EP0421979B1 - Process for diminishing the sulfur emission in sintering processes - Google Patents

Description

The invention relates to a method for reducing pollutant emissions in thermal processes, such as in particular sintering processes, in which a fuel-containing mixture, in particular a coke bed, is ignited.

From EP-A-0 39 305 a method for reducing pollutant emissions during sintering has already become known, in which a foreign rust coating is applied as an intermediate layer between the raw sintered layer and the sintered grate. The proposed intermediate layer consists of granular material, which should be able to remove pollutants. This granular sorbent material is moistened for significant pollutant emissions, with particularly large effects being achieved with basic slurries or liquids. Lime milk was preferably given as the basic slurry, a correspondingly larger volume of sulfur-containing waste products being formed in addition to the coke ash. In such sintering plants, for example, preliminary products for metal extraction from ores are produced, ore mixtures, concentrates, metallurgical recycling materials being fed onto the sintering plant together with coke breeze. For a layer thickness of approximately 40 cm of the sintered mixture, layer thicknesses for the additional rust layer between 2.5 and 15 cm are proposed in order to achieve an effective desulfurization.

From DE-A 36 35 206 a method for burning fossil fuels with the development of flue gases with reduced sulfur content has become known. In this process, the fuel particles are soaked in lime milk before being introduced into the furnace and then introduced into the furnace dry.

The invention now aims to provide a method of the type mentioned in the introduction, in which, without increasing the amount of materials to be handled, enriched with pollutants, a reduction in pollutant emissions, in particular an almost complete desulfurization of the exhaust gases can be achieved without complex flue gas desulfurization. To achieve this object, the invention consists essentially in the fact that the pore-rich fuel fraction, in particular the coke, of the mixture is rolled with Ca (OH) ₂ before the fuel-containing mixture is fed in. The measure of rolling the pore-rich fuel component, in particular the coke, of the mixture with Ca (OH) ₂ before the fuel-containing mixture was put in place was in no way obvious, since the prejudice of the experts was in particular that such additives to the pore-rich fuel component, coke, in particular, have a sensitive influence on the ignition temperature and in particular an increase in the ignition temperature. Surprisingly, it has been found that when dry hydrated lime is used, the ignition temperature of the coke remains almost unchanged. With the maintenance or lowering of the reaction temperatures, the conditions for effective desulfurization were significantly improved and well over 90% sulfur incorporation could be achieved with ease.

The process according to the invention is preferably carried out in such a way that, based on the amount of coke, 5-30% by weight Ca (OH) 2, preferably 10-25% by weight, are used in dry form and the coke is rolled together with the hydrated lime . In compliance with the specified limit values for the addition of hydrated lime, an almost unchanged ignition temperature of the coke could be maintained when rolling coke with dry hydrated lime.

Sinter coke is primarily used for such sintering purposes, as mentioned above, and in the process according to the invention it is preferred to use sinter coke in a grain size of 0.5-5 mm, preferably 1 to 3 mm.

It has proven to be particularly advantageous if the sinter coke to be used is dried before rolling, temperatures around 100 ° C. appearing to be particularly suitable as drying temperatures.

As already mentioned above, the sulfur incorporation could be further optimized by controlling the process temperatures and, according to a preferred procedure, the sintering temperatures are kept low.

The invention is explained in more detail below with the aid of examples.

Example 1:

1000 g of pre-dried sintered coke with a grain size of 1 to 3 mm were rolled with 125 g of dry lime hydrate and the non-adhering lime was removed by sieving with a 0.5 mm sieve. away.

Example 2:

1000 g of pre-dried sintered coke with a grain size of 1 to 3 mm were rolled with 250 g of dry lime hydrate and the non-adhering lime was removed by sieving with a sieve with a mesh size of 0.5 mm.

Carrying out the experiment:

Each of the coke and hydrated lime mixtures prepared in Examples 1 and 2, as well as a comparative sample consisting of 1000 g of untreated coke, was subjected to the following procedure.

