GB2216243A

GB2216243A - Method of melting solid ballast materials

Info

Publication number: GB2216243A
Application number: GB8903898A
Authority: GB
Inventors: Franz Thelen
Original assignee: Steag GmbH
Current assignee: Steag GmbH
Priority date: 1988-02-25
Filing date: 1989-02-21
Publication date: 1989-10-04
Also published as: GB8903898D0; DE3805943A1

Description

1642-43 METHOD OF MELTING-SOLID_BALLAST_MATERIALS

BACKGROUND OF THE INVENTION

1. Field of the invention

The invention is concerned generally with the melting of solid ballast materials,, particularly of foreign materials which are difficult to dispose of and which comprise either exclusively ashes from special furnaces or low calorific value materials rich in ballast. The invention relates in particular to a method of melting solid ballast materials in a slag-tap furnace with a low energy input.

2. Prior art

DE-A-3324411 describes a steam generating installation with a combined wet-bottom and dry-bottom furnace. The fly ash separated from the exhaust gases from the drybottom furnace is fed back into the slag-tap furnace chamber and melted. The wet-bottom furnace has the advantage that the flue dust which is produced is fed back into the slag-tap furnace chamber, melted and after withdrawal from the slag-tap furnace chamber can be processed into a coarse granulate which can be easily transported and can be used in many ways for building purposes.

The separate melting of flue dust from coal dust drybottom furnace installations is also knownfrom the paper by W Barho "Decarbonisation and Feeding of Flue Dust". VGB Kraftwerkstechnik, December 1980. Only a limited energy utilisation was possible with this separate melting process despite relatively high capital cost.

in a steam generation installation known from DE-A- 3607896, a dust filter is arranged in the exhaust gas flow from the steam generator behind an air preheater (Luvo). The separated ash is supplied to an ash melting chamber. The ash melting chamber is cooled e.g. by means of heat exchanger tubes.

A particular problem is the processing of flue dust from coal dust drybottom furnace installations when using coal mixtures which contain high proportions of lean coal. Installations of conventional construction, i.e. those with a cooled slag-tap furnace chamber, can only be constructed for a content of at most 55% ballast. At this point the change to a central grinding installation is necessary. The volatile components of the combustible materials of the fuels must be above 22% waf (water ash free).

THE INVENTION Experiments by the applicant have shown that relatively high unit power outputs are necessary for the retention of an amount of foreign ash of 100 t/h, particularly if the fuel which is used already contains a high proportion of ballast. With a fuel with 35% ballast the gross electrical output when melting a foreign ash quantity of 100 t/h is about 400 MW. Unit power and foreign ash quantities are approximately linearly related.

It is the object of the invention substantially to 9 increase the ratio of the amount of solid ballast materials to be melted to the amount of heat supplied to the system.

The melting of solid ballast materials occurs in accordance with the invention in an uncooled slag-tap furnace chamber which has a lining of refractory? preferably ceramic,, material. The combustion reactan ts, preferably particulate or granulate fuels and combustion air, and solid ballast materials are fed to this slag-tap furnace chamber. The combustion parameters, that is to say particularly the amount and the colorific value of the fuel used, relative amounts of oxygen carrier gas and ballast materials, heat supply by the added materials etc.., are so selected that the combustion chamber temperature in the slag-tap fuirnace chamber is above the maximum flow temperature of the solid ballast materials to be melted. The removal of the heat produced during the combustion process is effected - in a manner different to thLt in known ash melting methods only by way of the melt when it is drawn off from the slag-tap furnace chamber and by way of the exhaust gas containing ash and dust particles.

Surprisingly, it has been found that a substantial increase of the proportion of solid ballast to be retained, particularly the foreign ash, is possible if the slag-tap furnace chamber is practically uncooled. The heat produced in the slag-tap furnace chamber is removed practically only by the discharge of the melt on the one hand and the discharge of exhaust gas on the other hand but not by way of cooling means for the - 1 slag-tap f urnace chamber. In this manner the major proportion of the heat supplied with the fuel is utilised in a temperature range above the melting point of the ash. The ratio of the retained foreign ash to the gross electrical output from the waste heat system connected downstream of the melting furnace is substantially increased by comprison with conventional installations. A further advantage of the invention resides in that the slag-tap furnace chamber, which is substantially freed of special cooling systems, renders possible a more simple and favourable wall configuration in order to achieve a better degree of ash separation. The slag-tap furnace chamber can thus be constructed, for instance, in the shape of a cyclone.

