Recovery furnaces, Kraft

Atmospheric particulate matter samples can be analyzed routinely for more than 50 trace elements. Trace element emissions arise from a large number of different source types in urban areas. For example, motor vehicles burning leaded fuel, electric arc steel furnaces, Kraft recovery boilers, and secondary lead smelters contribute to atmospheric lead concentrations. The wide spectrum of sources, together with the fact that trace metals often are only a minor fraction of the mass emissions from each source, obscure the relative importance of the contributors to atmospheric trace element levels. 

The smelt-water explosion problem is of primary interest in kraft recovery furnaces where, from operational error or an equipment failure. 

Waste-to-energy boilers Coal-fired Boilers Pulp and paper digesters Kraft recovery boilers Municipal waste containing chloride, sulfur, alkali metals, zinc and lead Sulfidation attack boiler tube wastage (50-60 mpy) Thiosulfate and polysulfides Sulfate, thiosulfate Chloride 59 (alloy 625 overlay weld metal used) 8 (alloy 625 and 309 SS) 21 (overlaid with 309 SS) 11 (309 L on lower furnace sidewalls 625 on floor tubes and smelt opening)... 

A furnace similar to the Tomlinson kraft recovery furnace is used for the combustion of magnesium-based sulfite spent liquors. In this case, however, no smelt is obtained instead the base is completely recovered as magnesium oxide in dust collectors the sulfur escapes as sulfur dioxide and is absorbed from the combustion gases in scrubber towers. However, because magnesium hydroxide has a very low solubility in water, a complete recovery of sulfur dioxide meets difficulties. 

Pulp and paper manufacture black liquor and Kraft recovery furnaces. 

The sulfur dioxide in kraft recovery furnace gases can be readily scrubbed with sodium carbonate to produce sodium sulfite or sulfate as makeup for cooking liquor preparation. The absorbent liquor can be used in the scrubber units that collect part of the sodium carbonate and sulfate particulates that escape collection by the electrostatic precipitator. Particularly in Sweden, such afterscrubbers are sometimes used also to recover low level heat (110), and they could be extended to recover sulfur dioxide as well (111). The use of afterscrubbers to collect sulfur dioxide from kraft recovery furnace gases appear to be fairly common in Japan (27). 

GREEN LIQUOR - The liquor resulting from dissolved molten smelt from Kraft recovery furnace in water. 

Wessel, R.A., Parker, K.L. and Verrill, C.L.,1997 "Three-Dimensional Kraft Recovery Furnace Model Implementation and Results of Improved Black-Liquor Combustion Models," Tappi J. 80(10) 207. 

Manufacture. The oldest method for producing Na2S is by the reduction of sodium sulfate with carbon in a refractory oven at 900—1000°C. Whereas this method is no longer used commercially in the United States, a variation is used to produce sodium sulfide captively during kraft pulp processing to replace lost sodium and sulfur values that were initiated into the system by merchant-suppHed sodium sulfide. In this method, sodium sulfate is added to the system in the recovery furnace, where it is reduced by carbon from the wood pulp to produce sodium sulfide. 

The most widely used pulping process is the kraft process, as shown in Fig. 6-11, which results in recovery and regeneration of the chemicals. This occurs in the recovery furnace, which operates with both oxidizing and reducing zones. Emissions from such recovery furnaces include particulate matter, very odorous reduced sulfur compounds, and oxides of sulfur. If extensive and expensive control is not exercised over the kraft pulp process, the odors and aerosol emissions will affect a wide area. Odor complaints have been reported over 100 km away from these plants. A properly controlled and operated kraft plant will handle huge amounts of material and produce millions of kilograms of finished products per day, with little or no complaint regarding odor or particulate emissions. 

Fig. 6-11. Schematic diagram of the kraft pulping process (6). 1, digester 2, blow tank 3, blow heat recovery 4, washers 5, screens 6, dryers 7, oxidation tower 8, foam tank 9, multiple effect evaporator 10, direct evaporator 11, recovery furnace 12, electrostatic precipitator 13, dissolver, 14, causticizer 15, mud filter 16, lime khn 17, slaker 18, sewer.

Three major sources in the kraft process are responsible for the majority of the H2S emissions. These involve the gaseous waste streams leaving the recovery furnace, the evaporator and the air stripper, respectively denoted by R), R2 and R3. Stream data for the gaseous wastes are summarized in Table 8.8. Several candidate MSAs are screened. These include three process MSAs and three external MSAs. The process MSAs are the white, the green and the black liquors (referred to as Si, S2 and S3, respectively). The external MSAs include diethanolamine (DBA), S4. activated carbon, Sj, and 30 wt% hot potassium carbonate solution, S6. Stream data for the MSAs is summarized in Table 8.9. Syndiesize a MOC REAMEN that can accomplish the desulfurization task for the three waste streams. 

