Updated: July 2002

1. Purpose

It is more advantageous to treat exhaust gas under low temperature condition when removing at high efficiency the toxic substances such as HCl, SOx, heavy metals, dioxins, etc. in the exhaust gas from waste incinerator. On the other hand, for NOx removal by use of catalyzer, an operation at a higher temperature side of more than 200ºC is considered effective.
Up to now, however, in order to remove them in a well-balanced condition, operation thereof has been restricted to a limited temperature range, thereby making it inevitable for the both to sacrifice the important operating efficiency to a certain extent.
The activated coke has an activity to function as a denitration catalyzer at a comparatively low temperature and yet an absorptivity, thus enabling trace toxic substances as well as NOx to be simultaneously removed at a low temperature.

2. Activated Coke

The activated coke is a carbon material developed for denitration/desulfuration exhaust gas, and has delivery results to dry-type exhaust gas treatment plants in electric power plant and refineries. It works as an absorbent having an intermediate characteristics between those of activated carbon and coke, and has a capacity not only to perform removal by absorption of trace toxic substances such as Hg, dioxin, etc., but also to conduct reducing denitration as catalyzer in a comparatively low temperature range (110-200ºC). The specific surface is 15-400 m²/g, which corresponds to ½-1/3 compared with these of the activated carbons. It has a large number oxygen-contained functional groups on the inside surface of the pore, which conduct the denitration thereof.
When operated at a low-temperature in connection with the bag filter, the Hg, dioxins, etc. can be removed in a high degree by the function of both the bag filter and the activated coke, while NOx is subjected to reducing decomposition and then is reduced down to a low concentration level by the activated coke.

3. Performance Data

Results shown below are of the experiment obtained by use of a pilot plant.
1) Denitration effect
The higher the temperature, the higher the denitration ratio can be exhibited in a trend but a denitration ratio of 59% was obtained at a considerably low temperature of 120ºC. As seen from a 86% that reached at 180ºC, a high denitration ratio was exhibited for activated coke even in a sufficiently low temperature range of below 180ºC.
2) Removal effect of dioxins
Since almost all the dioxins can be removed by a bag filter, the concentration at inlet is low (20-80 mg/m³N), however, a removal ratio of more than 50% was obtained at a temperature of up to 150ºC. Combination with a bag filter has made it possible to achieve a removal ratio of 97%.
3) Removal effect of Hg
Regardless of a temperature range, Hg was removed by absorption up to a level below the detective critical (threshold) value under every condition. Although almost all the heavy metals contained in the exhaust gas can be eliminated by more than 99% by the bag filter, perfect elimination of Hg alone was a bit difficult.
With the pilot plant employed, it was proved that an activated coke over tower installed at a pot-stage of the bag filter could eliminated the amount of Hg almost completely and yet stably.

Results of Hg removal (Unit: mg/m³N, O2: 12%)

4) Summary
It was proved that the activated coke has a sufficient denitration capability in a low temperature range (120-180ºC), and, in addition, a performance capability to remove by absorption the dioxins and Hg.