Prevention and abatement options
With regard to prevention and abatement of Acid deposits, there are some technical and economical , social options that generally relates to planning for abatement and prevention plannings are subject to a series of alternatives, the most important of which are economical power, technical an technologic development, management, laws and regulations , geographic and cultural conditions. Plannings should have the following specifications:
The short term options in different activities are as follows:
A. Electricity Generation
Improved appliance efficiency
Improved operation and maintenance
Improved efficiency of cook stoves ( in developing countries).
Improved energy storage systems:
Use of solar , wind, geothermal, tidal energy, … with regard to present prices of fossil fuels, are not economical from the point of economy especially subsidies belonging to them , no motivation for innovation in the field of new energies and efficiency increase does not bring about.
Use of radio active energy and related technology , particularly for the developing countries is not useful enough.
The problem of disposal of radioactive wastes is one of the difficulties which has involved developed countries. Therefore some of the primary actions about the reduction of acidic pollutants in developed countries, are managerial actions in producing energy and the change of consumption pattern.
Information science is one of the actions which can be done on acid deposits-awareness containing research and development, demonstrative programs about innovative technologies and the training and education of experts in all the sections.
These programs should be taken place in special groups of people with emphasize on present and likely future expences and advantages.
The transfer of expert information among countries requires providing special facilities with regard to local conditions . This affair also applies to technology transfer to developing countries.
The proceedings on economical management that can decrease the consumption of emitter fuels of acidic pollutants and other producing activities are taxing on consumption and production, the issue of tradable emission permits, price-fixing, establishing protective funds and bestowal to developing countries to enjoy modern technologies and the increase of efficiency and control of acidic pollutants emission.
Although these economical proceedings cause consumption decrease ,the increase of financial ability of producer and also using this ability to research and development, but for the sake of the decrease of purchase ability of people of low income, can cause problems .
Among other managerial proceedings are performance of regulations and criteria about the permitted limits of efficiency, electrical and thermal tools, choice of fuel, permitted limits of acidic pollutants, structural criteria that performance of these criteria has an effective role in fuel consumption decrease , other primary substances a and acidic pollutants emission.
The development of people’s participation, and reinforcement of non-governmental organizations can be useful in controlling the emission of pollutants, following up of environmental problems , environment protection from impacts of acidic deposits,
Improvement of behavioral pattern of the society and the consumption of substances and the expansion of services can have effective role in reducing acidic deposits .
The expansion of public transport and the encouragement of people to use such services, production of industrial long-lasting and multipurpose products, common use of some household equipments like washing machine can have an effective part in reducing of energy use and primary materials with the regard to consumption section, there managerial tools can be used:
In relation to information science that is applicable in short term
most negotiate with the consumer about the sources of and methods of energy
and materials reservation and saving in their consumption and environmental
impacts of irregular consumption.
With regard to motivation 4 projects can be used as follows:
For reduction of acid rains, these can be done:
1. Fuels desulfurization
1.1 Gas
During the last 25 years, the rapid growth of the natural gas industry has led to simultaneous development of purification process and, in particular, processes for the desulfurization of combustible gases. Consequently, there now exists a wide range of industrial techniques for the efficient (i.e. up to 99 per cent) elimination of any sulfur compounds that may be contained in natural, refinery or synthesis gases (i.e. H2S, mercaptans, COS and CS2).
The acidic gases contained in the gas to be desulfurized (i.e. hydrogen sulfide and carbon dioxid) are usually absorbed simultaneously, partially or totally from the sour gas. The hydrogen sulfide removed by gas- sweetening solutions can be either recovered as sulfur or incinerated.
The standard process- combination for desulfurization of sour gas is a washing unit ( for desorption) and a Claus kiln ( for sulfur recovery) often followed by a tail-gas treatment unit.
1.2 Oil
The different fractions contain different types of sulfur compound; and the naphtha contains primarily mercaptans and sulfides whereas the high molecular weight ring structures in the residues contain more complex molecules with sulfur. The removal of sulfur components becomes more and more difficult the heavier the residual fraction, resulting in carp increases in energy requirements and costs and significant decreases in the net product yield.
