Process for energy recovery in carbon black production

Abstract

A combustible gas from carbon black production is utilized in a gas engine by adding an oxygen-containing gas to the combustible gas, passing said mixed gas over a selective catalyst, which is active for oxidizing H.sub.2S to SO.sub.2 but substantially inactive for oxidation of CO, H.sub.2 and other hydrocarbons with less than 4 C-atoms, passing the converted gas through an SO.sub.2 removal step, and passing the cleaned gas to a gas engine or to an energy recovery boiler. This way, the tail gas from carbon black production, which is normally combusted in a CO boiler or incinerated, can be utilized to good effect.

Claims

1. A method for utilization of a combustible gas from carbon black production in a gas engine, said method comprising the steps of (a) adding an oxygen-containing gas to the combustible gas in an amount corresponding to an O.sub.2 content of 0.5-5% in an outlet of a carbon black production facility, (b) passing said mixed gas over a selective catalyst, which is active for oxidizing H.sub.2S to SO.sub.2 but substantially inactive for oxidation of CO, H.sub.2 and other hydrocarbons with less than 4 C-atoms, (c) passing the converted gas from step (b) through an SO.sub.2 removal step, and (d) passing the cleaned gas from step (c) to a gas engine.

2. The method according to claim 1, wherein the cleaned gas from step (c) is passed to an energy recovery boiler.

3. The method according to claim 1, wherein the selective catalyst consists of one or more metal oxides, in which the metal is selected from the group consisting of V, W, Ce, Mo, Fe, Ca and Mg, and one or more supports taken from the group consisting of Al.sub.2O.sub.3, SiO.sub.2, SiC and TiO.sub.2, optionally in the presence of other elements in a concentration below 1 wt %.

4. The method according to claim 3, wherein the selective catalyst comprises from 1 wt % to 50 wt % V.sub.2O.sub.5.

5. The method according to claim 1, wherein a monolithic type catalyst is used with the further purpose of avoiding plugging in the system.

6. The method according to claim 1, wherein the selective catalyst is integrated inside the wall of a ceramic candle dust filter.

7. The method according to claim 1, wherein the selective catalyst is integrated inside the wall of a fabric dust filter bag.

8. The method according to claim 1, wherein said SO.sub.2 removal step is carried out by using a wet scrubber and an alkaline reagent, an oxidative reagent, or combinations thereof.

9. The method according to claim 8, wherein said SO.sub.2 removal step is carried out by using a dry scrubber and an alkaline sorbent.

10. The method according to claim 1, wherein said SO.sub.2 removal step is carried out by using a wet scrubber and an alkaline reagent, an oxidative reagent, or combinations thereof, or wherein said SO.sub.2 removal step is carried out by using a dry scrubber and an alkaline sorbent.

Description

EXAMPLE

(1) According to the method of the invention, a carbon black tail gas containing 400 ppm NH.sub.3 and 2000 ppm H.sub.2S with added air is passed over a selective catalyst. The effluent from the catalyst, containing 400 ppm NH.sub.3 and 2000 ppm SO.sub.2 is fed to a scrubber. Three different feed scenarios are considered as shown in Table 1 below.

(2) TABLE-US-00001 TABLE 1 Feed stream to scrubber scenario 1 2 3 Temperature 40 C. 40 C. 40 C. Pressure 1 atm 1 atm 1 atm Flow (Nm.sup.3/h) 10000 10000 10000 NaOH (kmol/h) 0.8 1.55 0 H.sub.2O.sub.2 (kmol/h) 0 0.83 0.89 H.sub.2O (kg/h) 1500 400 100

(3) In the calculation with H.sub.2O.sub.2 without any NaOH (scenario 3) it has been assumed that all SO.sub.2 is oxidized to H.sub.2SO.sub.4. In reality, a surplus of H.sub.2O.sub.2 is required. This will lead to a very acidic liquid and an equilibrium partial pressure for SO.sub.2 and NH.sub.3 around 0. In practice, a few ppm SO.sub.2 will most likely remain, dependent on H.sub.2O.sub.2/SO.sub.2 ratio and contact efficiency.

(4) In the scrubber, NaOH and/or H.sub.2O.sub.2 is added at pH around 6. The liquid effluent from the scrubber contains salts, i.e. ions (NH.sub.4.sup.+, Na.sup.+, HSO.sub.3.sup., SO.sub.3.sup.2, HSO.sub.4.sup., SO.sub.4.sup.2), in water. The emissions from the scrubber are SO.sub.2 and NH.sub.3. Regarding the NH.sub.3 emissions it is noted that at pH<1, the equilibrium vapour pressure is practically 0. The actual value depends on the scrubber design.

(5) The calculated results are summarized in Table 2.

(6) TABLE-US-00002 TABLE 2 Liquid effluent and emissions scenario 1 2 3 Liquid effluent liquid (kg/h) 1594 523 189 salts (wt %) 5.9 23.6 47.1 pH 6.23 5.75 <1 Emissions SO.sub.2 (ppm) 20.9 17.2 20 NH.sub.3 (ppm) 5.3 5.4 5