Education Charter International


CCLP Worldwide renews its Commitment towards RIO+20 Vision
05/04/2012, 9:20 am
Filed under: World
Official Campaign poster of CCLP Worldwide to RIO+20 Vision

Official Campaign poster of CCLP Worldwide to RIO+20 Vision

CCLP Worldwide publish its official statement to renew its commitment to RIO+20 Vision.
“At the time when world is witnessing global economic expansion at the cost of fragile environment ,ecological footprints of humanity pose great threat to the bio capacity of this planet and where three of nine boundaries that define the safe operating space for human life has been breached, there remain urgent need to bring balance between Environment, sustainability, development and protection of planet.
The Vision and objective of the RIO +20 Conference is to secure renewed political commitment for sustainable development and address new and emerging challenges with special focus on a green economy in the context of sustainable development and poverty eradication; and the institutional framework for sustainable development.
CCLP Worldwide completely endorse the vision of RIO +20 and reiterates that Higher Education and its progress is one of the key instrument which unlocks opportunity to deal with the emerging challenges in the area of Sustainable Development and Environment and fill the gap created by over expansion of economic activity and human footprints.”

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Queridos compañeros y colegas de CCLP Worldwide, (consejo Académico).
Mi más sinceras felicitaciones por esta iniciativa tan importante y vital para nuestro planeta y los ciudadanos que lo habitan.
Ante esta coyuntura el Comisario para Europa, y su oficina en Madrid España, y su equipo de expertos.
Alza el procedimiento para el proyecto de la creación de la Dirección de Ciencias Ambientales, en la Oficina del Comisionado Prof. Dr. Jenaro Rosemary Shepherd.
En función de su potestad Jurídico-Profesional legal para Europa según su titulación de postgrado universitario.
Otorgado por Suffolk University Campus de Madrid-España fechado y sellado en Madrid , a 16 de Marzo de 2002, como Auditor de Gestión de Prevención en Sistemas de Gestión, de Seguridad, Calidad y Medio Ambiente, y acreditado y registradas las competencias del ejercicio profesional en toda Europa.
El proyecto y la información será remitido a la secretaria General de CCLP Worldwide, en la India , a la mayor brevedad..
Cordialmente.
Prof. Dr. Jenaro Rosemary Shepherd
Comisario Internacional-Europa (wwww.cclpworldwide.com/esi)
Decano-Rector del Consejo Académico.
ID. Nº 10001045.

Comment by Jenaro Rosemary Shepherd

Pollution

1. processes

In a wet process, we may have physical absorption of SO2 or absorption accompanied by a chemical reaction. The absorbent should have a high capacity to absorb SO2 so that the size of the absorption equipment is not very large. At the same time, and wherever possible, we should be able to regenerate the absorbent so that it can be used back again in the process. Thus, we have some processes, where we can regenerate the absorbent and also those where the absorbent is used only once in the process and is then thrown away. SO2 is only very slightly soluble in water and as such the use of water as an absorbent is very limited. In actual practice, a chemical reaction is incorporated in the process since the rate of mass transfer accompanied by a chemical reaction is always higher than the physical absorption alone.

Sulphur dioxide, being an acidic gas, is readily soluble in alkaline solutions. Some of the commonly employed scrubbing solutions are limestone slurry/lime solution, NaOH solution, MgO slurries/Mg(OH)2 solution, Na2CO3 solution, NH3 liquor solution, sodium sulphite – sodium bisulphite solutions, ammonium sulphite solutions, dimethylaniline solution, xylidine – water solution, citrate solution and other scrubbing solutions.

Let us now describe the use of some of these scrubbing solutions for the removal of sulphur dioxide from effluent gases.

Lime/limestone scrubbing

Of all the chemicals listed above, lime/limestone is the cheapest and readily available from nature. A typical flow sheet for lime scrubbing process for the removal of sulphur dioxide from effluent gases is shown.

In a process called the Calsox process, the scrubbing of SO2 containing gases is carried out by lime/limestone solution slurry in contacting devices such as gravity, venturi, floating bed or turbulent bed scrubbers.

