The following article will guide you about how to control emissions of nitrogen oxides in air.
1. Absorption by Liquids:
The oxides of nitrogen can be absorbed by water, hydroxide and carbonate solutions, sulphuric acid, organic solutions and molten alkali carbonates and hydroxides. Several scrubbing techniques were developed which initially used solutions of sodium and calcium hydroxide.
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Some of the mostly adopted methods are:
(i) Absorption by Alkaline Solutions:
NOx absorption by using aqueous alkaline solutions like NaOH and Mg(OH)2 also yields good results. The removal efficiency is high if one half of the ‘NO’ is oxidised to NO2 or if NO2 is added to the gaseous stream so that the optimum NO/NO2 molar ratio of 1:1 is maintained.
This is a prominent method adopted during desulfurisation of power plant emissions by such alkaline solutions. In the desulphurisation process about 10% of NO is oxidised to NO2 before the flue gas reaches the scrubber. The scrubber then removes about 20% of the total NOx in equal parts of NO and NO2.
(ii) Absorption by Lime:
Aqueous suspension of calcium hydroxide can be used as the scrubber to reduce NOx levels to 200 ppm. The calcium nitrite in the solution can further be converted to more valuable calcium nitrate by treating with sulphuric acid.
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2H2SO4 + 3 Ca (NO2)2 → 2CaSO4 + 4 NO + Ca (NO3)2 + 2H2O
The NO evolved may be recycled to the nitric acid plant and calcium nitrate can be used as a fertiliser. Thus the process reduces NOx, recovers NO and gives a valuable fertiliser simultaneously.
(iii) NOx Absorption by H2SO4:
H2SO4 reacts with NOx to form violet acid, H2SO4NO and nitrocyl sulphuric acid, NOHSO4
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NO + NO2 + 2H2SO4 → 2NOHSO4 + H2O
The NOHSO4 formed is very stable in concentrated acid. Any moisture in the flue gas is picked up by the acid and hence drives the equation to the left. To avoid this problem high temperatures (120°C) are maintained, so that vapour pressure of water in the solution is equal to the partial pressure of water in the flue gas. The use of sulphuric acid to remove SO, and NOx simultaneously as well as separately is currently under active investigation.
(iv) NOx Absorption by Magnesium Hydroxide:
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In this process as shown in Fig. 3.21 oxides of nitrogen are absorbed by hydroxide liquor in an absorption tower. The magnesium nitrite solution leaving the absorber is taken to a pressure reactor where the nitrite is converted to nitrate. The by-product NO is oxidised to NO2.
The liquid leaving the pressure reactor, consisting of Mg (NO3)2/Mg (OH)2 is sent to a settling chamber where nitrate is separated from the hydroxide. Part of the NO, from the oxidiser is sent to the absorber to maintain equimolar concentrations of NO and NO2 while the rest of NO2 is used for nitric acid production.
A continuous catalytic absorption process using stripped nitric acid as the absorbing medium has been reported. The advantage of this is that it not only reduces NOx in the tail gas to a tolerable level but also recovers NO as nitric acid.
Some of the scrubbing techniques, correctly used are:
(i) Two-stage absorption, first in water and then in sodium hydroxide yielding nitrite and nitrate salts.
(ii) Absorption in various types of ammoniacal solutions such as waste caustic ammonia liquor, ammonium bicatronate and ammonium bisulphate.
(iii) Absorption with an aqueous suspension of lime, where calcium nitrite and nitrate can be recovered for use as fertilisers.
2. Adsorption by Solids:
The adsorbents that show some capacity for oxidising NO to NO2 and for adsorbing nitrogen dioxide are activated carbon, silica gel, molecular sieves, ion-exchange resins, and certain metal oxides, particularly manganese and alkalised ferric oxides. The use of activated carbon (char) to adsorb oxides of nitrogen has been studied extensively. Activated carbon has a high adsorption rate and capacity compared to other materials. However, regeneration may be a problem.
A potential fire and explosion hazard may be another difficulty with this material since O2 is usually present in most stack gases. Thus the efficiency of char decreases with quantity of O2 present in flue gas. Manganese oxides and alkalised ferric oxides show technical potential. However, sorbent attrition is a major technical stumbling block.
The most suitable adsorbent for NOx is the one which can be regenerated and at the same time which does not preferentially react with water vapour or with CO2 in the flue gas. The most promising adsorbent is ferrous salt. Molecular sieves also can be used for NOx control, particularly for NOx from nitric acid plants. In this process two beds operate batch-wise one adsorbs NOx from the tail gas while the other bed is regenerated.
3. Catalytic Reduction:
Selective Catalytic Reduction (SCR):
Selective catalytic reduction refers to a process that chemically reduces NOx with NH3 over a heterogeneous catalyst in the presence of O2. The process is termed selective because the reducing agent NH3 preferentially attacks NOx rather than O2. However the O2 enhances the reaction, and is indeed a necessary part of the reaction scheme.
Thus SCR is potentially applicable to flue gas from fuel-lean firing combustion systems, that is, the flue gas is under oxidising conditions (e.g. greater than 1% O2). Non-selective catalytic reduction (NSR) processes are applicable to flue rich firing combustion conditions (i.e. reducing conditions in the flue gas).
