Furnaces – Process – Incinerating refuse
Patent
1991-08-22
1993-03-09
Yuen, Henry C.
Furnaces
Process
Incinerating refuse
110224, 110229, F23G 500, F23G 700
Patent
active
051918455
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a process for the reprocessing of sewage sludge.
Because of its organic components, the reprocessing of sewage sludge is performed mostly by combustion whereby the very high water content proves, however, to be a great impairment to the combustion process.
Thus, it has already been suggested that, prior to its combustion, the sewage sludge be subjected to a mechanical partial dehydration or that the sewage sludge be mixed with corresponding volumes of a dry fuel. Both methods have the disadvantage that heat required for the evaporation of the still large volume of residual water is provided disadvantageously at an unnecessarily high temperature level during the combustion.
Thus, it has also already been suggested that the sewage sludge be subjected to a thermal drying prior to its combustion. For this reason, atmospheric fluidized bed dryers e.g. have been introduced in which the heat necessary for the drying process is generated in an atmospheric fluidized bed combustor and is added to the fluidized bed dryer via an intermediate heat transfer cycle. The exhaust vapors produced during the drying process are recycled in part as a carrier medium for the fluidized bed into the fluidized bed dryer and are, for the remainder, condensed outside the installation and are processed correspondingly as an exhaust vapor condensate. Disadvantages of this known process are the relatively high expenditure for process-specific installation parts such as exhaust vapor purifier, exhaust vapor concentrator, exhaust vapor condenser, exhaust vapor condensate processor, and generation and transfer of the required drying heat via pipe walls which are in contact with the exhaust vapors on both sides.
SUMMARY OF THE INVENTION
The invention is based on the task of developing a simple and economical process for the thermal reprocessing of sewage sludge.
According to the invention this task is solved in that the sewage sludge is dried first by indirect heat exchange with condensed steam from the water-steam cycle of a power plant process operated with fossil fuels; that the exhaust vapors produced during drying are passed into a combustion chamber downstream from the furnace zone of the steam generator of the power plant process in order to reduce the nitric oxides formed in the furnace zone and that the dried sewage sludge is burned.
The suggestion according to the invention permits both an economically and ecologically advantageous reprocessing of sewage sludge through close combination with a power plant process operated with fossil, i.e. solid, fluid, or gaseous fuels. In this way the drying heat required for the sewage sludge may be provided at the required temperature level in a simple manner through low temperature steam, e.g. in the form of drawn off steam which is essentially already worked off in a steam turbine and stems from the water-steam cycle of the power plant. In the case of sufficiently large sewage sludge water volumes it is possible to adjust the necessary temperature level in the dryer by means of the pressure of the steam which is used as heating medium, e.g. by way of a counter-pressure turbine or also with a regulated bleeding condensation turbine.
In the case of comparatively low sewage sludge water volumes the necessary heating steam may be taken from the bleeder at the temperature and pressure level suitable in each case. For this purpose the supplying bleeder may also be alternated by switchover.
The exhaust fumes produced during the drying of the sewage sludge contain, in addition to the water steam, combustible components and other organic compounds, e.g. nitrogen compounds. According to the invention, these exhaust fumes are fed under precisely defined conditions to a reduction zone which is downstream from the furnace zone of the fossil power plant process, in order to reduce the nitric oxides in the flue gas of the fossil power plants, possibly together with other reducing substances, such as reducing gases or reducing vapors. They react with t
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patent: 4583470 (1986-04-01), Hirose
patent: 4656955 (1987-04-01), Kuo
patent: 4760650 (1988-08-01), Theliander et al.
patent: 4930429 (1990-06-01), Ryham
patent: 4938155 (1990-07-01), Williams
Creighton Wray James
Saarbergwerke Aktiengesellschaft
Yuen Henry C.
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