The overall reactions for lime and limestone are:

CaO + SO2 + 2 H2O  CaSO3.2 H2O
CaCO3 + SO2 + 2H2O  CaSO3.2H2O + CO2

The reaction of lime and SO2 is more favourable than limestone and SO2. The calcium sulphite formed gets easily oxidised to calcium sulphate. The latter can settle in the scrubber and conveying lines. This difficulty can be overcome by the incorporation of a delay tank where the deposition of calcium sulphate is increased by the addition of CaCO3 followed by its separation.

Dual-alkali process

The second commonly used method for desulphurisation of flue gases (FGD) is the dual-alkali process in which two alkalies, namely sodium hydroxide (NaOH) and calcium oxide, are used. The advantage of using NaOH is that no solids are formed during neutralisation of SO2. Reacting the scrubbed liquid with lime/limestone can carry out the regeneration of NaOH. This concept is quite useful as the chemical that is consumed is lime/limestone, which is commonly available indigenously in India. Coagulants will be required for the removal of calcium sulphate particles.

Ammonia scrubbing

In this process, aqueous ammonia solution is used to scrub the gases containing SO2. The concentration of SO2 can be reduced to below 100 ppm by this method. This method is ideally suitable in fertilizer industries where ammonia or its aqueous solution is readily available. The product of neutralisation, ammonium sulphate is itself a fertilizer and can be recovered after concentrating the solution followed by crystallisation.

Scrubbing by MgO slurry

More than 95% of SO2 in the stack gases can be removed by scrubbing the gases by MgO – water slurry. The scrubbing liquor is centrifuged and the hydrated salt obtained and dried. The anhydrous sulphite is calcined to obtain back MgO for use and SO2, which can be converted to sulphuric acid.

Absorption by sodium carbonate/bicarbonate solutions

Sulphur dioxide can be absorbed in a highly alkaline solution of soda ash. The product of the reaction, viz., sodium sulphite, can be used in paper and pulp industry. An oxidation-inhibiting agent will be required to avoid/reduce the formation of sulphate from sulphite.

Citrate process

The absorption of sulphur dioxide is highly dependent on the pH of the scrubbing solution, which keeps on decreasing. This lowers the rate of absorption. In the citrate process, citric acid or its sodium salt provides the buffering action.

Other scrubbing solutions

Very high efficiencies have been claimed using some priority scrubbing solutions. In this connection, we can mention potassium sulphite – bisulphate solution. Some processes make use of organic solutions for scrubbing. Dimethyl amine, xylidine and dimethyl amine have been used in applications where SO2 concentration in the effluent gas is more than 2%.

Dry processes

Broadly, there are two kinds of dry processes that are employed for the removal of sulphur dioxide from the effluent gases.

(i) Oxidation/reduction: The catalytic oxidation (CAT-OX) or the Monsanto method, a fixed or fluidised bed reactor packed with vanadium pentaoxide catalyst and kept at 450oC is employed. In a slightly modified method, SO2 and oxygen present in the exit gases are adsorbed on the surface of an activated carbon catalyst that catalyses the oxidation of sulphur dioxide to sulphur trioxide. The latter reacts with moisture to form sulphuric acid. In the Scot process, where almost complete removal of sulphur and its compounds is possible, the gases are reduced when the sulphur or its compounds are converted to H2S, which can be, in turn, absorbed by an amine solution. Sulphur can be regenerated from the spent liquor by the low temperature Claus process.

(ii) Use of metal oxides: We can use calcium and magnesium oxides to adsorb sulphur dioxide from the effluent gases, where part of the oxides react with sulphur dioxide pollutant, forming the corresponding sulphate. In another process, sodium aluminate is used to remove SO2 in a fluidised bed reactor.

From a description of different methods used for the removal of SO2 pollutant, it seems that the economics and the applicability of the process chosen depend on the availability of the raw materials and markets for products/by products formed. These products/by products include sulphur, ammonium sulphate, calcium sulphate. The capital cost of equipment is the least for lime scrubbing and most for MgO scrubbing, where the final product formed is elemental sulphur. The operating cost is least for ammonia scrubbing as the ammonium sulphate formed in this case has good marketability. For smaller plants, lime scrubbing is recommended.

