WO2004057299A2

WO2004057299A2 - Method and equipment for measuring the composition of gas from a fluidised bed

Info

Publication number: WO2004057299A2
Application number: PCT/FI2003/000937
Authority: WO
Inventors: Juha Parkkinen; Launo Lilja; Maija-Leena Metsärinta; Jarmo Saarenmaa
Original assignee: Outokumpu Oyj
Priority date: 2002-12-23
Filing date: 2003-12-09
Publication date: 2004-07-08
Also published as: FI20022266A0; WO2004057299A3; AU2003285373A8; PE20040631A1; FI116161B; FI20022266A; AU2003285373A1

Abstract

The invention relates to a method and equipment with which gas composition can be measured continuously from the fluidised bed of a roasting furnace, fluidised bed boiler or any other kind of process. The method and equipment have enabled the measurement of the gas composition of a gas with a considerable amount of dry solids. The gas composition measurement facilitates the regulation of for instance the reaction degree of the reactions occurring in the bed.

Description

METHOD AND EQUIPMENT FOR MEASURING THE COMPOSITION OF GAS FROM A FLUIDISED BED

The invention relates to a method and equipment with which gas composition can be measured continuously from the fluidised bed of a roasting furnace, fluidised bed boiler or any other kind of process. The method and equipment have enabled the measurement of the gas composition of a gas with a considerable amount of dry solids. The gas composition measurement facilitates the optimization of for instance the reaction degree of the reactions occurring in the bed.

Fluidised bed technology is used in many different processes in the metallurgical industry and in energy and environmental technology. The gas pressures of the bed affect in what compound and phase the reacting materials exist in the bed. The control of fluidised bed processes requires that the conditions of the bed can be controlled to prevent molten phases or other sintering compounds that lead to an unstable state from being generated. The reactions of the bed cannot be controlled entirely by the feed, since some of the reactions occur in the bed and some above it. The reactions can be controlled better by measuring and adjusting the gas composition in the bed.

There follows a more detailed example of the use of the method in a roasting process occurring in a fluidised bed reactor: The roasting of fine-grained material generally takes place with the fluidised bed method. The material to be roasted is fed into the fluid bed furnace through feed units in the wall above the fluidised bed. There is a grate in the base of the furnace, through which oxygen-cόntaining gas is fed in order to fluidise and oxidize the concentrate. The oxygen-containing gas generally used is air or oxygen- enriched air. When the concentrate fluidises, the height of the feed bed grows relative to the height of the bed of solid material. The back-pressure of the furnace is formed from the grate resistance and the bed resistance. The bed resistance is mainly the mass of the bed, when it is in the fluidised condition.

As far as roasting is concerned, it is important to control the bed i.e. the bed should be stable in structure and other fluidizing properties should be in order, with controlled fluidization. Oxidation should occur as completely as possible i.e. when processing zinc concentrate for instance the sulphides should oxidize into oxides, because sulphides form molten phases at lower temperatures than the said oxides. When the amount of sulphides remains too high the bed becomes unstable.

Zinc sulphide concentrates processed by zinc roaster have over the course of time become more and more impure. Concentrates are no longer anything like pure zinc blende, or sphalerite, instead they may contain a considerable amount of iron. Iron is either dissolved in the sphalerite lattice or present as pyrite or pyrrhotite. In addition concentrates often contain sulphidic lead or/and copper. The chemical composition and mineralogy of concentrates varies considerably. In that case the amount of oxygen required to oxidize the concentrates also varies, as does the amount of heat produced when the concentrates are burned. In accordance with the technology used at present, concentrate feed of the roasting furnace is adjusted according to the temperature of the bed using fuzzy logic for example. This leads to the danger that the amount of oxygen in the roasting gas may drop too low, in other words the amount of oxygen may be insufficient to roast the concentrate. As a result the bed does not agglomerate normally but remains too fine and at the same time the back-pressure of the bed may fall too low, because a fine bed stops fluidizing and channelling occurs.

During fluidised bed roasting, agglomeration of the product normally occurs i.e. the calcine is clearly coarser than the concentrate feed. However, the formation of molten sulphides described above increases agglomeration to problematic levels, since larger agglomerates with their sulphide nuclei are left to move around to the grate. Agglomerates cause build-ups on top of the grate and over time they block the gas nozzles under the grate.

