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Åtgärder för bättre utbränning på fast rost - studie av Rockhammars bruks barkpanna
2004 (Swedish)Independent thesis Advanced level (degree of Master (One Year))Student thesis
Abstract [en]

A common way of burning solid fuels is to burn them on a grate. The supply of primary combustion air is divided into zones to be able to control the airflow to the three combustion processes drying, devolatilisation and char combustion. Within the pulp and paper industry bark is burned in bark boilers. These boilers are either equipped with a grate or a fluidisized bed. The operation conditions for these boilers consist of large and fast load variations and are mostly fired with bio fuels of varying kind. In recent years environmental demands has been introduced for the boilers, where the latest is that the ash may not be deposited, if the heating value exceeds 6 MJ/kg. The purpose of the work which is the basis for this essay was to study the bark boiler of the pulp mill Rottneros Rockhammar in order to improve the combustion on the grate with the intention to reach better burnout of the fuel and thereby lower energy content in the bottom ash. The aim has been to find the reasons to the problem, recommend how the boiler should be controlled for better burnout and give suggestions for measures in form of repairs and reconstructions to further increase the burnout.The boiler in the mill of Rockhammar has got a cold slanting grate, which is divided into three zones. It’s dimensioned for 10 t/h saturated vapour at 30 bars. Combustion air is supplied through four air registers from one air fan. The air is not preheated and flue gases is brought back to two of the zones. The bottom ash is fed out with a screw which is placed in a shaft in the end of the grate.The study has been carried through as four test series whose results partly have been completed with calculations. The test series were based on the assumption that if the fire were moved further up the grate and the residence time for the fuel on the grate increased, the burnout would be improved. Temperatures on the grate and in the oven have been used as process outcome during the tests. The calculations were needed to achieve better understanding to the problem with poor burnout and give basis for possible measures in form of reconstruction.The four test series were supposed to analyse the distribution of combustion air and exhaust gases under the grate and the operation of the screw. Slagging on the grate has been a major problem during the test series, but the results that were found showed that the foremost problem is that the main part of the fire is too far down on the grate. The lack of air-preheating is considered as main cause, but even that the screw of different reasons is in operation too long and too often. The fire could not be moved upwards noticeable by redistribution of the primary combustion air.The calculations showed how combustion efficiency can become low if too much unburned fuel is allowed to leave the boiler. By exemplify with a model of the drying zone it were shown how better burnout demands lower drying power which in turn further increases the conditions for good burnout.To speed up the burning of the solid fuelremain (the char) has shown to be a bad alternative. Measures should instead be taken to extend the time and the surface area at which the char combustion is in progress. The time is increased by setting down the operation time for the screw and bigger surface area is achieved by render the zones above more effective. Recirculation of hot flue gases (400?C) is considered to be the most suitable measure to render the drying zone more effective.Suggestions for further work are to investigate the fuel feeding is affecting the fuelbed and the burnout. Another suggestion for further work is to develop and project a control system for the screw which is based on temperature measuring on the last zone on the grate or in the shaft.

Place, publisher, year, edition, pages
2004. , 51 p.
Identifiers
URN: urn:nbn:se:kau:diva-54430Local ID: EMI-7bOAI: oai:DiVA.org:kau-54430DiVA: diva2:1103151
Subject / course
Environmental and Energy Systems
Available from: 2017-05-30 Created: 2017-05-30

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