CONTROL FOR OXIDES OF NITROGEN

Broadly, we have three major sources of oxides of nitrogen, and these are:

(i) Stationary sources such as power plants that use coal or other fossil fuels.
(ii) Mobile sources such as automobiles and aircraft.
(iii) Chemical industries where nitric acid and other nitrogen bearing compound are utilized or produced.

There are seven different forms of oxides of nitrogen, viz., NO, NO2, NO3, N2O, N2O3, N2O4, and N2O5 that can theoretically exist in the ambient atmosphere but only NO and NO2 are present in appreciable amounts. Both are grouped as NOx. During biological nitrification in nature, oxides of nitrogen are also produced but the quantity produced is several magnitudes less than that from anthropogenic sources. Pollution due to chemical sector is fairly localised but pollution from the automobiles and combustion of fossil fuels is fairly widespread. While coal after combustion may give between 4.5 to 9.5 kg/tonne of NOx, diesel about 20 kg/tonne and petrol about 10 kg/tonne. Table 10.1 below gives percentage contribution to total NOx in Mumbai (India) from different sources:

Table 10.1
NOx Emissions in Mumbai (India)

Source Mumbai, India U.S.A.
Steam/Electric Boilers
Chemical Industry
Mobile Sources
Uncontrolled Sources 30.6
36.4
28.3
4.7 50.0
0.3
45.4
4.3

From Table 10.1, we can see that the NOx pollution from chemical industries is very large and amounts to more than one-third of the total anthropogenic sources. The Indian standards specify less than 3.0 kg NO2 per tonne of nitric acid produced in the new plants.

The control measures for oxides of nitrogen include combustion modifications and remedial measures, which we will discuss, next.

Combustion modifications

Combination of oil, coal or gas in industrial furnaces, kilns, driers and boilers leads to the formation of oxides of nitrogen similar to the combustion in automobiles. The basic reaction is the oxidation of nitrogen gas in the presence of oxygen, at high temperatures. Usually, we use excess air for combustion. For 10% excess air used for combustion and at 1000 K, about 1.5 ppm of NOx will appear in the exit gases. On the other hand, at 1500 K, the NOx level will jump to 1600 ppm. With an increase in the concentration of oxygen and the time of combustion, the NOx levels increase further.

We can carry out the following combustion modifications in order to reduce the concentration of oxides of nitrogen in the combustion gases:

(i) Recirculation of flue gases: In this, a part of the flue gas is re-circulated and employed for combustion. The combustion takes place in an atmosphere that is deficient in oxygen. As a result, the peak combustion temperature gets reduced. We can reduce as much as 90% NOx in the exhaust gases by this method.

(ii) Low excess air: We use less excess air so that nitrogen to oxygen ratio is increased. This reduces the formation of NOx. We, however, require a very controlled combustion; otherwise the fuel will be left unburnt and CO will be formed in greater proportion. Burners that use gas or oil for combustion require less excess air compared to those, which make use of solid fuels.

(iii) Two-stage supply of air: Here, we supply only 90% of the stoichiometric of air to the burner. The remaining air required for combustion is supplied at a location that is above the burner flame. This reduces the formation of NOx, because the flame temperature decreases. The concentration level of oxygen also becomes less. Reduction in the NOx concentration by as much as 40% has been reported in oil-fired and gas-fired furnaces by incorporating this change.

(iv) Tangential firing: For tangential firing, the burners are located tangentially around the combustion chamber radiating heat to a concentric cooling area. The peak temperatures, as a result of this modification, are reduced thereby reducing NOx emissions.

(v) Using clean-up equipment: We employ the remedial measures, which make use of clean-up equipment such as absorption, adsorption and catalytic reduction.

