Cerafil/Dry Scrubbing Applied to Incinerators

With tightening emissions legislation gas cleaning technologies including fabric bag filters, electrostatic precipitators, cyclones and wet and dry scrubbing units have been used in various combinations. An emerging technology for cleaning waste incineration off-gases is a filter plant based on low-density ceramic filter elements. Initially launched in 1991 Cerafil elements are high porosity, monolithic ceramic tubes that are closed at one end and have an integral T-flange at the other. They are therefore applied in filtration plants in much the same way as fabric filter bags. 

At Filtration '99  Scapa Filter Media highlighted two applications where low-density ceramic filter elements are employed in conjunction with dry scrubbing to treat off-gases from incineration processes. The first plant serves a clinical waste incineration facility at a hospital in Bilbao, northern Spain. The Bilbao plant employs sodium bicarbonate/powdered activated carbon injection for the control of acid gas and dioxin emissions. Key plant and operating variables are given in Figure 1.

FIGURE 1 ― BILBAO PLANT DATA

Since commissioning the plant has operated well, meeting the expectations of the OEM and the incinerator operator. Particulate emissions are continuously monitored and have remained well within the authorized limits. Similarly the outlet HCl concentration is continuously monitored and maintained at 5 mg/Nm³ again well within authorized limits. Dioxin measurements have been made twice yielding 0.01 and 0.02 ng/Nm³, extremely low figures given the inlet burden which was estimated at 30-40 ng/Nm³. This removal efficiency is ascribed to the combination of sorbent employed, filtration temperature and the scrubbing which takes place in the dust cake on the surface of the elements.

The second of the applications is the Sarp UK chemical waste incineration facility at their Killamarsh site which is situated near Sheffield in England. The ceramic filter plant which serves the incinerator was commissioned in October 1995. The plant incinerates flammable organic solvents and non-flammable contaminated aqueous waste from all industry segments in the U.K., Ireland and some European countries. The broad spectrum of liquid waste that is incinerated produces a similarly broad range of inorganic chemicals which pass through the process as solid ash to be captured by the filter plant. The ash consists mainly of transition metal oxides.

The plant runs at 2.5 tonne/h input of liquid waste with a maximum ash content of 3 percent w/w. The sorbent employed at the plant is sodium bicarbonate which is injected into the gas stream at the rate of 80-100 Kg/h. Sodium bicarbonate is often the favored sorbent for small to medium scale incineration applications. It is chemically effective and yields NaCl when reacted with HCl, which is easier to handle than the CaCl₂ that is generated by the reaction with slaked lime.

Figure 2 gives key plant and operating variables. The operating pressure drop is high, reflecting the nature of the particulate being collected. The particulate emissions from the plant are low and well within those prescribed by the Hazardous Waste Incineration Directive.

FIGURE 2 ― SARP PLANT DATA

* Original element type installed

** Approximately 70% of which is sodium

    bicarbonate sorbent

 ^  Iso-kinetic testing carried out in December 1997