Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With regeneration – reclamation – reuse – recycling or...
Patent
1998-06-04
2000-06-06
Nguyen, Dean T.
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With regeneration, reclamation, reuse, recycling or...
162 3011, 110238, 110345, D21C 1112
Patent
active
060713771
DESCRIPTION:
BRIEF SUMMARY
AREA OF THE INVENTION
The present invention relates to a process for combusting black liquor in recovery boilers. The invention also includes a recovery boiler for implementing the process according to the invention.
STATE OF THE ART AND PROBLEMS
Recovery boilers for combusting black liquor which is obtained during cellulose cooking have been well known for many decades. Their task is, on the one hand, to generate energy by means of the combustion and, on the other hand, to recover chemicals which were used during the cellulose cooking and which are released in the smelt state during the combustion and tapped off from the bottom part of the recovery boiler. Recovery boilers are large installations and, apart from generating a large quantity of energy and recovered chemicals, also generate a large quantity of flue gases. These flue gases contain, inter alia, nitrogen oxides which have arisen during the combustion of the liquor.
During recent years, ever stricter requirements have been introduced with regard to the discharge of nitrogen oxides into the atmosphere. It is well known that these oxides contribute to acidification and other unfavourable effects on the natural environment. Nevertheless, the quantity of nitrogen oxides which is emitted from the recovery boilers of the wood-processing industry is low as compared with that originating from cars, etc. While the contents are normally within the range of 40-70 mg of NO.sub.2 /MJ (calculated as the effective heat value in a reducing medium), even these low contents have to be decreased substantially in future. The discharges have been shown to depend principally on the nitrogen content of the fuel, i.e. the black liquor, and this content will increase in the future as the chemical cycles are closed ever more stringently. This will result in the nitrogen oxide emissions increasing if countermeasures are not taken.
Factors which can effect the formation of nitrogen oxides are dwell time, temperature and oxygen content.
As a result of experience gained from conventional power boilers based on coal, oil and gas, it is known that substoichiometric conditions with regard to the oxygen supplied to the combustion zone, in combination with a final combustion, for the purpose of obtaining maximum energy evolution, in a so-called overfire air register which is placed directly above (or after) the combustion zone, result in a lower No.sub.x emission.
This technique has also, for a long time now, been used for other reasons in recovery boilers, where primary air and secondary air have been supplied below (before) the black liquor, and tertiary air has been supplied immediately above (after), in a similar manner to that in which overfire air has been used for power boilers. This is described in more detail by Anderson & Jackson in the TAPPI Journal for January 1991, pp. 115-118.
SE 468 171 discloses a method for decreasing the content of nitrogen oxides in the flue gas, in which method a part of the combustion air is supplied, as a final portion, at a very high level so that a higher grade of reducing atmosphere is maintained, without any extra addition of reducing substances, over a distance of 10-20 metres or greater, from the region of the liquor injection and up to the point of the final addition of air, corresponding to a dwell time under normal loading of 2.5-5 seconds or more. This method presupposes that the recovery boiler is relatively tall so that the reducing atmosphere can be maintained for a sufficiently long time.
Another method has been proposed for reducing the NO.sub.x in the recovery boiler, which method uses the technique, which has been developed for power boilers, involving thermal or selective non-catalytic reduction (SNCR), by means of supplying a reducing substance, in the form of natural gas, ammonia or urea, relatively high up the recovery boiler for the purpose of reducing nitrogen oxides which have been formed. When natural gas is used, large quantities of uncombusted residual products are formed which have to be finally combusted by m
Fasth Rolf
Kvaerner Pulping AB
Nguyen Dean T.
LandOfFree
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