Reducing smelting furnaces that produce a high-sulfidity, kraft-like green liquor are now employed at sodium-based sulfite mills. U.S. EPA anticipates that it would be necessary to replace the existing recovery boilers at ammonia-based mills if chemical substitution to a sodium base were employed. Additionally, it is likely that, because the heat value of sodium spent liquor is lower than ammonia spent liquor, evaporator modification may he required if excess capacity does not already exist. 

Recovery of inorganic chemicals is crucial to the cost effectiveness of the Kraft process. The black liquor which is obtained from delignification is rich in solubilised lignin and carbohydrate degradation products and, after concentration, is combusted in a recovery furnace. The Carbon dioxide which is produced during combustion converts unused sodium hydroxide into sodium carbonate. In addition, the sodium sulfate is converted, under the reducing atmosphere of the furnace, to sodium sulfide. 

Combustion Engineering, Inc. (1966). Summary Report on the Nature of and Remedies for Explosive Reaction(s) Between Smelt and Water in Kraft Chemical Recovery Furnaces. Inst. Pap. Chem., Appleton, Wisconsin. 

Kraft pulp mills Digesters batch and continuous Multiple-effect evaporators Recovery furnace Weak and strong black-liquor oxidation Smelt tanks Lime kiln Mercaptans, methanol (odors) H2S, other odors H2S, mercaptans, organic sulfides, and disulfides h2s Particulates (mist or dust) Particulates (dust), H2S Condensers and use of lime kiln, boiler, or furnaces as afterburners Caustic scrubbing and thermal oxidation of noncondensables Proper combustion controls for fluctuating load and unrestricted primary and secondary air flow to furnace and dry-bottom electrostatic precipitator noncontact evaporator Packed tower and cyclone Demisters, venturi, packed tower, or impingement-type scrubbers Venturi scrubbers... 

These calculations show that if renewable resources such as lignins are to find use as substitutes for phenol in wood adhesives, the cost differential between these materials must be maximized to attract investment capital. Clearly, the pulp lignins already being produced are likely to be the lowest cost renewable resource available for this purpose. Those generated in the kraft process are already recycled for their fuel value in the recovery furnace and are not easily accessible. There is only one U.S. company selling kraft lignin, and its lowest bulk price is 40 cents a pound (3). 

Liquors from Neutral Semichemical Pulping and from Kraft Pulping. In making neutral sulfite semichemical pulp, the black liquors may have about 10 parts of water to 1 part of total solids, of which about one-third is sodium acetate and sodium formate. After evaporation to about 1 part solids to 1 part water, sulfuric acid is added to the concentrate to free the acetic and formic acids. When the concentrate is extracted with acetone, the mixed acids are obtained, the acetone is separated for recycle, and the acids are concentrated and refined. The raffinate is stripped and is passed to the usual furnace to be burned for recovery of the inorganic salt values. This process gives a smelt of sodium sulfate, which may be used in the kraft process as chemical makeup. The loss of the fuel value of the acetic and formic acids is practically negligible. 

Hence, a part of the black liquors (10-50%) of a kraft plant may be processed to free, and subsequently to recover, the acetic acid as described by the addition of sulfuric acid and the formation of sodium sulfate. This new sodium sulfate from the sulfuric acid is present in the raffinate from the extractor it is thus the makeup for the balance of the liquors (50-90% ) that can be processed conventionally without recovery of the acetic and formic acids. The liquors from the raffinate are added to the other liquors before they go to the drier and the furnace. This part of the liquors (10-50%) recovers that much of the volatile acids that would otherwise be lost in the burner as salts. 

Both neutral sulphite and bisulphite pulping liquor can be reeovered by the Tampella process (Rimpi, 1983). The residual eooking liquor is burnt in a kraft type recovery furnace and the smelt of sodium earbonate and sodium sulphide obtained as described in the seetion on kraft pulping. The dissolved smelt is carbonated with flue gas to form sodium hydrosulphide and sodium bicarbonate and the partially carbonated liquor stripped with steam in order to liberate hydrogen sulphide gas. The gas formed plus make up sulphur is then burnt to sulphur dioxide for the preparation of sulphite eooking liquor. 

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