Under certain circumstancesi the requirements as regards service life of the ceramic brickwork of the slag-tap furnace chamber render a certain minimal cooling of the slag-tap furnace chamber walls necessary. The cooling capacity is however substantially lower than in the cooled slagtap furnace chambers of thd prior art.

The amount of the foreign ash to be mixed in is limited by the energy necessary for the heating of the products which are supplied and circulated. Melting is no longer possible if the combustion temperatures are lower than the melting point of the ash. in addition to the amount of the ash the temperature of the ash which is fed back and the temperature of the hot air also have a considerable influence on this.

A further increase in the useful power output may be achieved in the invention if at least a portion of the solid ballast materials are fed hot to the slag-tap furnace chamber. This may be achieved, for instance, if one extracts f lue dust from the hot region of the exhaust gas line and feeds it back into the slag-tap furnace chamber. in a preferred exemplary embodiment, the flue dust is firstly separated from the exhaust gas with the aid of an electrostatic filter and thereafter the exhaust gas is cooled in a Luvo whilst preheating combustion air. Additionally, the solid ballast materials which are supplied to the slag-tap furnace chamber, particularly foreign ash and/or recycled ash, can also be warmed, optionally with available process heat.

The manner in which the foreign ash is supplied is determined as a rule by the structure and properties of the ash or other solid ballast materials. The supply of foreign ash can be effected separately, optionally after crushing or grinding, and/or mixed with solid fuels. The method in accordance with the invention is not limited to a certain fuel. Rather any desired fuels can be used so long as their calorific value is sufficient to melt the solid ballast materials used.

The exhaust gases from the slag-tap furnace chamber can be fed into a built-in or a separate waste heat system. The built-in waste heat system can have a waste heat boiler directly connected to the slag-tap furnace chamber.

When leaving the slag-tap furnace chamber the exhaust gases are cooled to less than the melting temperature, for instance at a recovery grate through which water or steam flows or at a nozzle-injected carpet of air or flue gas.

The invention can be used in conjunction with known method steps, for instance known types of fuel and ash supply, ash recycling, the withdrawal of the melt from the slag-tap furnace chamber and with the use of different fuels or fuel mixtures and different oxygen carrier gases.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic representation of a wet-bottom furnace installation in combination with a waste heat system in accordance with a preferred embodiment of the invention; and Fig. 2 is a diagram which illustrates the dependence of the gross electrical power output on the ash retention and the temperature of the recycled ash.

DETAILED DESCRIPTION OF THE DRAWINGS

The f low points of the slags in coal are in the range of about 13500C and 15500C. A typical flow temperature of the ashes of coal which is mined in the Ruhr region is 15100C. The combustion chamber temperature in the slag-tap furnace chamber should be about 1000C above the maximum flow temperature of the solid ballast materials to be melted, for instance 16100C.

In the wet-bottom furnace installation schematically illustrated by way of example in Figure 1, a wet-bottom furnace is combined with a slag-tap furnace chamber 1 b,.

7 and the waste heat boiler 2 of an associated waste heat system. The fuels provided for the melting, in this case coal and additional fuels.. are supplied to the melting zone 3 in the interior of the slag-tap furnace chamber 1 by means of suitable supply means 4 and 5 known per se, preheated combustion air is supplied thereto by means of a line 6 and foreign ash is supplied thereto by means of supply means 7. Connected upstream of the slag-tap furnace chamber 1 there is also an ash recycling line 8.

The preparation of the coal constituting the fuel and the supply of the foreign materials to the melting zone 3 of the slag-tap furnace chamber 1 can be effected in a known manner. For instance, the foreign materials, namely the ash from the supply bunker 9, can be wholly or partially mixed into the coal before its supply into the slag-tap furnace chamber 1.

The melt or the liquid slag is permitted to run off at the floor end of the slag-tap furnace chamber 1 through a melt and slag discharge 10. The latter (10) can be constructed in a conventional manner as a wet slag remover or water bath in which the liquid slag is retained.

