The main gaseous levels that have to be controlled are SO2, SO3 + mist, NOx, carbon monoxide, fluorine, hydrocarbons and particulate matter. Any fertilizer production unit which has ammonia, urea and complex plants, coupled with the manufacture of intermediate/raw materials required for complex fertilizers invariably results in the emission of SO2, NOx and dust.
The emissions of SO2, NO2 and particulate matter, from the fertilizer plant at Bombay, before pollution control measures were adopted, was as follows:
Since then several antipollution measures have been implemented and the gaseous emission has been considerably reduced. The SO2 has been cut down to 7.5 tpd. Total dust emission even after the installation of 1200 tpd complex plant is only 2.5 tpd, which is lower than the emission from the old plant. As regards the NOx emission, this will be of the order of 3.7 tpd, even after the installation of a 750 tpd nitric acid plant.
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The various sources and method of control of gaseous effluents are as follows: Sulphur Oxides
Source 1: Sulphuric acid plants based on sulphur or sulphur based raw materials such as pyrites; ore roasters for recovery of metals as Zu, Cu, Lead, etc. The unabsorbed gases contain SO2 and SO3 + mist.
Source 2: Boiler houses where steam is generated by the use of fuel oil. The sulphur content in oil which may be 3 percent is converted into S02 and vented to the atmosphere through boiler stacks. Coals fired boilers also emit S02 depending on the sulphur content of the coal.
Source 3: The various furnaces or kilns based oil used for drying raw materials such as rock phosphate and granulated fertilizers, cement etc., emit S02.
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Source 4: H2S from ammonia plants.
The SO2 emissions from a large single-train, 900 Te/day ammonia plant, based on the latest technology and using fuel oil as feedstock, like the plant at Nangal, Sindri modernization, Bhatinda, Panipat, etc., are as follows:
The emission from a steam generation unit using normal fuel oil would be 33 Te S02 per day. However, if fuel oil with a low sulphur content of 1 percent, as available in LSHS, is used, the emissions could be brought down to one third at 11 Te S02 per day.
Control of H2S :
Feed stocks generating gas for the synthesis of ammonia are based on naphtha, fuel oil or coke. Since almost all the feedstocks contain sulphur constituents, the synthesis gas, H2S, is produced as an effluent gas. During its purification, the burning of H2S with air gives rise to S02 which causes environmental pollution problem in the neighboring areas.
The quantity of H2S depends on the type of feedstock and its origin, so that with varying amounts of sulphur in the feedstock, varying amount of H2S is produced in various locations.
Image Source : chemvironcarbon.com
Measures:
H2S could be converted into S02 by controlled combustion with air and finally catalytically converted into sulphuric acid. This method is preferable in locations where a sulphuric acid plant is already installed. Alternatively, H2S could be converted back to sulphur by the well proved ‘Claus Process’. The sulphur can, then, be sold as such, thus, increasing the economy of the industry.
Control of S02 and S03 + Mist
Emissions from sulphuric acid plants could be controlled by the use of a double contact- double absorption technology for manufacture of sulphuric acid as against the conventional technology which is currently common in India. The latter is now described.
Conventional Process:
In this process, the reaction of SO2 being oxidized to SO3 is carried out in a converter using a catalyst containing V2O5. The SO2 yield is governed by thermodynamic equilibrium and kinetics.
Due to adiabatic conditions and the exothermic reaction heat (42342 Btu/lb mol at 77°F), the temperature of the reaction gas rises until its composition approaches the equilibrium conversion and heat must be removed if further reaction is required.
Thus, at lower feed temperatures, conversions may be obtained but will be accompanied by lower reaction rates.
At lower reaction rates, the catalyst requirement increases, thus, increasing pressure drop through the catalyst bed, resulting in higher operating costs. Hence, higher conversions and lower catalyst requirements, the optimum balance between which has to be ensured, are both dependent upon temperatures.
The usual commercial practice is to use a feed temperature of438-446°C, in a four stage conversion, with 170-180 litres of catalyst per day, per tonne of sulphuric acid product. Below 438° C, the reaction rate slows down and is negligible around 3 99°C. Typical plant data are given below: 1. 8 percent S02 from sulphur burning 2 13 percent 02 and 79 percent N2.
The overall sulphur conversion efficiency is 97 percent. Based on this, the consumption of sulphur per tonne of H2SO4 produced works out to be 0.336 tonne (theoretical 0.326) and, hence, the total loss in the stack would be 20 kg of SO2 per tonne of sulphuric acid produced.
The emissions from the conventional plant stack, depending upon their performance, dependent on various factors, vary from 0.15 percent to 0.25 percent as S02 (v/v). The S03 + acid mist varies from 300-500 ppm as 100 percent H2SO4.
Double-Contact Double-Absorption Process (DCPA) it is similar to a conventional plant up to the conversion step. The S02 formed after the 2nd or 3rd stages is removed in a primary absorber and the remaining gas, now with a very high O2/SO2 ratio, is returned to a second converter.
A heat exchanger or a series of heat exchangers reheats the gas from the absorber and cools the converted gas going into it.
The second converter provides additional conversion and acid recovery in a second absorber. By this process, the effluent SO2 content can be brought down to 500 ppm (v/v) and acid mist to 100 ppm (v/v). The sulphur efficiency is 99 percent.
Since the SO2 content in the gas to the 1st converter could be raised to 13 percent (conventional 8 percent), the plant capacity of the older plants on conversion could be raised by 40-50 percent. In view of very high sulphur efficiencies, this must be considered the best solution to the problem of emission control from a sulphuric acid plant.
The fertilizer factory at Trombay has converted their 200 tonne per day sulphuric acid plant to the DCDA technology at a cost of Rs. 14.0 million. The plant was commissioned in April 1977 and its capacity was raised to 300 tonne per day while emissions of SO2 are 400- 500 ppm and SO3 + acid mist to less than 30 mg/m3 (100 ppm).
Control of Nitrogen Oxides:
Source 1: From nitric acid plant effluent gas
Source 2: From the oil pipe effluent of automobiles and boiler houses fired on coal.
Nitric Acid Plant:
In the nitric acid plant, the main effluent is from the absorption towers of the acid plant where the tail gas contains unabsorbed NO + N02. Normally, the NO + N02 content of these gases used to be fairly high (from 2500 to 3000 ppm) depending on the process employed. In a nitric acid plant, with a capacity of 300 tonnes per day, N, vented per day, could be as high as the equivalent of 5-6 tonnes of nitric acid per day.
Measure:
By selection of the latest technology, it has been possible to reduce the NOx content in the stack gases to below 200 ppm by the following methods:
1. Increasing the absorption volume in the absorber. In case of medium pressure plants, this could bring down NOx level to 5000 ppm. In the case of dual pressure processes, some designers claim to reduce it to 200 ppm level.
2. By use of a catalytic converter, to reduce the NOx to 200 ppm level, by catalytically treating NOx with a hydro carbon gas.
3. By use of caustic/soda as scrubbing.
4. By use of a chilled nitric acid wash.
Due to these innovations, the acid which was formerly going to waste as a pollutant could be converted into a useful product. The recovery would depend on the NOx level that could be tolerated, in particular locations, and the regulations applicable in that area.
The emission could be reduced to the following levels for every 100-tonne-per-day capacity nitric acid plant with improved technology.
This could bring down the consumption of ammonia, per tonne of nitric acid, so that nitric acid can be made more economically.