Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
  • B01D53/34—Chemical or biological purification of waste gases
  • B01D53/46—Removing components of defined structure
  • B01D53/48—Sulfur compounds
  • B01D53/50—Sulfur oxides
  • B01D53/508—Sulfur oxides by treating the gases with solids

Definitions

• a process for the purification of flue gases and an apparatus for it
• the present invention relates to a process for the purifica ⁇ tion of flue gases. It relates in particular to a process for the purification of flue gases wherein a dust-like substance which absorbs the gaseous sulfur compounds present in flue gases, primarily sulfur dioxide, or becomes absorbent while in the furnace, is fed into the furnace of a boiler, the partly reacted dust-like absorbent is separated from the flue gases at a point after the furnace, is hydrogenated and returned to the flue gas flow at a point after the separation point, but before the reaction zone, in which additional water or steam is fed into the flue gas, whereafter the dust which contains solid sulfur compounds is ultimately separated from the flue gases thus purified.
• the invention also relates to an apparatus for use in the process referred to above, for activating at a point before the reactor the partly reacted dust-like absorbent traveling together with the hot flue gases emerging from the boiler fur ⁇ nace, and in particular to a separation device, intended to be connected to the flue-gas duct between the boiler furnace and the reactor, such as a cyclone having means for hydrogenating the partly reacted absorbent separated from the flue-gas duct and for returning it to the flue gas duct at a point after the separation apparatus, before the reactor.
• a separation device intended to be connected to the flue-gas duct between the boiler furnace and the reactor, such as a cyclone having means for hydrogenating the partly reacted absorbent separated from the flue-gas duct and for returning it to the flue gas duct at a point after the separation apparatus, before the reactor.
• the object of the present invention is therefore to provide a process for the purification of flue gases, and an apparatus for use in it, by means of which an absorbent present in the dust traveling together with the flue gases emerging from the boiler furnace is activated before it is fed into the actual reactor.
• this is accomplished by dividing, in a separation zone, the hot, dust-bearing flue gas emerging from the furnace into a dust-poor main flow and a dust-rich secondary flow, into which steam and/or water mist is fed at a point at which the temperature of the secondary flow is still so high that the reaction among the sulfur com ⁇ pounds, the absorbent and water is still drastic enough to break up the surface layer of the partly reacted dust par ⁇ ticles, thereby activating them.
• the flue gases are thus not entirely separated from the dust at a point before the actual reactor, as is done in the above-mentioned FI Lay-open Print 76931; instead, the hot flue gases which contain partly reacted dust-like absorbent are divided into two fractions, namely a dust-poor main flow which, in a manner known from the LIFAC process, is directed, possibly via an air preheater, into the actual reactor, and a dust-rich secondary flow into which steam and/or water mist is fed.
• the problematic solids-moistening systems which include moving parts (cf. FI Lay-open Print 76931) are entirely avoided, and the entire flue gas volume need not be moistened with steam or water at a point before the actual reactor, as is necessary in the above-mentioned LIFAC process.
• an oxide, hydroxide or carbonate of calcium or magnesium, preferably limestone, which is known as an inexpensive raw material, can be fed into the furnace of a boiler.
• the limestone breaks down into calcium oxide, which partly reacts with the gaseous sul ⁇ fur compounds, mainly sulfur dioxide, present in the flue gases, whereby a gypsum crust is formed on the calcium oxide particles, preventing the calcium oxide in the inner part of the particle from reacting with sulfur dioxide.
• steam or water mist is fed into the dust-rich secondary flow of flue gases, either after it has been separated from the main flow or, preferably, into the separation zone itself, at a point at which the flue gases already have a significant concentration of dust but at which there, nevertheless, still prevails effective mixing.
• the dust-rich secondary flow is made to join the dust-poor main flow of flue gases, either in the reactor or at a point before it, but preferably only after the main flow has been brought into an indirect heat-exchange contact with the air to be fed into the boiler furnace.
• Part of the secondary flow can, however, be introduced into the main flow even before the above-mentioned indirect heat-exchange contact.
• a portion of the dust-poor main flow separated from the flue- gas flow can be introduced into the tail end of the actual re actor in order to raise its temperature.
• a portion of the preheated air may be introduced into this section of the reactor.
• the separa tion device used is preferably a cyclone, the structure of which is, however, such that the dust traveling together with the flue gases emerging from the boiler furnace will not sepa rate out from the gas phase as a separate solids phase, but the flue gases will divide into two separate gas fractions, i.e.
• a dust-poor main flow and a dust-rich secondary flow which secondary flow is removed from the lower end of the cy ⁇ clone, there being in the lower section of the cyclone, in th vicinity of the secondary-flow outlet, nozzles for feeding in steam and/or for atomizing water into the cyclone, the secondary-flow outlet in the lower end of the cyclone being connected by means of a connecting duct to the flue-gas duct which is between the cyclone and the actual reactor and* in which the main flow travels, in order to make the activated, dust-rich secondary flow to join the dust-poor main flow at a point after the separation device.
• the furnace of the boiler is generally indi ⁇ cated by reference numeral 1.
• a sulfur-containing fuel 8 is burned in the boiler furnace 1, usually with air 9.
• the flue gases which contain gaseous sulfur compounds, mainly sulfur dioxide, cool on the heat surfaces 20 in the upper section of the boiler and in the air 9 preheater 10 located at a point after the boiler.
• the flue gases are directed into an oblong reactor 4, which is followed by a dust-separation apparatus 5 and a flue 6.
• the finely-divided reagent for absorbing the gaseous sulfur compounds present in the flue gases, or its precursor, prefer ⁇ ably calcium carbonate, is fed from a storage container 2 by means of batching feeders 17 along a pipe 16 into the air flow of a pneumatic air-conveying blower 18, and it travels to ⁇ gether with this air flow along a pipe 7 to the upper section of the boiler.
• water is sprayed into the oblong reactor 4 via nozzles 15, which are installed at the beginning of the reactor 4.
• the hot, dust-bearing flue gases emerging from the boiler are divided in a cyclone 3 into two fractions, namely a dust-poor main flow 11 of flue gases, which is directed via an air preheater 10 into the reactor 4, and a dust-rich secondary flow 12, which is made to join the main flow at a point after the air preheater 10, preferably at a point before the reactor 4.
• a portion of this dust- rich secondary flow 12 can be directed along a pipe 13 branch ⁇ ing from the pipe 12 and be introduced into the main flow even at a point before the air preheater 10.
• the temperature of the dust-rich flue gases is approx. 200-600 °C, at which temperature the formation of calcium hydroxide and hydrous calcium sulfate is very drastic, causing the dust particles to swell rapidly and to break up into smaller particles with reactive calcium hy ⁇ droxide on their planes of fracture.
• the process and apparatus according to the invention the following advantages are gained in comparison with the state of the art: the need for limestone is reduced, the de ⁇ gree of sulfur removal is improved, the steam quantity re ⁇ quired for the moistening is low and can well be replaced by water spraying, and in addition, the apparatus required is simple and does not include parts which move and are therefore prone to wear and disturbances.
• Limestone powder is fed into the furnace of a boiler in order to remove sulfur dioxide from the flue gases by using a lime- to-sulfur ratio of two.
• the moisture content of the flue gases leaving the boiler is increased, whereby the reactivity of the lime remaining in the flue gases is improved through the formation of calcium hydroxide.
• a secondary flow is sepa ⁇ rated by means of a solids-separating apparatus from the flue gas flow leaving the boiler, 50-80 % of the dust present in the flue gases concentrating in this secondary flow.
• This dust consists of fly ash and of lime which has formed from the limestone powder blown into the boiler and on the surface • of which a calcium sulfate layer has formed in the boiler fur ⁇ nace.
• the temperature t(in) of the flue gas entering the dust separation system is usually approx. 400 °C after the heat surfaces of the boiler. No substantial cooling takes place in the dust-separation apparatus, and thus the main flow of the flue gases returns at the same temperature to the flue-gas duct.
• Water and/or low-pressure steam is atomized into the secondary flow of the flue gases, and its temperature is low ⁇ ered.
• the calcium sulfate layer on the particle surface preventing the binding of sul ⁇ fur dioxide, breaks up, and the more active lime returning to the process together with the secondary flow of the flue gas improves the capacity of the entire process to bind sulfur dioxide.

