脱硫与脱硝论文

同济大学热能与动力工程专业 燃烧与环保期末论述报告 姓名 学号 院系 机械与能源工程学院 专业 热能与动力工程 General discussion about De sulfurization and De nitration Abstract De sulfurization and De nitration project have been a key for preventing and recovering atmospheric pollution which has been one of current global deterioration In this paper we will discuss more about fundamental principles of the two processes finally describe a new relevant technological project with its brief operating system Key Words De sulfurization Hydrodesulfurization De nitration Henry s Law Claus process Integration technology for flue gas dry technique de sulfurization de nitration and de dusting from glass furnace 1 General description of De sulfurization 1 1 Removal of sulfur from solid Sulfur may be present in solid fuels in three forms as inclusions of pyrite FeS2 sulfur in molecules of the organic mass of the fuel and sulfate sulfur The pyrite and organic sulfur can be removed from coal by hydrothermal desulfurization which consists in processing of ground coal in autoclaves at a pressure of 1 75MPa and temperature around 300 by alkaline solutions containing sodium and potassium hydrates The process gives coal with a rather low sulfur content which is centrifuged from the liquid and dried The liquid which contains sodium and potassium sulfides is regenerated by treating it with carbonic acid and the hydrogen sulfide thus obtained is used to produce elemental sulfur 1 2 Reducing sulfur oxides emission Upon combustion of a fuel almost all the sulfur contained in it passes to flue gases in the form of SO2 and SO3 oxides The bulk of sulfur oxides is present in the form of poorly reacting gas SO2 and only 1 in the form of SO3 Flue gas desulfurization is commonly known as FGD and is the technology used for removing sulfur dioxide from the exhaust flue gases in power plants FGD systems can be categorized as dry or wet SO2 is an acid gas and the typical sorbent slurries or other materials used to remove the SO2 from the flue gases are alkaline In wet FGD scrubbing systems the scrubbing liquid contains an alkali reagent to enhance the absorption of SO2 and other acid gases More than a dozen different reagents have been used with lime and limestone being the most popular The reaction taking place in wet scrubbing using a CaCO3 slurry produces CaSO3 the reaction expressed CaCO3 SO2 CaSO3 CO2 When wet scrubbing with a Ca OH 2 slurry the reaction also produces the CaSO3 the reaction expressed Ca OH 2 SO2 CaSO3 H2O Some FGD systems go a step further and oxidize the CaSO3 to produce Marketable CaSO4 2H2O CaSO3 1 2O2 2H2O CaSO4 2H2O 1 3 Cleaning of combustion gases from hydrogen sulfide Natural Sources Hydrogen sulfide is one of the principal compounds involved in the natural sulfur cycle in the environment As indicated in Fig 1 it occurs in volcanic gases and is produced by bacterial action during the decay of both plant and animal protein Many bacteria fungi and actinomycetes release hydrogen sulfide into the environment during the decay of compounds containing sulfur bearing amino acids and in the direct reduction of sulfate The heterotroph Proteus vulgaris is an example of a common bacterium that produces hydrogen sulfide when grown in the presence of protein Sources Associated with Human Activity There are various circumstances under which naturally occurring hydrogen sulfide is released by human activity For example hydrogen sulfide occurring in association with natural gas and crude oil deposits in some areas may be released during extraction and drilling operations The sulfur content of crude oils ranges from 0 to 5 and some natural gas deposits have been reported to comprise up to 42 hydrogen sulfide Coals can contain sulfur levels of up to 80 g kg and occasionally conditions arise in which hydrogen sulfide is formed within such deposits Thus special precautions must be taken in some mining operations as well as in the drilling and extraction of natural gas and crude oils with significant sulfur content In our industry like refineries for instance one of the desulfurization process is Hydrodesulfurization during which a big amount of hydrogen sulfide produced below is the brief introduction of it Hydrodesulfurization Hydrodesulfurization HDS is a catalytic chemical process widely used to remove sulfur S from natural gas and from refined petroleum products such as gasoline or petrol jet fuel kerosene diesel fuel and fuel oils the purpose of removing the sulfur is to reduce the sulfur dioxide SO2 emissions that result from using those fuels in automotive vehicles aircraft railroad locomotives ships gas or oil burning power plants residential and industrial furnaces and other forms of fuel combustion Another important reason for removing sulfur from the naphtha streams within a petroleum refinery is that sulfur even in extremely low concentrations poisons the noble metal catalysts platinum and rhenium in the catalytic reforming units that are subsequently used to upgrade the octane rating of the naphtha streams The industrial hydrodesulfurization processes include