It is important for the purpose of the invention that the slag-tap furnace chamber 1 is uncooled as substantially as possible. The cooling which is always provided in the wall region in conventional steam generators with a wet-bottom furnace,, e.g. by way of heat exchange tubes or finned tube walls, is omitted or maintained minimal. In special cases certain cooling 2 measures can be necessary in order not to excessively thermally load the ceramic lining of the combustion chamber 1 or the insulating materials and/or carrier wall materials surrounding it. The internal space of the slag-tap furnace chamber 1 is preferably so shaped that an optimum degree of separation is achieved. For instance, a cyclone-shape of the inner walls is suitable. The special shaping of the slag-tap furnace chamber 1 is favoured by the substantial omission of cooling means in the wall region.

The exhaust gas discharge from the slag-tap furnace chamber 1 is effected in the described example via a recovery grate 11 through which water or steam flows and which is arranged in the transition region between the slag-tap furnace chamber 1 and the downstream waste heat boiler 2. Additional burners, which are supplied via a supply line 12, can be arranged in the waste heat boiler 2. The waste heat system is constructed in a known manner and indicated here as a whole with 13.

The flue gas leaving the waste heat boiler 2 is fed to a hot electrostatic filter 14 via a flue gas line 18. Such filters have been proved at the temperature of 4500C given in the drawing. A denox installation 15 is arranged behind the filter 14 and in front of the primary side of an air preheater (Luvo) 16. The arrangement of a separator or filter and a denox installation on the cold side is basically possible; it is however preferred in the context of the invention to arrange the electrostatic filter 14 in the illustrated manner on the hot side since the temperature of the ash has an effect on the desired high ratio of the I.

retention efficiency to the amount of heat supplied to the slag-tap furnace chamber 1. The exhaust gases cooled in the Luvo 16 are supplied in the described example to a flue gas desulphurising installation 17 and from there flow into a suitable chimney.

A balance sheet for the described installation is given in the following table for an input of 100 t/h foreign ash. In particular, the table shows the thermal capacity proportions and the gross electrical power outputs in association with differing ash retention efficiences and two different ash recycling temperatures.

11 TABLE (Input fuel 35% ballast) per 100 t/h foreign ash ash retention % 100 75 50 temperature ash recycling oc - 50 450 50 450 necessary fuel input kg/s 8.15 10.83 10.19 16.58 14.70 fuel heat MWth 175.23 232.85 219.09 356.47 316.05 melt: heat MWth 56.17 57.44 57.14 60.15 59.26 useful heat steam generator MWth 107.01 157.70A45.62 271.82 232.19 exhaust gas losses, other losses MWth 12.05 17.71 16.33 24.50 24.60 gross electrical output MW 39.6 58.4 53.9 100.6 85.9 1 1 i v I I The strong dependence of the necessary fuel input and the power outputs on the efficiency of the ash retention is apparent from the table and Figure 2. If the retention efficiency is low the circulating quantities of ash are high. The circulating quantities of ash must be heated up again and also tranfer heat again in the steam generator whereby a correspondingly increased gross electrical output is produced (Figure 2).

In modern slag-tap furnace chambers with U-shaped firing systems retention efficiencies of around 50% can be achieved. It is however to be expected that the slag-tap furnace chamber which is provided in accordance with the invention and is substantially uncooled can be constructed more efficiently as regards its flow technology so that retention efficiencies of between 60 and 75% may be achieved. Thus reduced unit power outputs of between 60 and 8( MW gross are achieved per 100 t foreign ash melting.

The method in accordance with the invention and the associated apparatus thus open up additional and economically favourable possibilities for retaining considerable quantities of foreign ash with a comparatively low electrical power. The invention increases the efficiency of the ash melting principally when there are few distribution possibilities for the electrical power produced.

It will be clear to the expert that the present invention is not limited to the specific examples given above and that there are many modifications and 11 variations within the scope of the present invention. The invention is particularly suitable for melting fly ash and flue dust from coal dust dry-bottom furnace installations; the invention may however basically be used without restriction when using liquid or gaseous fuel or other oxygen carrier gases. Foreign ash can ' be supplied to the slag-tap furnace chamber separately from or mixed with the fuel or other combustion reactants. The exhaust gases leaving the slag-tap furnace chamber can also be fed to a separate waste heat system.

