Quality control

Electricity companies in the Netherlands have long put the fly ash and bottom ash produced by coal fired power plants to good use in the construction industry, but the key question is whether co-firing with biomass and other secondary fuels reduces the quality of the fly ash. Extensive research performed by KEMA has proven that co-firing fly ash can meet the strictest standards, depending on the nature of the secondary fuel.

Ruud Meij, Angelo Sarabàƒ¨r, Henk te Winkel, Kema, The Netherlands

As a result of the Kyoto Protocol and agreements between energy companies and the Dutch government, coal fired power plants are required to contribute to the reduction of the greenhouse effect. Indirectly, that is already taking place. Dumping of the produced ash is not allowed in the Netherlands, and fly ash has already been used for decades as a substitute for cement in concrete, resulting in a substantial reduction in the consumption of cement per m3 concrete.

By substituting coal with biomass, the energy sector’s contribution to environmental improvement can also be increased. Agreements with the Dutch government are in place to reduce CO2 emissions by 3.2 million t between 2008 and 2012 by replacing coal with biomass fuels. This amounts to 13 per cent of the coal in terms of energy and about 20 per cent in terms of dry mass.

Specific composition

The composition of biomass and other secondary fuels can vary greatly in terms of net heating value, ash content and ash composition. This means that at increasing co-firing percentages, thorough research into the effects of co-firing on the quality of the powder coal fly ash is essential. In assessing the suitability of biomass it is not just the technical requirements but also the regulations, the occupational exposure, the environment and perception that play an important role. Only when all of these conditions are met will all parties see fly ash as a satisfactory substitute material.

Increasing co-firing percentage

Since 1993, Kema has investigated fly ash produced by co-firing with increasing percentages of various types of biomass. In 2001, co-firing tests with demolition wood, refuse derived fuels (RDFs) and chicken manure were performed in Kema’s 1 MWth test boiler to generate fly ash for pilot scale research purposes. Thereafter, co-firing tests with very high percentages of co-firing fuels in coal fired power plants were performed to investigate these types of ash produced in real-scale situations.

In technical materials research, the highest-grade application serves as the starting point. The key questions to consider are whether fly ash used as a partial replacement for cement can make the same contribution to the hardening of the concrete, its workability and its durability. The research has shown that, depending on the nature of the fuel, high co-firing percentages are possible without a negative effect on the quality.

Figure 1. Shredded wood
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Taken as a whole, it appears that fly ash with high percentages of biomass can meet the European standards. Recent real-scale tests have shown that in some cases even with a co-firing percentage of 40 per cent (fuel based) the quality requirements can be easily met.

Figure 2. Gelderland power plant, Electrabel Nederland
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The chemical and mineralogical characteristics of the fly ash appear to be more important than the co-firing percentage. To gain proper insight into the relationship between biomass and fly ash, it is therefore important to analyse all the sequential steps in their proper interrelationships: fuel – combustion – fly ash – performance. The chemical and mineralogical aspects of the ash from the co-fired fuel is the determining factor.


The legislation of the Netherlands for the use of fly ash in concrete and mortar has since been adapted to accommodate this new information (CUR-Recommendations). No restrictions are placed on the maximum co-firing percentages, but requirements are established pertaining to the material level (physical, chemical and mineralogical characteristics) and the performance level (examples include: at least 25 per cent reactive SiO2; only certain minerals; max. 5 per cent P2O5, Na and K; and max. 4 per cent MgO and reactive CaO). On this basis, fly ash from co-firing processes can be used, just like coal fly ash, in a responsible manner as a high-grade material in concrete.

European Waste Catalogue

In addition to the technical research, investigation of the environmental and occupational exposure impacts of co-fired ash is also important. The governments of the Netherlands and the European Union both have established standards for the environment and working conditions. The Dutch Building Materials Decree is intended to limit the leaching of heavy metals and other compounds into the ground. That applies to both the bound and unbound application of construction materials. Furthermore, the European Waste Catalogue has an important role. This concerns a list of more than 800 waste types that indicates whether or not they have an impact on the environment. Fly ash from 100 per cent coal firing, by definition, is classified as a non-hazardous material. Co-firing fly ash has to be examined. Limits have been established for 14 classes of hazardous materials varying from toxic and carcinogenic to corrosive and mutagenic. The co-firing ash is analysed and tested for all relevant classes. Whether a material is hazardous depends not so much on the element itself as on the compound in which it is present (speciation). It makes a difference, for instance, whether a material is present in the fly ash as a free element, an oxide or as a sulphate. That knowledge is required (to nano level) in order to produce a good assessment.


All types of ash examined so far appear to be non-hazardous according to the EWC criteria. The co-firing of biomass sometimes has a positive rather than a negative environmental effect. For normal co-firing practices, the energy producer can therefore assume that the environmental effects are far under the limits. Whereas for new secondary fuels with high heavy metal concentrations or higher co-firing percentages, further research is required. Here, too, the chemical characteristics and presence of minerals appear to be more significant for the quality than the co-firing percentage.

Figure 3. Microscopic picture of fly ash
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Research into occupational exposure in situations involving fly ash from coal fired power plants dates back to the 1980s. Kema is now carrying out similar research into the occupational exposure for situations involving co-firing ashes in the energy and building sectors. The assessment is based on TLV-values, the Threshold Limit Values, for hazardous compounds. Related questions that must be answered include: which compounds are found in fly ash, which of these have an associated TLV-value, and are these TLV-values exceeded at a maximum exposure to fly ash? The results of the tests indicate that the co-firing ash types must be considered nuisance (non-toxic) dust. No extra health risks are encountered under conditions that generally meet the requirements established for nuisance dust in the workplace. The tests also indicate that the co-firing ashes should be classified as non-carcinogenic materials. Even 40 per cent co-firing (on a dry mass base) of paper sludge, waste wood, chicken manure and RDF of average composition does not lead to exceeding the limits for individual compounds, at a maximum acceptable exposure of 10 mg of inhalable fly ash particles per cubic metre.

Expert model

It is of little economic value to determine after the fact, once the quality of the ash is known, whether the quality of the fuel mixture was suitable or not. For this reason, Kema has developed an expert model that predicts the composition of the fly ash and the resulting emissions. The complicated European regulations are integrated into this Kema Trace Model (KTM). With the model, energy companies can determine in advance which mix of fuels they can burn and in which proportions. The KTM model has been validated for 100 per cent coal firing and co-firing. Kema is carrying out large-scale measurement campaigns to determine whether elements continue to behave in the same manner when the percentage increases to levels as high as 40 per cent on a dry mass base.

Figure 4. Composition of fly-ash sample presented with the KEMphase viewer
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