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• Engineering & Computer Science (AREA)
• Chemical & Material Sciences (AREA)
• Health & Medical Sciences (AREA)
• Biomedical Technology (AREA)
• Environmental & Geological Engineering (AREA)
• Analytical Chemistry (AREA)
• General Chemical & Material Sciences (AREA)
• Oil, Petroleum & Natural Gas (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Treating Waste Gases (AREA)

Applications Claiming Priority (2)

Publications (1)

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Cited By (1)

Citations (2)

• 1989
  • 1989-07-19 FI FI893494A patent/FI83167C/fi not_active IP Right Cessation
• 1990
  • 1990-07-18 CA CA002064210A patent/CA2064210A1/en not_active Abandoned
  • 1990-07-18 DE DE19904091281 patent/DE4091281T/de not_active Withdrawn
  • 1990-07-18 ES ES9150011A patent/ES2038952A6/es not_active Expired - Lifetime
  • 1990-07-18 HU HU92165A patent/HUT63073A/hu unknown
  • 1990-07-18 WO PCT/FI1990/000184 patent/WO1991001174A1/en not_active Application Discontinuation
  • 1990-07-18 DD DD342901A patent/DD300075A5/de unknown
  • 1990-07-18 IT IT067565A patent/IT9067565A1/it unknown
  • 1990-07-19 PL PL28612690A patent/PL286126A1/xx unknown

Patent Citations (2)

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