maintenance.
Another option is to use the concentrated ammonia liquor or anhydrous ammonia, i.e., obtained from the former, for power purposes. A study has examined the potential of using ammonia in both its ammonia vapour and recovered anhydrous ammonia forms, to produce power using gas turbine technology. The priority of the study was to minimize NOx concentrations and other harmful emissions while optimizing operational performance. After analyses using different soft-wares with various combinations of ammonia and coke oven gas blends, the study concluded that ammonia from steel plant can be used to produce power at efficiencies up to 46 %. According to numerical analyses, adding 15 % coke oven gas to both the ammonia vapours, and anhydrous ammonia gives the optimal balance of reactivity and lower pollutant products. Hence, the potential of using ammonia product from coke processes can have a viable application to support energy storage in steelwork complexes.
Another option presented by the industry is to destroy ammonia by cracking the molecule. Instead of producing contaminants (NOx, SOx) or adding extra costs to the production of ammonium sulphate, the concept seeks to recover hydrogen from the ammonia molecule, hence enabling the use of hydrogen in other parts of the processor as a doping agent to increase combustion performance. The process uses coke oven gas which in conjunction with vapours, air, and ammonia are fed to a burner with high flowrate controls. The reaction favours the combustion of coke oven gas, hence creating an environment consisting of ammonia vapours and coke oven gas combustion products at around 1,150 deg C. A catalytic nickel-based bed is used, which cracks the ammonia into hydrogen and nitrogen. Hydrogen cyanide reacts with the water vapour producing nitrogen, hydrogen, and carbon di-oxide. The gases then pass through a waste heat boiler to be cooled to 300 deg C. The gases can pass directly to the raw coke oven gas main, hence closing the loop, or they can be further cooled by water for other purposes. This process enables a reduction of NOx or SOx (oxide of sulphur) emissions.