GB2186562A - A method of precipitation of harmful substances from flue gases - Google Patents

Abstract

In a first stage flue ash as well as a part of the SO2 are separated, then, in a second stage, a further part of SO2 as well as NOx is separated onto active charcoal and/or active coke. The active charcoal/active coke thereby charged with harmful substances is finally fed as fuel to the furnace, so that the need for expensive regeneration is abolished.

Description

SPECIFICATION A method of precipitation of harmful substances from flue gases This invention relates to a method of precipitation of solid and gaseous harmful substances from flue gases which accrue upon the operation of a furnace and which contain flue ash as well as SO2 and NOX. In afirst stage ofthis method, only flue ash as well as a part oftheSO2 is separated from theflue gas stream, and only in a second stage is afurther part ofthe SO2 as well as NO, separated from the flue gas stream.In the second stage these harmful substances are precipitated onto active charcoal and/or active coke, in which respect, in this second stage, a reducing agent is added and NO, is removed catalyticallyfrom the flue gas, while SO2 is removed from the flue gas partly adsorptively and partly by reactive with amonia.

A method of this kind is described in German Patent Specification No. DE-OS 34 23 744. In this respect, the precipitation of SO2 and NOxfromthefiue gases is effected by the combination ofthefollowing process steps: a)splitting-up of the overall stream of flue-gas from the furnace into a flue-gas main stream and into a flue-gas partial stream; b) Precipitation of a large proportion of the 502 and a lesser proportion ofthe NO, from the flue-gas main stream by wet washing along with the addition of Ca(OH)2 or CaCO3 and subsequent re-heating ofthe flue-gas main stream; c) Bringing-together of the flue-gas main stream with the flue-gas partial stream which has been conducted so as to bypass the wet wash;; d) Precipitation of a large proportion of the NO, a nd of the residual 502 from the overall flue-gas stream onto active charcoal/active coke, in which respect ammonia is added and the NO, is separated catalytically while the residual SO2 is separated from the flue gas partly adsorptively and partly by reaction with the added ammonia.

e) Regeneration of the active charcoal/active coke in a wet wash with subsequent drying of the active charcoal/active coke and return of same into the reactor; f) Reconditioning of the washing liquid oftheAC-wash by the addition ofCa(OH)2orCaCO3 (Ca-precipitation) and recycling of the ammonia; and g) Further joint processing ofthewashing liquid from the SO2wash and from the AC wash into gypsum.

The regeneration of the active charcoal or, respectively, ofthe active coke, which is necessary in step e) of this known process, is comparatively expensive. Depending on the quality ofthe active charcoal or, respectively, of the active coke, the regeneration costs may, in fact, be consderably higherthan the cost of procurement of same at the outset. The regeneration is also complicated and causes considerable additional costs as a result of the necessary furthertreatment of the washing water. Thus, for small furnaces particularly this process represents an uneconomic mode of operation.

The object ofthe invention is to propose a process which makes it possible to separate flue ash 502 and NOxfrom the waste gases without expensive regeneration of active charcoal or active coke being necessary and without any need to use washing liquids.

With this object in view, the invention provides a method as set out in the introductory paragraph, but characterised in that the active charcoal and/orthe active coke, after being specifically charged with harmful substances,isfedasfueltothefurnace.

As a result of the combustion, in accordance with the invention of the active charcoal/active coke which has been enriched with harmful substances, not only is the expensive regeneration thereof abolished, but also the harmful substances released upon combusion, in the first process step, can be introduced into a product, for example a mixture offlue ash and gypsum, which can be removed without difficulty. In addition, a further advantage of the combustion of the active charcoal/active coke which has been enriched with harmful substances, is that the calorific value of these materials which were previously used onlyforflue-gas cleaning are now utilised in the furnace.Altogether the method in accordance with the invention is a considerably more cost-favourable mode of operation of a furnace as compared with the previously known modes of operation.

In the first stage ofthe method ofthe invention wherein the partial desulphurization ofthefluegas isto be achieved, a powdery additive which contains alkali oxides is preferably fed to the flue gas stream,the resulting reaction products which are in the form of solid particles then being separated from the flue gases together with the flue ash.

The aforesaid first stage is considerably simpler in its mode of execution than the corresponding state of the known process wherein theflue-gas stream is split up into a flue-gas main stream and a flue-gas partial stream and a considerableamountofthe 502theS02anda lesser portion ofthe NOxfrom the flue-gas main stream is precipitated in a wetwash, which is followed by re-heating oftheglue-gasmainstream as well as bringing together again of the flue-gas main stream and the flue-gas partial stream which has bypassed the wetwash, and expensive purifying of the washing water.

Finally, in accordance with a development of the process ofthe invention it is additionally proposed that in the second step a multiway sorption filterthrough which active charcoal and/or active coke has travelled fromtopto bottom is used. In this second stagetheflue gases are, to begin with, conducted rightthroughthe lower section of the multiway sorption filter and the active charcoal and/or active coke remaining in thefilter serves as adsorption agentforthe SO2. Subsequently, after addition of the reducing agent, the flue gases are conducted right th rough the upper section of the multiway sorption filter where the active charcoal and/or active coke serves as catalystforthe NO, decomposition. The multiway sorption filter can also be multistage in design.

