FI64997B - FOERFARANDE FOER TILLVARATAGANDE AV VAERME UR GASER INNEHAOLLANDE VAERMEYTOR NEDSMUTSANDE AEMNEN - Google Patents

Description

Method for the recovery of heat from gases containing pollutants on the heating surface - Method for the recovery of heat from gases containing pollutants in the heat exchanger

The present invention relates to a method for recovering heat from a gas containing vaporized, molten and eutectic components by contacting it with the heat surfaces of a heat exchanger.

The process industry produces large quantities of hot gases, the heat recovery of which is substantially hampered by the vaporized or molten components contained in the gases, which badly contaminate the heating surfaces. A typical example is waste gases from the pyrometallurgical industry. The cleaning of heating surfaces is in many cases extremely difficult with the methods currently used, which results in a reduction in the availability of the plant and thus in high costs.

Experience has shown that purification problems are greatest in a process-specific temperature range where some of the solids are in the eutectic state. For example, in non-ferrous smelting processes, low concentrations of Zn, As and Pb are often sufficient to produce the eutectic behavior of all dust. Dust in the eutectic state adheres to thermal surfaces and, especially when crystallized, forms a layer of dirt, the removal of which by known cleaning methods (blowing or mechanical soot removal) is in some cases an impossible task. Field studies have shown that the best usability values have been achieved in steam boilers where, due to the nature of the process, natural erosion of the deposits has occurred. It has also been clear from the shape of the dirt layer that even effective blowing or mechanical sootstocks have not been able to significantly affect the dirt layers. Instead, erosion has kept the heat surfaces in the flow direction reasonably clean.

From these observations, the idea has emerged to take advantage of controlled erosion to clean heating surfaces that are otherwise difficult to keep clean. The process according to the invention for heat recovery based on controlled erosion described below is characterized in that the gas temperature is dropped below the eutectic temperature range of the melt droplets before the heat transfer by mixing cooled, gas-separated, recycled solids and possibly other particles in the heat exchanger.

The figure shows an implementation of the heat recovery method according to the invention.

In the device shown in the figure, a hot gas containing vaporized and molten components flows through a channel 1 provided with radiating surfaces. The gas temperature as the heat exchanger 2 approaches is close to the upper limit of the eutectic region. The temperature after the heat exchanger is chosen sufficiently below the eutectic temperature range so that the dust contained in the gas is powdery in nature and does not adhere to the heating surfaces. The soot effect required to clean the heating surfaces is achieved when the soot dust is enriched in the heat exchanger to such an extent that, when mixed with the dusty gas entering the heat exchanger at point 3, it drops the temperature of the mixture close to the eutectic range.

After stirring in step 3 and dropping the temperature, the suspension containing sufficient abrasive particles flows through the heat exchanger 2, preventing the growth of dirt layers on the heating surfaces by erosion.

After the heat exchanger 2, the suspension has cooled below the eutectic region and is passed tangentially to the channel 4.

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3 to the flow-through cyclone 5, from which the almost dust-free gases leave through the central pipe 6 and the separated solid is returned through the pipe 7 to the gas flow in the channel 1 at point 3 before the heat exchanger. A return solids outlet 8 is provided in the return pipe 7, so that the circulating solids flow and the erosion effect can be adjusted. The circulating material can preferably be the process's own solid material or other inexpensive material, such as, for example, sand, which is fed to the device via a pipe 9. The method according to the invention achieves the following advantages: 1. The controlled erosion effect ensures that the heating surfaces remain clean.

2. Stirring causes the temperature to drop rapidly.

3. A so-called dry cleaning effect because circulating solid particles condense compounds that have entered the surface in the vapor phase.

4. Sulfur emissions can be reduced by arranging, for example, Ca-based circulating material.

5. Radiation and convection heat transfer become more efficient.

The operating ranges of the method according to the invention are as follows:

Gas velocity 3 - 20 m / s

Gas particle content 10 - 500 g / mol

Gas inlet temperature 300 - 1500 ° C

Gas outlet temperature 500 - 1200 ° C

The average particle diameter is 100 to 2000 μm

Example 1:

Exhaust gas values of the Cu smelter after the melting furnace gas flow mol / s 1740 dust content g / mol 2.7 temperature ° C 1400.: I ’4 The heat capacity of the dusty exhaust gas is approx. 1.7 kJ / (Nm3 ° C) = 38 J / (mol ° C) r, ie the heat capacity flow is 66.1 kW / ° C. The suitable temperature before the heat surfaces of the heat exchanger 2 is 700 ° C and after the heat surfaces 550 ° C. The circulating heat capacity flow is thus 88.1 kW / ° C. The specific heat capacity of the circulating material can be estimated to be about 0.8 kJ / (kg ° C), which results in a value of 110 kg / s for the circulating mass flow, i.e. after mixing the solids content of the gas 3 is 63 g / mol (= 2.81 kg / (Nm)). In practice, solids concentrations of 900 to 1400 g / mol have been used in the so-called circulating mass reactor, and concentrate, sand or a mixture of these can be used as a circulating material in the smelters, and particles in the exhaust gas are enriched in the cooling circuit.

Example 2:

The black liquor flow of the soda boiler is 5.6 kg / s and its dry matter content is 60%. A typical analysis of dry matter is as follows: C 35.5% (by mass)

Na 20.8% S 5.2% 0 35.1% H 3.4%

If the combustion is carried out in a normal recovery boiler, flue gases from the furnace follow about 30% of the introduced sulfur and 10% of the introduced sodium, partly as gaseous compounds and partly as small melt droplets. If combustion is carried out in a separate combustion chamber, the flue gases may contain up to 50% sulfur and 30% sodium after the combustion zone. As the flue gases cool, the inorganic chemicals mainly form sodium sulfate and sodium carbonate, as well as sulfur dioxide. Depending on the composition of the lye and the driving conditions, this may in some cases lead to the formation of a difficult layer of sodium pyrosulphate on the thermal surfaces.

Em. in the case of a combustion chamber, the values of the exhaust gases are gas flow mol / s 840

Na current mol / s 4.56 S current mol / s 2.75 temperature ° C 900 dust (cond) g / mol 0.23 (10.3 g / Nm ^)

Gas heat capacity flow 29.4 kW / ° C

Gas temperatures:

before the transfer 870 ° C

after stirring 700 oc

after the transfer 550 ° C

Circulating heat capacity current 33.0 kW / ° C

Circulating mass flow (0.8 kJ / kg ° C) 41.7 kg / s

Dust content of the gas in the converter 50 g / mol

The circulating stream consists of Na 2 O 2 -based dust from the flue gases and ^ 2 (30 ^ or Na 2 SO 4) added to point 3.

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Claims (5)

A method for recovering heat from a gas containing melt droplets by contacting it with the heat surfaces of a heat exchanger, characterized in that the gas temperature is dropped below the eutectic temperature range of the melt droplets before mixing the heat by mixing other gas-cooled particles. A method according to claim 1, characterized in that the temperature of the exhaust gas of the Cu smelter before the heat exchanger is lowered to about 700 ° C. Method according to Claim 2, characterized in that sand is added to the circulating stream. A method according to claim 1, characterized in that the temperature of the gas coming from the combustion chamber of the recovery boiler is reduced to about 700 ° C before the heat transfer. Process according to Claim 4, characterized in that sodium sulphate and / or sodium carbonate are added to the circulating stream. • j;.; 3 II