facilities for the capture and removal of the resulting hydrogen sulfide gas In petroleum refineries the hydrogen sulfide gas is then subsequently converted into byproduct elemental sulfur or sulfuric acid H2SO4 In fact the vast majority of the 64 000 000 metric tons of sulfur produced worldwide in 2005 was byproduct sulfur from refineries and other hydrocarbon processing plants Then we could see that desulfurization being an important level of our industrialization for its dedication on both safety and sulfur production However for instance we removed the sulfide away by hydrodesulfurization after then how do we handle a mass of hydro sulfide which is considered as a toxic gas with a considerable danger to human In contrast to combustion products where sulfur is mainly present as SO2 and SO3 there is contained chemical element of sulfur predominantly as hydrogen sulfide in combustion gases before combustion principle of scrubbing process after the desulfurization that produces hydro sulfide as a reactive production during a variety of operation the hydro sulfide must be recovered from the sour gas streams by a scrubbing process Henry s law is the most initial process of scrubbing H2S in which H2S can be removed by dissolving it in liquid with water or more effectively with a weak alkali Equilibrium showed below H2S gas H2S dissolved in water H HS H2O H OH As a result of existing of two reaction equilibrium can be shifted to the right by a result of consumption of H A weak alkali allows then to remove the acid gas from it via heating or pressure reduction The using of alkali can save amount of liquid needed However if there is exists CO2 in stream being an acid it competes with H2S for the use of alkali to remedy this we will use alkanolamine process instead of using Henry s law In alkanolamine process the sour gas can be treated with an alkanolamine to remove the containing gas such as H2S CO2 COS and CS2 each alkanolamine has at least one hydroxyl group and one amino group the hydroxyl group increases water solubility and reduce vapor pressure while the amino group provides the alkalinity to cause absorption of the acid gas Principle of Claus process Once H2S has been isolated from the gas it is further reacted in controlled condition which is showing below H2S 1 2O S H2O heat If there is exists excess O2 and high temperature will occur a unwanted reaction H2S 3 2O2 SO2 H2O Discretion of Claus process acid gas and a controlled stoichiometric quantity of air are fed into a reaction furnace where 1 3 of the H2S is burned to SO2 the SO2 further react to form elemental sulfur thermally in the furnace and catalytically in downstream reactors according to the Claus reaction SO2 2 H2S 3S 2H2O heat Absorption of hydrogen sulfide Cleaning of the gas from hydrogen sulfide is usually effected by absorption the effective absorbents being mono and diethanolamine the absorption of hydrogen sulfide by monoethanolamine which takes place at a temperature of 30 40 is described by the following reaction RNH2 H2S RNH3HS At a temperature of 105 the reaction proceeds in the reverse direction with the formation of monoethanolamine and the hydrogen sulfide which is desorbed from the solution The regenerated solution is returned into the absorber In practice monoethanolamine is often used since it is stable and more active The cost of ethanolamines is relatively high but the loss of absorbents in the process is rather low 2 General description of De nitration 2 1 Formation of Nitrogen Oxide in combustion Nitrogen oxides belong to harmful impurity present in the air Even in low concentrations they can irritate respiratory organs destroy equipments and materials and promote the formation of smog and impair visibility in cities Nitrogen oxides form by oxidation of the nitrogen contained in the fuel and of atmospheric nitrogen and for that reason they are found in combustion products of all types of fuel such as coal fuel oil or natural gas The phenomena occurring in oxidation of atmospheric nitrogen in steam boiler furnaces can be described most fully on the basis of the theory follows For the oxidation of atmospheric nitrogen molecules of the oxygen should be dissociated at a high temperature the reaction occurring with the absorption of heat O2 O O 495 kj mol Atomic oxygen reacts with the molecules of nitrogen and the atomic nitrogen formed by this reaction enters into the exothermic reaction with molecular oxygen N2 O NO N 314 kj mol O2 N NO O 134 kj mol N2 O2 2NO 180 kj mol The reaction occurring in boiler furnace produce mainly nitrogen oxide NO of more than 95 The formation of NO2 by the oxidation of NO is appreciably long process which occurs only at low temperature At the exit from the stack the composition of nitrogen oxides remained almost unchanged compared with that in the furnace for example NO remains predominant and finally be oxidized only later in the cold atmosphere 2 2 Reducing nitrogen oxides emission 1 Selective catalytic reduction SCR Selective catalytic reduction is a means of converting NOx with the aid