FI81970B - Method and installation for removal of sulfur from flue gases from a power plant using solid fuel - Google Patents

Description

81 970

Method and apparatus for removing sulfur from the flue gases of a solid fuel power plant

The invention relates to a method for removing sulfur from the flue gases of a solid fuel power plant according to the preamble of claim 1.

The invention also relates to an apparatus according to the preamble of claim 5.

In power plants that burn solid fuel, sulfur-containing fuel is often used. Desulphurisation has been carried out in many ways and there are currently at least 30 different technical solutions on the market that can substantially remove sulfur and, in some cases, nitrogen oxides from flue gases. Desulphurisation usually results in new waste again. In some cases, sulfur can be reduced to an element or the waste sold as such. However, most processes generate waste, the recovery or further treatment of which poses a new problem.

Normally, the power plant removes ash and dust with electric filters after the actual heat transfer process. In power plants, which in modern terminology are called combined cycle power plants, where combustion takes place in a pressurized state and where the first process is a gas turbine process and then the heat recovery boiler space and steam turbine process, the flue gases are often filtered hot before the gas turbine. However, such power plants are only becoming more common and suitable filters are being developed for them.

In the least costly desulfurization processes, desulfurization is carried out by injecting alkaline earth metal carbonate, usually ground limestone or 2,81970 dolomite into the furnace, whereupon it is converted by heat to alkaline earth metal oxide, which in turn reacts with sulfur dioxide and sulfur trioxide. From now on, there are various process variations, such as the LIFAC method developed by Tampella Oy. In said method, the efficiency of sulfur recovery has been improved by performing post-wetting, i.e. the so-called activation, which increases the recovery efficiency by almost 100% compared to alkaline earth metal carbonate injection alone.

Power plants that typically use peat, coal, or similar fossil, solid fuels as their fuel also have problems other than the sulfur contained in the fuels. Considering that typical such solid fuels contain about 10% non-combustible material, commonly referred to as ash, the utilization or disposal or storage of this ash poses a new problem. Obstacles to the exploitation of this ash include the non-combustible carbon it contains. In addition to this irreversible carbon, the fact that some of the lime is bound to calcium sulphite and not all is sulphate are obstacles to the utilization of desulphurisation ash. However, this sulfite is subsequently oxidized to sulfate, and in some applications, such as the concrete industry, this oxidation subsequently results in deformation of the product and loss of strength.

The object of the present invention is to eliminate the drawbacks associated with the prior art and to provide a completely new method and apparatus for removing sulfur from flue gases.

Ideal waste from power plants would ideally be one in which, after desulphurisation, the ash is a) dry, b) all the sulfur is bound to the sulphate form and c) this ash does not contain free non-combustible carbon.

3,81970 The present invention starts from the idea that a substantial part of the solid is separated from the flue gases in a known manner. In this respect, the procedure is similar to that in combined heat and power plants before the gas turbine process.

According to the invention, the ash is treated in a separate afterburner by decarbonizing it at a temperature which, however, is so low that the calcium sulphite and sulphate do not decompose again. The substantially particle-free stream, in turn, is treated with a catalytic oxidant and additional air, whereby the sulfur dioxide in the gas stream is oxidized in a known manner by catalysts (see Sheldon Lee, Irving Johnson, Kevin Myler et al., Argonne National Laboratory, Annual Contaminant Control in Hot Eolation Derived Gas Streams, Washington PA, USA 1983, May 24 - 26). To ensure the long life of the catalyst, it is important that the flue gases are substantially free of solids.

After separate treatments, the streams are combined.

The invention is thus characterized in that the flue gases are divided into two parts, i.e. a solids-free gas part and a solids stream, both of which are treated separately. The additional air is used to burn the free carbon of the fly ash, however, using such a low temperature, typically 800-900eC, that the gypsum and calcium sulfite do not substantially decompose. The second stream is catalytically oxidized so that sulfur dioxide is oxidized to sulfur trioxide. In both cases, additional air is introduced into the oxidation because the oxygen content of the flue gases in the power plant process is usually very low. For the oxidation of the gas free of solids, e.g. iron salts or activated aluminate or combinations thereof are used as catalysts. Examples are activated bauxite or synthetic and natural zeolites.

Such a process as described above is naturally suitable for a pressurized combustion combined cycle power plant, in which case, in any case, 4,81970, the solid particles of flue gases have to be substantially filtered off before the gas turbine. The process is also technically suitable for a normal power plant, since the same filtration can be applied there and in both cases the particulate flue gas can be catalytically treated a) to oxidize sulfur dioxide or b) by various means to reduce nitrogen oxides before or after this oxidation of sulfur dioxide.