(3

Claims

CLAIMS 1. In a wet-bottom furnace installation including at least one

slag-tap furnace chamber, a melt and slag discharge and an exhaust gas discharge, a method of melting solid ballast materials, said method including the steps:

(a) providing an uncooled slag-tap furnace chamber with a lining of refractory, particularly ceramic, material; (b) supplying combustion reactants and solid ballast materials to the slag-tap furnace chamber; (c) burning the combustion reactants whilst producing heat in the slag- tap furnace chamber in a temperature range in which the solid ballast materials are substantially melted; (d) discharging exhaust gas containing dust particles from the slag-tap furnace chamber; (e) drawing off the melt downwardly from the slag-tap furnace chamber; and (f) removing heat produced in step (c) substantially only by way of the melt and the exhaust gas in conjunction with the steps and (e).
2. Method as claimed in claim 1 wherein step (b) includes supplying at least a portion of the solid ballast materials to the slag-tap furnace chamber at a temperature of at least 4000C.

v
3. Method as claimed in claim 2 further in cluding the step of separating flue dust in the hot region from the exhaust gas and recycling the hot f lue dust into the slag-tap furnace chamber.
4. Method as claimed in claim 3 wherein the exhaust gas is firstly passed through a filter for the purpose of separating dust and is thereafter cooled whilst preheating combustion air.
5. Method as claimed in claim 2 wherein foreign ash is warmed and thereafter supplied to the slag-tap furnace chamber.
6. Method as claimed in claim 5 wherein the foreign ash is warmed with available process heat.
7. Method as claimed in claim 1 wherein foreign ash is mixed with at least one of the combustion reactants and is thereafter supplied to the slag-tap furnace chamber.
8. Method as claimed in claim 1 further including the step of cooling the exhaust gases on leaving the slagtap furnace chamber.
9. Method as claimed in claim 8 wherein exhaust gases are cooled at a recovery grate through which fluid flows.
10. Method as claimed in claim 8 wherein the exhaust gases are precooled by injecting a gaseous fluid through nozzles.

Is
11 Method as claimed in claim 8 wherein the exhaust gases are fed from the slag-tap furnace chamber to an integrated waste heat system.
12. Improved wet-bottom furnace installation for melting solid ballast materials, e.g. ash and flue dust, having at least one slag-tap furnace chamber, a melt and slag discharge at the bottom thereof and an exhaust gas discharge system, said improvement residing in that the at least one slag-tap furnace chamber of the wet-bottom furnace is uncooled and has a lining of refractory, particularly ceramic, material.
13. Improvement as claimed in claim 12 wherein connected downstream of the exhaust gas discharge there are: a waste heat system, at least one dust filter and ash recycling means discharging into the slag-tap furnace chamber.
14. Improvement as claimed in claim 13 wherein the filter is incorporated in an exhaust gas line before an air preheater.
15. Apparatus as claimed in cl,im 12 wherein the walls of the slag-tap furnace chamber are constructed in the shape of a cyclone such that a good degree of ash separation is achieved.
16. Method of operating a wet-bottom furnace installation, said method including the following steps:

(a) supplying predetermined amounts of combustion 16 reactants and solid ballast materials to at least one slag-tap furnace chamber; (b) burning the combustion reactants to produce heat in the slag-tap furnace chamber; (c) maintaining a combustion temperature in a predetermined temperature range which is above the melting temperature of the predominant proportion of the solid ballast materials and substantially melting the solid ballast materials; (d) discharging exhaust gas containing dust particles from the slag-tap furnace chamber; (e) withdrawing the melt downwardly out of the slag-tap furnace chamber; and (f) removing the heat from the slag-tap furnace chamber substantially exclusively by way of and in conjunction with the outflowing melt and the discharged dust-containing exhaust gas; whereby the fuel input necessary for meltipg a predetermined amount of solid ballast materials can be reduced.
17. Method as claimed in claim 16 wherein the combustion reactants include a solid fuel of coal rich in ballast and combustion air.
18. Method as claimed in claim 17 further including the step of separating flue dust in the hot region from the exhaust gas and recycling the hot flue dust into the slag-tap furnace chamber. - W 17
19. Method as claimed in claim 18 wherein foreign ash is warmed and thereafter supplied to the slag-tap furnace chamber.
20. Method as claimed in claim 18 wherein foreign ash is mixed with at least one of the combustion reactants and thereafter supplied to the slagtap furnace chamber.

V.

Published 1989 at The Patent Office, State House, 66 71 High Holborn, London WC 1E 4TP. Further copies maybe obtained from The Patent OMce. Was Branch, St Mary Cray, Orpington, Kent BR.5 3RD. Printed by Multiplex techniques Itd, St Mary Cray, Kent, Con- 1187