Distillate fuel desulfurization is a well- established process which is commonly used to reduce the sulfur content of lighter fractions. This process, capable of 90 per cent sulfure removal, consists of treating fraction with hydrogen in the presence of a catalyst. The energy penalty is about 3.5 per cent of fuel intake and the operating costs are estimated at around $US 5 per tonne of product.
In order to meet increasing demand for lighter products (i.e. naphtha and distillate) there is a trend towards increased conversion of the residues to lighter fractions. A number of different processes are currently available for virtually complete conversion of existing residues(i.e. atmospheric and vacuum residues). Examples of such processes are catalytic cracking, thermal cracking, vis-breaking and coking.
In conversion processes the sulfur in the feedstock is redistributed between the resulting gas, light products and residues. Sulfur removal from gases and light products is relatively easier than sulfur removal from the feedstock to the conversion processes. However, the sulfur content of the remaining residue increases. The total amount of sulfur removed from refined products varies with the type of process used but may reach 90-95 per cent for a refinery using a coking conversion process compared with 17 percent sulfur removal for a traditional hydro skimming refinery with only gas-oil desulfurization.
Residue desulfurization is a relatively new process and only limited commercial experience has been gained. Sulfur removal from atmospheric residue can be as high as 90 per cent, whereas removal efficiency from vacuum residue is unlikely to be higher than 80 per cent with current catalytic fixed-bed technology.
In this process, the residues are treated with hydrogen at high temperature and pressure in the presence of a catalyst. The process can achieve desulfurization up to 80 per cent. The main problems associated with this process are de-activation of the catalyst and fouling of the reactor.
Typical heavy oils (or bitumen) generally contain sulfur at a concentration above 4 per cent by weight and the amount of nitrogen compounds is much higher than in conventional crudes. Furthermore, the very high metal content, which reaches 1,200 ppm in Boscan crude, is one of the main objections to the processing of these heavy oils by conventional methods.
1.3 Coal
During the production of synfuels from coal, two desulfurization steps are involved. The first one is the liquefaction itself. One type of coal liquefaction process is, in principal , a liquid phase hydrogenation of a slurry of coal and coal-derived oil with the use of a suspended one-way catalyst, preferably red mud from alumina production.
The product oil is separated into heavy, middle , and light oil in a distillation unit. The heavy fraction and part of the middle distillate are recycled for the preparation of the coal / oil slurry.
The liquid phase hydrogenation of coal normally results in an oil of very low sulfur content. However, the raw oil leaving the hydrogenation unit needs further upgrading before stable , marketable products can be obtained. The necessity of upgrading the coal-oil fractions is due to their quality characteristics which are quite different compared with those of mineral oils. The first upgrading step for both the light oil and the middle distillate fraction will be hydro treatment with much greater intensity than is required for the corresponding fractions from mineral oil. By this procedure, the remaining sulfur is converted completely to hydrogen sulfide, leading to completely desulfurized synfuels (process integrated costs).
1.4 Oil shale
The production of syncrudes from oil shale can also be divided into two main procedures: (i) the retorting of the shale, and (ii) the upgrading of the primary oil. Unlike coal liquefaction, the ratio of organic material to inorganic is such that no hydrogen is used in the first step. As a result, the distribution of sulfur is quite different and the oil products still contain high amounts of sulfur.
After the retorting step, approximately 20 per cent of the total sulfur is found in the shale oil and the retort gas- in the latter as hydrogen sulfide. The sulfur content of the different shale oil fractions is very high and does not increase with the boiling point but is nearly constant over the whole boiling rang-which is typical for shale oils. In addition, it should be noted that this oil contains a high amount of unsaturated compounds as well as small amounts of arsenic.
Upgrading therefore consists of an intense hydro treatment step preceded by mild hydrogenation when the arsenic (which would otherwise poison the hydro treating catalysts) is removed. In order to comply with a maximum nitrogen concentration of 1,000 ppm in the syncrude , the required hydrogenation level obtains desulfurization greater than 99 per cent. For the middle distillate and heavy oil fractions , the level of the remaining sulfur is sufficiently low for all further uses. However, if the naphtha fraction is fed to a reformer, a refining step must be included in the process. All the costs involved are process- integrated.