A sample of the coke mixed with hydrated lime was placed in a retort and heated in a tube furnace with air supply at a heating rate of approx. 5 ° C./min. The exhaust gas composition was continuously measured during the heating phase. The ignition point of the sample mixtures was characterized by a significantly faster rise in the sample temperature and the beginning of C0₂ development. The ignition temperature of the sample of Example 1 was not determined and that of Example 2 was 495 ° C. For comparison, the ignition temperature of the untreated coke with a grain size of 1 to 3 mm is 490 ° C.

In a further test, the sintered coke samples pretreated according to Examples 1 and 2 were subjected to ashing. The ashing was carried out at three different temperatures, namely 900 ° C, 1000 ° C and 1100 ° C. The incorporation of sulfur into the ashes was determined by carrying out a sulfur analysis before and after ashing. The results of the sulfur incorporation in the ashes are shown in Figure 1, which is a block diagram of the sulfur incorporation in the ashes. In Fig.l the percent sulfur inclusion is shown on the ordinate and arranged on the abscissa, side by side, the results of the three ashing tests carried out with untreated coke and with the sintered coke breeze treated with hydrated lime. With 1 the sulfur incorporation is shown, which at one Ashing temperature of 900 ° C was reached, with 2 the sulfur binding, which was achieved with ashing at 1000 ° C, and with 3, the sulfur binding, which was achieved with ashing at 1100 ° C. From Fig. 1 it can be clearly seen that the untreated coke showed the lowest sulfur incorporation in the ash at all three ashing temperatures. The highest sulfur incorporation value, namely 20.7%, was achieved at the lowest incineration temperature (900 ° C).

Relatively balanced sulfur binding values at the three ashing temperatures could be achieved with the sinter coke pretreated according to the example.

In the ashing of the sinter coke pretreated according to Example 1, an average value of the sulfur incorporation of 86.8% could only be achieved at an ashing temperature of 900 ° C. In the ashing at higher temperatures, the sulfur incorporation values of around 64% are well below the values that could otherwise be achieved with sintered coke pretreated with hydrate lime.

Particularly high values for the incorporation of sulfur into the ash could be achieved with the ashing of the mixture of coke breeze and dry lime hydrate produced according to Example 2. In the Ashing at 900 ° C resulted in a sulfur binding of 97.1% and in that at 1000 ° C a sulfur binding of 93.8%.

Overall, it can be seen that the largest percentage of sulfur incorporation is achieved at the lowest ashing temperature and the relatively lowest percentage of sulfur incorporation can be achieved at the highest ashing temperature. In any case, however, the percentages of sulfur incorporation in the sinter coke pretreated according to the invention are at least four times as high as those which could be achieved in the ashing of untreated coke.

The sulfur emission via sintered exhaust gases depends on the sulfur content of the sintered coke. This sulfur burns primarily to S0₂. A reduction in S0₂ formation is theoretically possible even through a sulfidic bond in the sinter.

Rolling with hydrated lime ensures optimal distribution of the sulfide-forming calcium and thus close contact with the sulfur of the fuel. When the coke is burned during the sintering process, not only S0₂ is produced, but also CaS, because reducing conditions are also possible in the coke particle.

The temporary setting of the coke sulfur in the sinter reduces the S0₂ emissions through the sintered exhaust gas and helps to comply with or fall below the prescribed emission values.

Claims (4)

1. A method of reducing pollutant emissions in thermal processes, such as sintering processes in particular, in which a mixture containing fuel, in particular a coke bed, is ignited, characterised in that the pore-rich fuel component, particularly the coke, of the mixture is rolled with Ca(OH)₂ before the charging of the fuel-containing mixture.
2. A method according to Claim 1, characterised in that, in relation to the quantity of coke, 5-30 % by weight of Ca(OH)₂, preferably 10-25 % by weight, in dry form is used and the coke is rolled together with the hydrated lime.
3. A method according to Claim 1 or 2, characterised in that the sintered coke is used in a particle range of 0.5-5 mm, preferably 1 to 3 mm.
4. A method according to any one of Claims 1 to 3, characterised in that the sintered coke is dried prior to the rolling with hydrated lime.

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