After these operations, the streams are recombined and passed to a known method (e.g. (Tampella LIFAC method) in which steam vapor activation causes a gas phase solid reaction.

More specifically, the method according to the invention is mainly characterized by what is set forth in the characterizing part of claim 1.

The apparatus according to the invention, in turn, is characterized by what is set forth in the characterizing part of claim 5.

The invention provides considerable advantages. Thus, the process produces almost completely gypsum and not substantial amounts of calcium sulfite as in previous normal procedures. At the same time, dry, carbon- and calcium sulfite-free fly ash is obtained, which consists essentially of calcium silicates, gypsum, calcium carbonate and calcium oxides in the form of fly ash particles. The product is suitable for use, for example, as a cement admixture or in various concrete products.

The invention will now be examined in more detail with the aid of the accompanying drawings, in which Figure 1 shows a schematic diagram of a desulphurisation arrangement suitable for a normal power plant, and Fig. 81970 shows a schematic diagram of a desulphurisation process according to the invention.

In Fig. 1, reference numeral 1 denotes a normal fuel boiler into which fuel, air, water and flue gases and steam are supplied. To bind the sulfur contained in the fuel, finely ground limestone, or similar alkaline earth metal carbonate, is also fed to it. The flue gases from the boiler 1 are led to a filter 2, where the solids are separated. The filter used may be of any type known per se, such as a sand filter, a fluidized bed filter, a cyclone, a ceramic filter or a combination thereof, or some other filter capable of operating at high or normal temperatures.

The solids from the filter are passed to a separation device 3, where the ash and lime are separated from any filter material (e.g. sand), after which the solids are passed to an afterburner 4, where the ash is burned free of carbon. The firing temperature is kept below 1000eC, preferably 800-900C, to prevent the decomposition of calcium sulphate and sulphite. As the afterburner, a fluidized bed furnace or a rotating fluidized bed furnace can be advantageously used.

The solid-free flue gases from the filter 2 are fed to the reactor 5, i.e. to the catalytic oxidizer. The reactor used is, for example, a packing tower or a similar gas-solid contact device. The reactor is also fed with the additional air required for the oxidation reaction of the sulfur compounds. Activated aluminates and / or ferric oxides are preferably used as catalysts.

The product streams from the afterburner 4 and the oxidizer 5 are led to an activation reactor 6, where they are combined and at the same time wetting water is fed, whereby the aim is to place the moisture of the gas as close as possible to its dew point. The reactor used is a substantially empty reactor vessel with suitable dwell and flow dimensions.

Within the framework of the process according to the invention, the product streams can of course also be combined before the reactor 6.

The gas / solid mixture from the reactor 6 is divided into its components by an electrostatic precipitator marked with the reference number 7 or a similar gas-solid re-separation device.

In order to heat the additional air required in the afterburner 4 and the oxidizer 5, the fresh supply is passed through two countercurrent heat exchangers, HE2 and HEI, as shown in Fig. 1.

The heat exchanger HE2 can be placed after the oxidizer 5, as shown in the figure, or before it. High temperature promotes the oxidation reaction. On the other hand, delivery at a lower temperature reduces the risk of corrosion.

Figure 2 is a schematic diagram of the desulfurization process applied to a combined cycle power plant. The same nomenclature as in Fig. 1 is used in the figure and the same devices are arranged in a similar manner as in the previous embodiment.

In particular, however, it can be noted that the oxidizing device in this embodiment is located downstream of the gas turbine (GT) and the steam boiler (HRB). The oxidation of the sulfur dioxide contained in the flue gases to sulfur trioxide is carried out only after the steam boiler, as the sulfur trioxide formed could promote corrosion of the turbine blades.

In both process alternatives, it is advantageous to introduce to the afterburning space 4 a suitable amount of water vapor to promote the combustion of the unburned carbon in the ash by a known aqueous process.

II

7 81 970 by a gas reaction, although this is not an absolute requirement according to the invention.

In the arrangement according to Fig. 1, it is, of course, necessary to introduce additional heat into the afterburner to ignite the combustion event, which is not required by the process shown in Fig. 2.

Example;

The power plant burns coal with an ash content of 11%, containing 1.0% sulfur and an effective calorific value of 26 MJ / kg. Combustion is carried out with an over-air factor of 1.20.