2. Modification of combustion processes for controlling sulfur emission
2.1 Fluidized bed combustion
In a fluidized bed furnace with sulfur reduction, the fuel is burned in granular bed of mineral material-generally limestone or dolomite, together with residual ash from previously burned coal. Solid, liquid, or gaseous fuel can be fed continuously to the bed while sufficient air for combustion is forced through. The quantity of fuel feed determines the unit’s thermal output.
Fluidized bed combustors can be used efficiently with almost any fuel, including municipal wastes, paint residues, sewage sludge and oily wastes. A multi fuel system permits switching to solid , liquid, or gaseous fuels, and a fluidized bed operator could, therefore, quickly and efficiently take advantage of beneficial changes in fuel costs and fuel availability. The two major categories of fluidized bed systems are those in which the bed operates at atmospheric pressure and those in which the bed operates at pressures up to 10 atmospheres or higher.
2.1.1 Atmospheric fluidized bed
There are two types of atmospheric fluidized bed boilers, which are briefly described below.
A. Conventional atmospheric fluidized bed (AFB)
This type of fluidized bed has been put into operation and has proved tits worth for units up to 35 MW. One 125 MW plant is presently being completed: the main area of application will be small units of up to 200 MW thermal. In applications of this kind, the simple design overcomes disadvantages with regard to inefficient use of additives, part- load performance, etc.
B. Fast fluidized bed (FFB)
This type of combustor promises very flexible operation and excellent utilization of additives. The overall plant is , however, somewhat complicated and the range of application should therefore extend from 30 MW to large power plants. The first commercial plant is in operation (65MW) and others are under construction. From one manufacturer at least, the FFB is available for commercial use in sizes up to 250 MW (thermal). So far no experience has been gained of sulfur removal in full- scale practice.
2.1.2 Pressurized fluidized bed (PFB)
This process offers the advantage of using a combined cycle which results in a significantly higher efficiency. In addition, the utilization of additives is good and the part-load performance is acceptable. The degree of sulfur removal ranges from 75 to 99 per cent, depending on the operating conditions and the quality of the added limestone. It is possible to use any solid fuel or fuel oil installations.
At present, naturally occurring limestone and dolomite are used as SO2 sorbents, and are fairly satisfactory. However, further improvements in sorbent capacity would favorably affect both the technical and environmental feasibility of the fluidized bed process. Pre-treatment methods are being investigated in order to increase the sorption and the amount of solid waste generated.
Precalcination of the limestone of the limestone for pressurized fluidized bed units makes it as effective as dolomite on a Ca/S molar basis. At the same time, fundamental studies on sorption phenomena-chemical, physical, and structural characterization; attrition behavior; kinetic properties- are being conducted. The relative capacities and costs of several alternative sorbent are being evaluated.
2.2 Dry additive procedure
CaO, CaCO3, or Ca(OH)2 can be added to brown coal before processing in coal mills. In the combustion chamber of the boiler, SO2 reacts with the CaO to CaSO4. The gypsum thus formed, as well as fly-ash, is then precipitated and finally deposited with other combustion residues. Such processes have been developed or tested in Austria, the German Democratic Republic and the Federal Republic of Germany. Dry additive processes achiever 40 to 60 per cent desulfurisation or in specific cases, more than 70 per cent. The consumption of calcium is 2 tonnes/tonne of sulfur removed. The advantages of the dry additive procedure are low investment costs and easy operation. Good results and high availability have been obtained in brown-coal-fired power plants.
2.3 Pulverized coal combustion
Modifications of dimensions and firing conditions are being investigated and also made in practice in order to reduce NOx emissions. These measures involve reduction of excess air, low NOx burners, multiple stager combustion and fuel gas recirculatin. Some of these modifications mean lower temperatures and, therefore, in the combustion process the possibilities of injection of limestone for SO2 absorption. In general, SO2 removal in the range of 50 to 70 per cent is possible with continuous low NOx formation but the technology is in an early stage of development. It is also possible that modifications could be made to existing boilers. Preliminary cost estimates suggest a range of $15 to $34 per kW and total revenue requirements of 2.9 to 3.6 mills 1/ per kWh.
3. Desulfurization of combustion gases for controlling sulfur emission
