data required to design a flue gas processing unitReading time:
We need to know the nominal flow rate and composition of the flue gases to be processed together with the temperature of these flue gases as they enter into the unit. The flow rate is expressed in Nm3 ·h–1 (or in m3 ·h–1 effective with the temperature). The composition will be quoted by volume or by weight and must be used to calculate CO2, H2O, N2, O2 steams rates (from which can be deduced relative humidity and saturation temperature). The flow of pollutants can be provided or estimated. These pollutants vary in nature and must be qualified and quantified because they will have an impact on the choice of treatment and/or the technologies applied.
The dust level must be known (in g ·Nm–3). This dust level will be fundamentallyn different depending on whether we have dedicated fluidised bed incineration (20 to 50 g · Nm–3) or pyrolysis/thermolysis (1 to 5 g · Nm–3). Dust levels will also depend on the nature of the sludge (whether or not digested).
With regard to sludge, this type of pollution is primarily caused by chlorine and, to a lesser extent, by fluorine. In the absence of any specific data on urban wastewater sludge, the default value will be 1 g of Cℓ–per kilo of sludge dry solids content. Empirically, the fluorine content will be one tenth of the Cℓ– content.
This is one of the most important items of data although frequently poorly defined. In this case, the default value will be 5 g of S per kilo of dry matters. N.B. in the case of dedicated fluidised bed incineration, a proportion of the SO2 and SO3 will be adsorbed direct by the mineral ash that forms the flue gas dust content.
volatile heavy metal pollution
This is mainly Hg and, to a lesser extent, Cd (we should also mention Thallium). Given the sensitive nature of this treatment and of the low levels accepted in emissions, we strongly recommend specific analysis are undertaken. Once again, when these are not available, we can refer to an empirical 4 mg Hg · kg–1 of dry matter. A non-negligible proportion of volatile mercury will chelate during the gas phase with Cℓ– in order to form an HgCℓ2 that is not volatile but that is soluble in water, especially under acid pH conditions.
non-volatile heavy metal pollution
Although volatile, when Pb, Zn and As have been chelated with other compounds, they behave like the other non-volatile compounds. All these metals (table 14) are fixed by mineral ash. Therefore, this creates ash problems and, more particularly, problems associated with behaviour in leaching tests that the ash will need to undergo in order to establish its leachable fractions and, consequently, how it will be disposed of (in France, governed by the provisions of the December 1992 decree).
We need to highlight the special case of Cr (tannery sludge) where pyrolysis processes allow the Cr to stay as Cr(III) whereas it would be oxidised as toxic and very easily soluble by Cr (VI) incineration.
nitrogen oxide (NOx) pollution
This type of pollution is difficult to anticipate and the choice of treatment tends to be based on feedback. It should be noted that some local regulations are more stringent than the recommendations laid down by European directives (200 mg ·Nm–3). It is acknowledged that this threshold is rarely reached in sludge heat treatment plant and that an SNCR type treatment is enough to guarantee this threshold (see section flue gas treatment types). On the other hand, should more stringent requirements apply and, more specifically, to the 70 mg · Nm–3 thresholds (equating to the Dutch standard), only SCR type solutions will apply.
This type of pollution is equally difficult to anticipate; however, the compulsory 0.1 ng ·Nm–3 emission threshold will always require the use of a treatment.
the flue gas plume
Specifications frequently demand that this be suppressed. The appearance of a plume is governed by the positioning of the humid air diagram point that is characteristic of flue gas relative humidity and by hygrometry parameters defined by meteorology. Therefore, if necessary, this point has to be shifted by, for instance, heating the flue gases.