Flue gas 9.3 NTP m3 / kg fuel is obtained. The flue gas is filtered at a temperature of 900 ° C with ceramic so-called with candle filters, whereby a total of 113 g of ash and unburnt carbon is recovered from the original amount of ash (100 g) in the filters. 1070 mg / m3 of solids remained in the gas, with a maximum particle size of 2 microns. Some of the ash remained at the bottom of the fluidized bed incinerator and is removed during sand cleaning.

The resulting ash is incinerated in a separate smaller fluidized bed burner with an excess amount of air 4, whereby water is fed to the incineration as a mist to gasify the carbon into monoxide. The temperature is maintained at 850eC. The ash resulting from the incineration contains 0.035% of carbon. Lime was injected in a molar ratio of 2 into the Ca / S furnace. The filtered flue gas is passed through the heat recovery devices in the normal way, whereby it is cooled to a temperature of 180 ° C. The pressure drop in the filter was 5% of the gas pressure.

After the main boiler, the hot gas, at a temperature of 250 ° C, was passed through a filter with activated bauxite in the form of granules about 1 cm in size and a layer 30 cm high at a gas velocity of 30 cm / s, and 8 81970 sa of pressurized air was added at this stage. % of gas volume (pressure 120 kPa, air preheated to the same temperature). Of the sulfur dioxide contained in the gas, 60% of the original amount remaining (40% had reacted with lime before), 92% was oxidized to sulfur trioxide.

The activated bauxite used contained about 4% ferric iron. The oxidized flue gas was combined with the burned ash at this stage at a temperature of 180 ° C and passed to a reactor where water was added to reach 5 ° C from the dew point. The fly ash was then separated from the flue gas with an electrostatic precipitator and the ash was analyzed. The sulfur in the ash represented 97% of the sulfur in the fuel and the sulfur was distributed so that 99% of it was as sulfate and 1% as sulfite (CaSO 4 and CaSO 3).

Claims (8)

9,81970 1. A method for removing sulfur from the flue gases of a sulfur-containing solid fuel power plant which, after incineration, leaves ash containing non-combustible organic matter, comprising: - feeding alkali metal carbonate to the furnace of the combustion process unit during combustion to bind a substantial amount of sulfur, to bind sulfur oxides as completely as possible, characterized in that - the flue gas solid, consisting essentially of ash containing non-combustible carbon and solid alkaline earth metal compounds, is separated after the incineration process, - the solid is - the oxidised gas stream and the afterburned solid are combined and treated with an appropriate amount of water to to react sulfur compounds with alkaline earth metal compounds in the solid. A method according to claim 1, wherein ground limestone is fed to the firebox to bind sulfur, characterized in that the solid in the flue gas containing gypsum and non-combustible carbon is burned with excess air at a temperature of up to about 1000eC, preferably 800-900eC. to prevent the decomposition of existing calcium sulphate. A method for removing sulfur from the flue gases of a combined cycle power plant according to claim 1 or 2, comprising a gas turbine and a steam boiler and a steam turbine, wherein the solids are separated from the flue gases before being introduced into the gas turbine, characterized in that the solid-free flue gas is catalytically oxidized through the gas turbine and steam boiler. Process according to one of the preceding claims, characterized in that the oxidation of the flue gases is catalyzed by iron salts and / or activated aluminates. An apparatus for removing sulfur from the flue gases of a sulfur-containing solid fuel power plant, the apparatus comprising - means for supplying alkaline earth metal carbonate to the furnace of the power plant combustion unit (1), and - an activation reactor (6) with water and flue gas supply means. that it further comprises - a combustion unit (1) connected to the outlet end of the separator (2) for separating solids from flue gases, - an afterburner (4) for burning solids from the separator (2), - a catalytic oxidizer (5) for quenching the flue gases, and - connecting elements 4 to the separating member (2), to connect the oxidizing device (5) to the separating member (2) and to connect the afterburner (4) and the oxidizing device (5) to the activation reactor (6). Apparatus according to claim 5, characterized in that the afterburner (4) comprises a fluidized bed or circulating bed device. Apparatus according to Claim 5 or 6, characterized in that additional air supply means are arranged in the afterburner (4) and in the oxidation device (5). 11 81970 Apparatus for removing sulfur from the flue gases of a combined cycle power plant according to Claim 5, characterized in that the catalytic oxidation device (5) is connected to the outlet end of the steam boiler. 12 81 970