Different ways of regulating roasting conditions have been attempted. US patent 5803040 concerns a method for stabilizing a fluidised bed in the roasting of metal sulphides, whereby stabilization occurs by adjusting the particle size of the feed. In US patent 3975484, stabilization occurs by feeding the concentrate as slurry. In the article by MacLagan, C. et al: Oxygen Enrichment of Fluo-Solids Roasting at Zincor, Lead-Zinc Symposium 2000, Pittsburgh, USA, October 22-25, 2000, pp .417-426, it states that the oxygen content of the exhaust gas of the roasting furnace is controlled by measurements that take place from the gas line after the boiler or cyclone. These measurements do not however tell of the state of the fluidised bed, because part of the material reacts above the bed and on the other hand air leaks are already included in the gas line measurements.

Patent application WO 02/40723 describes a method whereby roasting conditions are adjusted by controlling the oxygen pressure of the bed. However, the publication does not describe in detail the equipment with which the measurement is taken.

The regulation of gas composition is difficult, since measurement of the composition of dust-containing gases tends not to succeed. The dust contained in the gases clogs up the measuring devices and analysis cannot be made. Measurement of the fluidised bed is even less successful. In particular, continuous measurement has been impossible. However, analysing the composition of the fluidised bed gas is vitally important in order to regulate the fluidised bed. Particularly when the feed varies it is important to get continuous data about the composition of the gas, so that the pressures of the reacting gases can be adjusted to the right level. The method and equipment now developed in accordance with the present invention to measure gas composition continuously in a fluidised bed used in the processing of various materials, is intended to correct the shortcomings mentioned above. Measurement enables the monitoring of the content of at least one desired gas in the bed gas. In accordance with the method gas is sucked from the fluidised bed through a sampling tube using a pump. The diameter of the tip section of the sampling tube is made larger than the diameter of the rest of the tube. Since the flow rate of the gas to be sucked into the sampling line is lower in the tip of the tube than in the actual line, less dust is sucked in with the gas than the composition of the bed would suppose and this enables continuous measurement. The gas is scrubbed in the sampling line of the dust that came with it, is preferably dried and kept in a temperature range that prevents the formation of harmful compounds. The amount of gas flow is measured and the gas is analysed continuously in one or more gas analysers. One of the analysers is preferably an oxygen analyser.

The equipment according to the invention is made up of a sampling tube placed in the fluidised bed reactor bed, with the diameter of the tip of the tube widened in relation to the diameter of the rest of the tube, and a gas line with thermostat, which line is connected to at least one gas analyser. The equipment also includes a pump to suck the gas into the equipment, said pump being essentially gas-tight, and at least one filter. Preferably the equipment also contains a gas-drying element.

The essential features of the invention will be made apparent in the attached claims.

It has been stated that the gas composition of a fluidised bed can be measured continuously using the measuring method and equipment now developed. Thus for example it is possible to see during a feed transition period when the reacting gas contents are in the desired zone. The equipment according to the invention is described further in the attached drawings, where

Figure 1 is a sketch of the equipment according to the invention, and Figure 2 shows a graph of the gas contents in a fluidised reactor bed as a function of time.

According to Figure 1 , the sampling tube or probe 2 belonging to the gas measuring apparatus 1 in accordance with the invention is located inside the fluidised bed 4 of the partially shown roasting furnace 3. The roasting furnace is equipped with an opening (not shown in detail in the drawing) through which the probe is conveyed. The diameter of the tip 5 of the probe is essentially greater than the diameter of the rest of the probe. Thanks to the wider tip the flow rate of the gas to be sucked into the sampling tube is at first lower and this decreases the amount of dust brought with the gas from the bed.

The diameter of the probe tip depends on the properties of the material in the fluidised bed, mainly the coarseness and density of the bed material. Roughly speaking, the finer the bed material, the greater the diameter of the tip should be too. For instance, when zinc and iron oxides are in the fluidised bed with an average particle size of d₅₀ = 150 μm, the ratio of the tip diameter to the actual probe diameter should be in the region of 2:1 , so that the flow rate of the gas in the probe tip falls to a quarter of the gas flow rate in the rest of the probe. Advantageously the ratio of the diameter of the tip to the diameter of the rest of the probe is in the region of 1.3 - 3:1. Correspondingly, the gas flow rate in the probe tip is in the region of 0.6 - 0.1 times that of the gas flow rate in the probe itself. The material of the sampling tube is chosen to be durable in fluidised bed conditions.