Absorption in suitable solutions

Water is the cheapest liquid that can be used for absorbing out NOx but the solubility of NOx in water is very low. So, very tall absorption columns have to be used for this purpose. They form a part of the complete dual-pressure nitric acid plants designed and operated these days. The absorption columns can also be incorporated in the high-pressure nitric acid plants currently in use. The additional absorption units produce nitric acid of low concentration (usually between 5 and 8%), which can, in turn, be used as the absorbing medium in the original absorption column. Absorption of NOx is favoured by high pressure and low temperature. Therefore, a part of ammonia (available in a fertilizer complex) can be throttled to produce the necessary cooling, recompressed and sent to the inlet of the nitric acid plant.

For automotive exhausts, a molten-salt absorbent consisting mainly of sodium potassium, and lithium carbonates is made to enter a venturi-scrubber-type unit where it is brought into an intimate contact with the exhaust gases at high temperature. A mini-demister, after an open-ended diffuser, recovers the molten salt that trickles back into the salt trough. For absorption of gases containing NOx from nitric acid plants, other industries and boilers and furnaces we can use solutions of magnesium hydroxide, magnesium carbonate slurry, calcium hydroxide, ammonia, urea, ferrous sulphate among many more.

In what follows in this Subsection, we will discuss some of these.

(i) Magnesium hydroxide Magnesium carbonate slurry: The gases containing NOx are brought in contact with the liquid droplets of the solution/slurry. Magnesium hydroxide can be recovered from the used-up solution by heating it to 200oC with steam when magnesium nitrate decomposes into magnesium nitrate and magnesium hydroxide. Nitrogen oxide produced during the reaction can be recycled back into the nitric acid plant. This increases the throughput of the nitric acid plant, reducing the NOx pollution at the same time.

(ii) Sodium hydroxide and urea solution: NOx can be absorbed in solutions containing NaOH or urea. The absorption products containing NaNO2 or ammonium nitrate should be suitably utiliszed, or there will be loss of chemicals. A combined process using extended absorption in water and urea solution, in two stages, is called the Masar process. Here, the total gases from the nitric acid plant are scrubbed with water. The gases after scrubbing with water are led to a top section where scrubbing is carried out using urea solution. The used solution can be processed into ammonium nitrate fertilizer and can be marketed along with other fertilizers.

(iii) Ammonium sulphite – bisulphite solutions: The absorption of gases containing NOx is carried out in packed columns in a counter-current manner with scrubbing liquid that flows downwards. The concentration of NOx in the exit gases is reduced to 10 ppm from an initial concentration of more than 950 ppm. The ratio of bisulphate to sulphite is kept between 0.1 and 0.41. The used solution is processed for ammonium sulphate that can be used as a fertilizer.

(iv) Calcium hydroxide solution: The gases containing NOx are absorbed in calcium hydroxide solution, which is generally obtained from slaking of lime. The cost of the slaking unit adds to the cost of this process. The used solution can be converted to calcium sulphate by adding sulphuric acid in lead-lined tanks.

(v) Ferrous sulphate solution: Dilute ferrous sulphate solution can be used for removal of NO due to its high absorption rate, easy regeneration and low cost.

(vi) Absorption in the other media: Sulphur dioxide and NO2 can be simultaneously absorbed in alkalised alumina, Na2O. Al2O3. Sodium sulphate and sodium nitrate are the products of the reaction. Hydrogen peroxide is also used as an absorbing medium in some applications.

Having discussed the absorption of NOx in suitable solutions, we will take up for discussion the adsorption of NOx in Subsection 10.2.3.

Adsorption of NOx

In order to adsorb NOx, the NO in the mixture of nitrogen oxides is first oxidised to NO2, which can then be adsorbed by suitable adsorbents such as activated carbon, silica gel, etc. Heating regenerates the adsorbent; the NO2 evolved is reused for the manufacture of nitric acid. In addition, we need to consider a few other factors as well.

For example, dry silica gel has practically no adsorption capacity for NO. It is reported to catalyse the oxidation of NO to NO2. And, moist silica gel has no effect on oxidation. Therefore, silica gel should be used selectively when the gases contain appreciable quantities of moisture. You may be aware that silica gel has a high capacity to absorb moisture.

Natural zeolites can be used to remove NOx upto 200 ppm. However, their capacity to adsorb NOx is limited to 2.25-kg/100 kg of zeolite. The zeolite bed can be regenerated by hot air or steam whereby NO¬x or HNO3 would be respectively desorbed. They can, in turn, be used in the process.