The overall SCR reactions can be expressed as:
Equation (3) represents the predominate reaction since approximately 95% of the NOx in combustion flue gas is in the form of nitric oxide (NO). An NH3: NO molar ratio of about 1:1 has typically reduced NOx emissions by 80-90% with a residual NH3 concentration of less than 20 ppm.
The SCR processes require a reactor, a catalyst and an ammonia storage and injection system. Due to increased pressure drop across the SCR reactor, some increase in boiler fan capacity, or possibly an additional fan, may be necessary. The optimum temperature for the catalytic reaction is in the temperature range 570-720° K; to obtain flue gas temperatures in this range, the reactor is usually located between the boiler economiser and the air preheater.
A typical flow diagram is shown in Fig. 3.22 below:
Vanadium compounds have been found to promote the reduction of NOx and to be unaffected by the presence of SOx. Titanium dioxide (TiO2) has been found to be an acceptable carrier since it is resistant to attack from SO3. Thus, many SOx resistant catalysts are based on TiO2 and V2O5; however, constituents and concentrations of most catalysts are proprietary.
Reactor and catalyst configurations also vary with the application, primarily to accommodate the different particulate concentrations. Natural gas-fired boilers employ SCR catalysts as spherical pellets, cylinders, O-rings and reactor vessels as fixed packed beds. However designs for use with oil-and coal-fired boilers have to be capable of tolerating particulates (fly-ash) in the flue gas stream.
For these applications, a parallel-flow catalyst is preferred. Parallel flow means that the gas flows straight in channels parallel to the catalyst surface. The particulates in the gas remain entrained while NOx reaches the catalyst surface by turbulent convection and diffusion.
Even though much progress has been made in catalyst and reactor designs, some problems still remain. The catalysts may not be resistant to all contaminants in flue gas or be able to tolerate high particulate loadings. In addition, fine particulates, smaller than about 1 μm, may blind the catalyst surface.
Long-term operation without catalyst plugging or catalyst erosion needs to be demonstrated for coal-fired applications. Catalyst life also needs to be extended from the current guarantees of 1 to 2 year of applications with SOx and particulates in the gas stream.
The NH3: NOx molar ratio and the flue gas space velocity are the major operating variables affecting the level of NOx control achieved for a given boiler condition and SCR system. An NH3: NO molar ratio of 1:1 can achieve about 90% NOx reduction, with higher NH rates resulting in higher undesirable NH3 emissions.
One of the major concerns with SCR processes is the formation of solid ammonium sulphate {(NH4)2SO4} and liquid ammonium bisulphate (NH4HSO4). It is difficult to completely avoid them since some unreacted NH3 from an SCR system and some SO3 from combustion of sulphur-containing fuels are always present. The biggest problem seems to be the deposition of (NH4)2SO4, and NH4HSO4, on the air preheater.
These compounds are corrosive and can form deposits that plug the air preheater. Other concerns and potential problems include: emission of NH3 and NH3 compounds causing or increasing the emission of undesirable compounds such as SO3; lack of proven NH3 analytical control systems; sensitivity of the process to temperature changes due to boiler load swings and disposal or reclamation of spent catalyst in an environmentally acceptable manner.
Despite these potential problem areas and uncertainties, the processes have been successfully installed and operated in Japan on gas, oil and coal fired boilers. If 80% or more NOx reduction is required SCR is the only option.
Non-Selective Catalytic Reduction:
Reducing agents other than NH3 such as CO, H2 or CH4, completely reduce NO at temperatures of 300 – 400°C and space velocities in excess of 50,000 m3 per hour per m3 (space velocity is the volume rate of gas flow per volume catalyst). Non-noble metal catalysts such as supported copper oxides may also be used for NOx control.
These reductants act non-selectively, reacting with O2 and SOx present in the flue gas, in addition to NOx and thus huge amounts of reductants must be added. For non-selective catalytic reduction of NOx in stationary combustion sources stoichiometric or sub-stoichiometric air must be used in the primary combustion zone.
This reduces the cost of the reducing agent. NOx removal by CH4 is as follows: when hydrocarbons like methane are used for removing NOx higher oxides of nitrogen are converted to NO in the first stage. In the second stage NO is reduced to nitrogen. Oxygen in the tail gases reacts simultaneously with the hydrocarbons to produce water vapour and carbon dioxide as below –
In this process sulphur compounds must be removed first as they poison the catalyst. The tail gases are first heated to about 400°C and then mixed thoroughly with methane. The mixture is then sent to the reactor. The activity of the catalyst falls off with time. Its activity can be prolonged by maintaining higher inlet gas temperatures.
4. Electron Beam Irradiation:
This process simultaneously removes 90% of NOx and SOx with NH3 activated by electron beam irradiation, without the need for catalysts. The process first requires removal of fly-ash from the flue gas. Then ammonia is added and then the gas enters the electron beam reactors.
There, ammonia in the presence of electrons converts NOx and SOx into a dry powder of ammonium sulphate {(NH4)2 SO4} and ammonium nitrate sulphate {(NH4)2 SO4 2 NH4NO3)}. After the dry powder is removed by a second particulate collection device, the treated flue gas leaves the system hot enough to be exhausted through the stack without reheat. The waste generated also is a potential fertiliser feedstock.