The probe is connected to the thermostatically controlled gas line 6 to prevent the sample gas from condensing. For example, when measuring gases containing sulphur dioxide, the temperature in the equipment is kept constant in the range of 120 - 200°C, preferably around 170°C. Outside the furnace the temperature of the gas drops quickly, unless the gas line is heated e.g. with a heating resistor. If the sample gas entering the probe in the roasting furnace gas cools too much, the sulphur dioxide contained in it is absorbed into water droplets and this affects the sulphur dioxide analysis results. The condensation of sulphuric acid on the inside of the sampling tube can also damage the tube. Other devices used in determining flow measurement and gas composition can be connected to the apparatus, which devices are not shown here in detail. For instance, a rotameter measuring the amount of flow has been connected to the apparatus, but since its use is a normal technique, it has not been described here in more detail.

The gas containing a great deal of dust is sucked into the equipment using a gas-tight pump 7, but the majority of the fine dust included should ideally be separated from the gas even before the pump using the first filter 8. If required the gas line can also be equipped with another filter 9 before directing the gas to the gas analysers. The drawing shows two analysers 10 and 12, of which the latter is an oxygen analyser. One analyser can be used to analyse other gases such as the water vapour content and sulphur dioxide content in the gas etc. The drawing shows a dryer 11 before the oxygen analyser. The first gas analyser may be for instance a continuous FTIR gas analyser or equivalent and the oxygen analyser may be used e.g. in parallel with the previous, paramagnetism-based oxygen analyser. The analysers may also be other types than those mentioned above.

The graph in Figure 2 is a sample of gas composition monitoring of a roasting furnace bed performed with equipment according to the invention. It shows that it is possible to perform continuous measurement of gas composition directly from a fluidised bed furnace bed and not only from gas removed from the furnace space. The method and equipment for measuring gas composition from a fluidised bed reactor bed according to the invention is presented above for roasting furnace conditions, but of course the method and equipment can also be used in other fluidised bed reactors.

Claims

PATENT CLAIMS

1. A method for a measurement of the content of at least one gaseous substance from a gas of a fluidised bed reactor bed, characterized in that in the continuous measurement the gas is sucked from the fluidised bed reactor bed using a pump into a sampling tube, so that the flow rate of the gas in a tip of the tube is maintained at a lower rate than that in the other parts of the tube, the fine-grained dust coming with the gas is separated from the gas line by filtration and the gas is routed to at least one analyser, with which the content of the desired substance in the gas is measured.

2. A method according to claim 1 , characterized in that the gas flow rate in the tip of the sampling tube is adjusted on the basis of the coarseness of the bed material.

3. A method according to claim 1 , characterized in that the gas flow rate in the tip of the sampling tube is adjusted on the basis of the density of the bed material.

4. A method according to claim 1 , characterized in that the gas flow rate in the tip of the sampling tube is adjusted to be within the range of 0.6 - 0.1 times the gas flow rate used in the actual sampling tube.

5. A method according to claim 1 , characterized in that the temperature of the gas in the gas line is maintained between 120 - 200°C.

6. A method according to claim 1 , characterized in that the gas is dried before being routed to the gas analyser.

7. Equipment for a measurement of the content of at least one gaseous substance from a fluidised bed reactor (3) bed (4), characterized in that the equipment (1) intended for continuous measurement is composed of a sampling tube (2), which is connected to a gas line (6), a gas-tight pump (7) and at least one filter (8,9), which are located ahead of at least one analyser (10,12) for measuring gas content, whereby the diameter of the tip (5) of the sampling tube placed into the fluidised bed is made larger than the diameter of the actual sampling tube (2).

8. Equipment according to claim 7, characterized in that the ratio of the diameter of the tip (5) of the sampling tube to the diameter of the actual sampling tube (2) is between 1.3 - 3:1.

9. Equipment according to claim 7, characterized in that the gas line is equipped with heating.

10. Equipment according to claim 7, characterized in that number of gas analysers is at least 2.

11. Equipment according to claim 7, characterized in that the equipment is equipped with a dryer (11 ).