Activated carbon adsorbs NOx quite efficiently when the carbon bed is fresh. Repeated regeneration of the activated carbon reduces its adsorption capacity.

By far, the best adsorbents are available in different trade names. Besides NOx, these adsorbents can also adsorb other pollutants such as sulphur dioxide and mercury vapour. The basic requirement for the use of these adsorbents is that NO should first be oxidised to NO2. Unlike activated carbon, these adsorbents are not affected by the presence of oxygen in the gases. The operating cost is lower than that for catalytic reduction processes.
The use of adsorbents is, generally, practical only for exhaust gases from nitric acid plants. In the case of exhausts from other stationary sources, the disposal of regenerated NOx becomes a problem.

Catalytic reduction of NOx

During the manufacture of nitric acid, the absorption system releases as much as 0.33% of NOx in the exit gases. Oxygen is also present to the tune of 3%. For some inefficient absorption columns, the percentage of NOx can be even more. It has been reported that NOx can be reduced to nitrogen when the tail gases from the nitric aid plant are mixed with ammonia, hydrogen or other hydrocarbons and reduced catalytically. For catalytic reductions, we can use platinum, rhodium, palladium and copper oxide supported on alumina, silica, kieselguhr or diatomaceous earth in the shape of honeycomb structure. The concentration of the active component in the catalytic pallet is between 0.15 and 1% by weight.

The catalytic reduction of NOx possesses the following distinct advantages over other methods:

• The reduction of NOx to acceptable values is always possible and we can use more than one stage in series.
• The products of reduction present no disposal problems.
• The exothermic heat of reaction is sufficient to drive a turbine or an air compressor for pumping gases through the process equipment and the stacks.

If CO and NOx are both present in the gases (e.g., in automotive exhausts), both will be removed forming CO2 and N2, respectively.

We will next discuss some selected aspects of catalytic reduction of NOx:

• Catalytic reduction with methane: The methane plays two roles. The higher oxides of nitrogen are reduced to NO and the latter in turn to nitrogen. Oxygen in the tail gases reacts with the methane to produce carbon dioxide and water. A typical flow sheet of catalytic reduction using methane is shown in Figure 10.2 below:

Figure 10.2
Catalytic Reduction of NOx Using Methane

Sufficient heat of combustion is evolved to keep the temperature at around 400oC. The sulphur compounds should be removed first as they poison the catalyst.

• Catalytic reduction using ammonia: Ammonia reacts selectively with NO and NO2 in the presence of platinum catalyst, and nitrogen and water are the products of reaction. Twice the stoichiometric proportion of ammonia is added to the tail gases containing NOx under a 10 atmosphere pressure and temperature ranging from 300 to 400oC. As NO2 reduces the activity of platinum catalyst, it is recommended that NO2 be reduced to NO, using ruthenium as a catalyst, in the presence of hydrogen stream.

• Catalytic reduction of automotive exhaust: In India, the emission standards are becoming more and more stringent in the recent time. Most of the effort is confined to reduction of NOx levels in the automotive exhausts based on catalytic removal. The catalysts found useful are platinum coated aluminate, rare-earth oxides, manganese oxide, cobalt oxide, etc. and their combinations, supported on inert bases such as alumina, silica and aluminate. We have a wide variation in the quantity of catalyst in the pallet, ranging from 0.5 to 10%. The reducing gases and hydrocarbons provide the necessary reducing conditions. The compounds of sulphur poison the catalyst. Other pollutants, if present in the exhaust gases, will require the use of a dual removal system.

Most processes add a reducing agent to the exhaust gases to take oxygen from NOx. In modern auto engines, the NOx and CO combine to give nitrogen and carbon dioxide as the reaction products, in the presence of the platinum-rhodium catalyst. This is a very elegant way because two pollutants are turned into non-pollutants. A careful control on air-fuel ratio is necessary to produce NOx, CO and hydrocarbons in the proper ratio.

Comment by Jenaro Rosemary Shepherd




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