The process in accordance with the invention can be carried out particularly advantageously in the manner described hereinunder: Flue gas, coming from a furnace and containing harmful substances, mainly flue ash, SO2 and NOX is, to begin with, conducted through an additive reactor. In this reactor a powdery additive, which contains alkali oxides, e.g. Ca(OH)2, isfed into the flue gas and reacts with a part of the SO2 and thus partiallydesulphurizes the flue gas. The reaction products, such as CaSO3 and CaSO4 =gypsum, are solid particles which are then separated in a flue-gas filter, e.g. a hose filter, together with the flue ash of the flue gas.

The flue gas still containing an SO2 residue and the NOxthen flows through an active-coke filter, which is preferably a multiway sorption filter. In a first section of this filter, most of the SO2 residue is precipitated and in a second section, where a reducing agent, such as ammonia (NH3), is added, the NO, is decomposed into harmless nitrogen andwatervapour(N2 and H2O). The flue gas, which has been cleaned in the aforesaid manner to the permissible emission values, is then given off into the atmosphere.

The active coke used in the active-coke filter hastravelled in the opposite direction through the two sections. The fresh active coke is first of all used as catalystforthe NO, decomposition and then asadsorption agentforthe 502 residue, in which respect the SO2 is, for the most part, stored as sulphuric acid (H2S04) in the pores ofthe active coke.

The laden active coke is combusted in the furnace together with the actual fuel, such as coal. The SO2 stored in the active coke is released again here and, together with the SO2 which is formed during combustion ofthe actual fuel, passes into the flue-gas stream, which then travels through the above-described desulphurization, dust-removal residual desulphurization and nitrogen-removal stages.

Within the system, as a result ofthe SO2 recycling by means of active coke, a cycle exists for a part ofthe SO2, which increases the concentration of SO2 in front of the active reactor.

Since the SO2 residue removal follows, the efficiency in the additive reactor can be low, for example less than 60% desulphurization related to the SO2 partial pressure. The high partial pressure together with the sufficiency of low efficiency means that this stage can be constructed in a very simple manner.

The stages of the overall system are such that the amount of SO2 that is to be separated (i.e. the difference between the amount of SO2 fed with the sulphur content ofthe actual fuel to the furnace and the permissible amount of SO2 emission) is removed at the additive reactor and accrues as reaction producttogetherwith the flue ash. This mixture may be used, for example, as an admix substance in the production of cement.

Theactive-cokestageforthe residual desulphurization and for the nitrogen removal can in most cases be connected subsequent to furnaces which already posses a flue-gas desulphurization plant, but require an improvement in the degree of desulphurization and/or have subsequently had restrictions imposed relating to the NO, emission. This retrofitting is possible without expensive interventions into the existing system.

An active-charcoal filter may be used in place of an active-coke filter and where such an active-coke (or active-charcoal)stage is applied, a wet washing can still be used as first step for the precipitation of SO2.

In all cases, however, the active charcoal/active coke used for the gas cleaning and subsequently combusted is notsubstantially dearer and has a similar calorificvalue to, for example, a coal or coke. Thus, as a whole, a very cost-favourable process emerges.

Claims (4)

1. A method of precipitation of solid and gaseous harmful substances from flue gases which accrue upon the operation of a furnace and which contain flue ash aswell as SOP and NOX, in which respect in a firststage onlyflueash as well as a partofthe SOP is sepa rated fro mthe flue g as strea m, and only in a second stage isa further part ofthe 502 and the NO, separated from the flue gas stream by these harmful substances being precipitated onto active charcoal and/or active coke, in which respect, in this second stage, a reducing agent is added and NO, is separated catalytically from the flue gas, while SO2 is separated from the flue gas partly adsorptively and partly by reaction with ammonia, characterised in that the active charcoal and/orthe active coke, after being specifically charged with harmful substances, is fed as fuel to the furnace.

2. A method as claimed in claim 1, characterised in that in the first stage, forthe partial desulphurization of the flue gas, a powdery additive which contains alkali oxides is fed to the flue gas stream and the reaction products which accrue as solid particles are precipitated from the flue gases together with the flue ash.

3. A method as claimed in claim 1 or 2, characterised in that in the second stage the flue gases are, to begin with, conducted right through a lower section of a multiway sorption filter, through which active charcoal and/or active coke has previous been allowed to travel from top to bottom, the active charcoal and/or active coke remaining in the filterthus serving as adsorption agentforthe S02 and, after addition of the reducing agent, the flue gases are subsequently conducted right through the upper section of the multiway sorption filter where the active charcoal and/or active coke serves as catalystforthe NOx decomposition.

4. A method of precipitation of harmful substances from flue gases substantially as herein before described.