21. Combustion ashes and boiler deposits

E. Combustion residues: ashes, boiler deposits, cremation

The scope of investigation expressly covers the ash residues remaining on the grate, bottom ash, furnace residues and deposits on chromium-alloyed hot components, in addition to fly, filter and boiler ashes. These fractions are different sample matrices. Chromium may originate from the material being incinerated; in the event of contact with chromium-alloyed components, their surface/corrosion chemistry must also be taken into account. Reactive calcium and alkali components may come from the residues themselves. Insulation as a calcium source is not a prerequisite for this. [11, 26, 31-34]

Paoletti 2002: calcium chromate in waste incineration

As early as 2002, Paoletti investigated the behaviour of oxyanion-forming heavy metals in municipal solid waste incineration at the German research centre Forschungszentrum Karlsruhe (dissertation, University of Stuttgart, FZKA 6755). Thermogravimetric experiments with Cr₂O₃ and CaO in synthetic air showed a rapid increase in mass between 500 and 850 °C. After one hour at 850 °C, X-ray diffraction detected hexavalent CaCrO₄. In an inert atmosphere, by contrast, only trivalent Ca(CrO₂)₂ was formed. [50]

Paoletti concludes that hexavalent chromates can only be formed “if the oxygen partial pressure in the combustion chamber is sufficient”. She also expects similar reactions with sodium, potassium, magnesium, strontium and barium. In the TAMARA pilot plant, 85 to 100 % of the chromium dissolved in the DEV-S4 eluate of the grate ashes was present as Cr(VI) at air ratios of 0.7 to 1.5; at an air ratio of 0.6, the release fell by almost two orders of magnitude. [50]

Vehlow, Paoletti, Seifert and Sirini presented the results in 2004 at the WTERT Fall Meeting at Columbia University in New York, including the thermogravimetry on Cr₂O₃ and CaO with the product CaCrO₄. The formation pathway had thus also been presented at international specialist level since 2004. [51]

Thermal post-treatment of fly ashes

In 2015, Hu et al. describe the oxidation of chromium in waste incineration by alkali and alkaline earth metals. The publisher's abstract evaluated highlights the availability of free CaO; alkali components promote oxidation at lower temperatures in connection with flue gas cooling. Calcium bound in sulphates or aluminosilicates must be considered separately with regard to its reactive availability. Total calcium or an oxide content stated as CaO alone therefore does not yet describe the conversion to calcium chromate. [31]

Hu et al. (2013) had already investigated fly ashes from five Chinese waste incineration plants and their thermal post-treatment at 1,023-1,323 K, corresponding to approximately 750-1,050 °C, in air or nitrogen. The publisher's abstract describes the formation of CaCrO₄ from Cr₂O₃ and CaO during thermal treatment. The ashes initially investigated, by contrast, show only small Cr(VI) fractions and low leaching. The result concerns the change caused by post-treatment; it is not a general statement about all raw ashes. [32]

Gong et al. (2017) add to this aspect for a Chinese fly ash from flue gas cleaning: after thermal treatment, chromium leaching increases, while the leaching of several other metals decreases. The authors attribute the change to Cr(VI) compounds such as CaCrO₄. Total chromium was measured in the eluate; the calcium chromate pathway is the authors' mechanistic explanation, not an independent Cr(VI) speciation proof from this series of measurements. Improved immobilisation of individual metals therefore does not permit any general all-clear with regard to chromium. [33]

Grate ashes during shutdown: temperature profile and oxygen ingress

During shutdown for maintenance, ash residues remain on a chromium-containing grate or on chromium-alloyed hot component surfaces while temperature and atmosphere change. This process must be distinguished from the transport of fly dust during ongoing operation.

Paoletti provides the building blocks for this process: calcium oxide is present in household waste at around 3 %; calcium chromate forms between 500 and 850 °C and is reduced back to trivalent chromate above 850 °C; oxygen is an essential prerequisite. During shutdown, the grate and ash residues pass through precisely this temperature window, while additional oxygen enters through burnout and opening. Additional chromate formation or enrichment in the remaining residues is therefore chemically to be expected. [50]

The importance of the atmosphere is also confirmed by Leaching behavior of heat-treated waste ash (2006): the authors investigate bottom ashes from two Japanese municipal grate firing plants and vary temperature, oxygen concentration and treatment time. Thermal treatment with less oxygen reduces the leachable Cr(VI). [43]

That this effect is known is shown by a wood ash process that deliberately allows oxygen contact only below 200 °C in order to prevent Cr(VI) formation (Brill 2002, cited in the 2019 report of the German Environment Agency, p. 72). The report also considers an inert post-treatment of cremation ash. [44]

Direct comparative measurements at waste incineration plants before and after shutdown are not yet available; they would determine the order of magnitude, not the mechanism.

For a specific ash or deposit problem, the fraction, plant, operating state, sampling, Cr(VI) analysis and activity must be assigned.

Total chromium measurement, leaching tests and personal dust measurement each answer different questions.