WO1988003191A1

WO1988003191A1 - Method and apparatus for combustion of soda black liquor

Info

Publication number: WO1988003191A1
Application number: PCT/FI1987/000144
Authority: WO
Inventors: Erkki KIISKILÄ
Original assignee: Ahlstrom Corp
Current assignee: Ahlstrom Corp
Priority date: 1986-10-31
Filing date: 1987-10-29
Publication date: 1988-05-05
Anticipated expiration: 1989-04-30
Also published as: AU600048B2; JPH0253553B2; FI77278C; BR8707849A; FI77278B; KR880701801A; SU1766244A3; AU8174487A; FI864436A0; JPH01502761A; FI864436L

Abstract

When combusting sulfur-free soda black liquor in the presence of iron oxide in a fluidized bed reactor provided with a circulating bed, particle fraction of the size larger than 50 mum is separated from the flue gases and recycled to the reactor. In a preferred embodiment of the invention the separation is carried out in a cyclone separator (7) the separation capacity of which is controlled by an upwards directed air flow supplied to the bottom of the cone (10) of the separator.

Description

Method and apparatus for combustion of soda black liquor

The present invention relates to a method for combustio soda black liquor and to an apparatus for carrying out method.

U.S. patent specification no. 4,224,289 discloses a metho combusting sulfur-free soda black liquor in a bubbling fl ized bed reactor by using a bed of iron oxide at a tempera higher than the melting point of sodium carbonate. The bustion produces sodium ferrite. The sodium hydroxide in the digestion process is regenerated by immersing sodium ferrite in hot water. The sedimented iron oxid separated and recirculated to the fluidized bed.

A bubbling fluidized bed combustion has e.g. the follo drawback:

- the particle size of the iron oxide/sodium ferrite ha adequate (> 1 mm) to prevent the chemical from flowing ou the reactor with the gas flow

- as the particle size of the iron oxide is heterogeneou (1 mm to < 10 μ ) part of the iron oxide immediately fol the gas flow (< 200 μm) without reacting completely

- when course material is fed in, the diffusion of alkal the iron oxide particles is so slow that large quantitie iron oxide is required whereby only the surface layer rea The required Fe/Na mole ratio is then > 2.0

- when black liquor is injected in the bed, water evapora the bed is cooled and a temperature profile is created which the bed is cooler than the free-board_.sector.

Performed combustion tests have shown that the alkali con of the product ash depends on the particle size as illustrated in Figure 1, thus fine substance contains subst tially more alkali than coarse material.

The object of the invention is to provide a method and apparatus by which the volume of iron oxide required in combustion process can be remarkably reduces.

A characteristic feature of the invention is that the comb tion is carried out in a circulating fluidized bed reac and that particles substantially of the size larger than μm are separated from the lue gases-and recirculated to reactor.

By separating for example particles of the size of 200 to, μm from the flue gases and by recirculating them to the reac its reaction time can be prolonged and thus the volume iron oxide required in the combustion can be reduced t Fe/Na mole ratio of 1.5.

The separator has to be designed so as to allow the f fraction of less than 50 μm -to pass through for which a sh circulation time is adequate in order to be saturated w alkali. If the Fe/Na mole ratio approaches 1.0 the ash beco sticky and results in deposition in the hot separator.

The apparatus intended specifically for carrying out method of the invention is characterized in that there upwards directed nozzles provided at or underneath the lo end of the cone of the cyclone separator.

The invention is described further below with reference to accompanying drawings in which

Fig. 1 illustrates the alkalinity of the product ash a function of the particle size; 1

Fig. 2 is a schematical illustration of an apparatus carrying out the method of the invention;

Fig. 3 is a vertical section of a cyclone separator app to the method of the invention.

In Fig. 2, black liquor and Fe₂0₃ is supplied to a circula fluidized bed reactor 1 via ducts 2 and 3. Also ozygen- taining gas such as air required for combustion and flu zation is supplied to the reactor. The gas flow rate in reactor is maitained high enough to ensure that small parti follow the gas flow and are transported out of the rea with -the gas.

The reactor is provided with a separator 4 designed to a the fine fraction of less than 50 μm to pass through. coarser solid material, preferably^" of particle size 200 100 μm, is separated from the gas and recycled via duct 5 the reactor so that the iron oxide particles which are yet saturated with alkali are again brought to contact the black liquor and can react with the sodium compounds the liquor. The sodium ferrite produced in the combustion removed from the bottom of the reactor via duct 6 and sodium hydroxide is regenerated by immersing the sodium ferr compound in hot water.

Figure 3 illustrates a cyclone separator 7 applicable in method of the invention. The gases discharged from the reac 1, as illustrated in Figure 2, are guided to the upper par of the cyclone via an inlet duct 9 connected tangentially the upper part. The separated solid material is dischar via an outlet 11 provided at the conical bottom portion 10 the separator. Gas is removed via an outlet 12 provi centrally in the upper part of the cyclone. Below the bot of the cone there are nozzles 13 directed upwards and connec and connected to an annular feed chamber to which also inlet duct for air is connected. The inlet duct is n illustrated in the Figure. By blowing air in upwards direct jets to the lower end of the cone small particles can separated from the solid material which particles a transported via duct 12 with the gas flow out of the reacto The size and the volume of the particles transported with t gas flow out of the reactor and thus the separation capaci of the reactor can be controlled by changing the volume of a supplied to the reactor.

The invention is not limited by the embodiment presented he as an example, only, but several modifications and applicatio within the scope of protection defined by the appended pate claims are possible.

Claims

We claim :

1. Method of combusting sulfur-free soda black liquor circulating fluidized bed reactor in the presence of oxide in order to bind the sodium contained in the soda b liquor to the form of sodium ferrite, characterized in

- soda black liquor and iron oxide are supplied to circulating fluidized bed reactor, and oxygen-containing for the combustion and the fluidization;

- an adequate flow rate is maintained in the reactor by so as to cause the smallest particles to flow out of reactor with the gas flow;

- the gas flow is supplied to a particle separator in w the fraction substantially of the size less than 50 μm ha a higher sodium content is caused to be discharged with gas flow, and only the particle fraction of the size la than 50 μm containing iron oxide particles, which have reacted with the sodium compounds of the^~ black liquor, recirculated to the fluidized bed.

2. Method as claimed in claim 1, characterized in that particle size of the solid material separated from the f gases is 200 to 100 μm.

3. Method as claimed in claim 1 or 2, characterized in t the solid material is separated in a cyclone separator.

4. Method as claimed in claim 3, characterized in that separation capacity-of the cyclone separator is controlled an upwards directed gas flow supplied to the bottom of cone of the separator.

5. Apparatus for carrying out the method of any of the cla 1 to 4, the apparatus comprising a circulating bed reac provided with a cyclone separator, characterized in t upwards directed nozzles (13) are provided at the lower of or below the cone (10) of the cyclone separator (7).