of a catalyst into diatomic nitrogen N2 and water H2O A gaseous reductant typically anhydrous ammonia aqueous ammonia or urea is added to a stream of flue or exhaust gas and is absorbed into a catalyst Carbon dioxide CO2 is a reaction product when urea is used as the reductant Selective catalytic reduction of NOx using ammonia as the reducing agent was patented in the US in 1957 development of SCR technology continued in Japan and US in the Early 1960 s with research focusing on less expensive and more durable catalyst agents The first large scale SCR facilities was installed in 1978 The NOx reduction reaction takes place as the gases passes through the catalytic chamber Before entering the catalyst chamber the ammonia or other reductant is injected and mixed with the gases The ideal reaction has the optimal temperature range Catalyst cost constitutes 15 20 of the capital cost of an SCR unit therefore it is essential to operate at a possible high temperature to maximize space velocity and minimize catalyst volume At the same time it is necessary to minimize the rate of oxidation of SO2 to SO3 which is more temperature sensitive than the SCR reaction 2 Selective Non Catalytic Reduction SNCR Conception Selective Son Catalytic Reduction is a method for reducing nitrogen oxide emissions in the conventional power plants that burn biomass waste and coal The process injecting either ammonia or urea into the firebox of the boiler at a location where the flue gas is between 870 1150 to react with the nitrogen oxides formed in the combustion process The resulting of the chemical reaction is elemental nitrogen N2 carbon dioxide CO2 and water H2O No solid or liquid wastes are created in the SNCR process Other chemicals can also be added to improve performance reduce equipment maintenance and expand the temperature window within which SNCR is effective Reaction Urea NH2CONH2 is easier to handle and store than the more dangerous ammonia NH3 In the process it reacts like ammonia NH2CONH2 H2O NH3 CO2 The reduction happens according to the reaction 4NO 4NO O2 4N2 6H2O The reaction mechanism itself involves NH2 radicals that attach to NO and then decompose The reaction needs a certain minimum temperature to happen otherwise the NO and the ammonia don t react Ammonia that hasn t reacted is called ammonia slip and is undesirable At too high temperature ammonia decomposes 4NH3 5O2 4NO 6H2O In that case NO is formed instead of reduced Thus the reaction needs a specific temperature window to be efficient The reaction also needs sufficient reaction time in that temperature window 3 Application of flue gas purification 3 1 Integration technology for flue gas dry technique de sulfurization de nitration and de dusting from glass furnace Nowadays glass industry is an energy supplier in a meantime is a industry which has brought massive pollution to our environment The fuel combusting in glass furnace mostly are the heavy oil natural gases and coal which are the well known polluters in our industry Thus the purification of exhaust gas from glass furnace is always on top of industry project missions The exhaust gas purification process of glass industry is generally similar to the flue gas desulfurization process of power plant However glass furnace exhaust gas has its own characteristic like being on high temperature low volume and containing complicated contaminants involving SO2 SO3 NOx CO2 HF HCI Meanwhile the stability of operation must be taken into consideration Characteristics of glass furnace flue gas 1 high flue gas temperature the temperature of glass furnace flue gas at the outlet is 400 500 even after the exhaust heat exchange process in exhaust heat boiler its temperature still be around 180 250 2 there are contained more kind of acid gas in flue gas such as HCI HF besides SO2 SO3 3 high viscosity and complicated component of dust component in flue gas 4 the pressure of flue gas inside the furnace is demanded to be constantly stable 5 the amount of flue gas fluctuate at large scope 6 there is contained toxic organics in flue gas During the combustion there will be produced some cancerogens such as dioxins and furans that is deadly harmful to human body The principles of multi clean dry technique purification process Multi clean flue gas dry technique system constitutes with dry scrubbing Tower high concentration bag filter absorbant supply system material circulating system process water system the by product efflux system electrical instrumentation control system etc The process scheme showed in Fig below The high temperature flue gas out from glass furnace get into a heat exchange process when it passes through the exhaust heat boiler till the temperature declines to around 200 and then the cool flue gas into the dry scrubbing Tower from the bottom where it mixes with the absorbent and the circulating materials then to be rapidly suspended by Venturi effect that makes the friction and the colliding between flue gas and particles stronger as well as strengthen the heat transfer and mass transfer between them in a me