Since the problem of acid soil low fertility status has been known for long, many agricultural practices have been recommended to overcome the problem.

The fuels in Sweden are solid biofuels as well as different combustible wastes, including municipal solid wastes (MSW). The share of fossil fuels, coal or oil, is very small. The ash content in the fuels differs considerably. Clean heartwood as in wood pellets has an ash content of 0.2-0.5%. Wood chips obtained from forest residues have an ash content of 2-4% as the proportion of bark increases. Waste wood, such as construction and demolition debris, usually has higher ash contents. For peat, a figure of 5% is generally used, although the content can be much higher. Waste materials have comparatively high ash contents: 25% in MSW is quite common and various sludges in the pulp and paper industry have ash contents ranging between 10 and 50%.

Two scenarios have been examined and compared in this study. These scenarios are:

— Scenario 1 (Fig. 8.1): The cacao shells are combusted in a bioenergy plant in the Netherlands. The ash produced is transported to a sea port in the Netherlands. From this port the ash is shipped to Cote d’Ivoire. The ash is transported by truck to a cacao plantation. The ash is used as fertilizer. The ash contains no nitrogen and not enough phosphorous and potassium. Therefore, extra NPK-containing fertilizer is used to fulfill the nutrient demand. This NPK fertilizer is produced in Cote d’Ivoire.

— Scenario 2 (Fig. 8.2): The cacao shells are combusted in a bioenergy plant in the Netherlands. The ash produced is transported to a mine in Germany as filling material. The fertilization of the cacao plantation is performed by means of an artificial fertilizer (NPK). The fertilizer is produced in Cote d’Ivoire.

The system has to be bounded for the analysis. The boundaries determine the scope of the study. There are four subsystems in this study of importance:

1. The cacao plantation subsystem

2. The cacao shell combustion subsystem

3. The cacao shell ash subsystem

4. The fertilizer production subsystem

Ecotoxicity is a key property from a regulatory point of view, H14 in the European Waste Catalogue. Although the EU is expected to issue rules on criteria 2011, today there is no legally binding rule. There is no accepted test procedure for ecotoxicity yet, and an H14 value is computed using knowledge of the chemical composition of the waste and of the ecotoxicity of the pure chemical substances.

There is a difference of opinion in Sweden on the level at which a waste is to be considered ecotoxic. When a chemical product contains more than 2.5% of an ecotoxic substance, it is hazardous enough for organisms to warrant labelling as such. Analyses of ash usually yield the elemental composition, very seldom the concentration of chemical compounds. A study for the Ash Programme suggested a conservative selection of substances for the key trace elements and recommended that this value, 2.5%, be used to determine whether an ash is ecotoxic or not. The environmental authorities are of the opinion that the limit should be 0.25% because this is the limit for aquatic organisms.

The key element in this discussion is zinc, an essential nutrient. To be conserva­tive and not underestimate the ecotoxicity, the above-mentioned study assumed zinc was present as zinc oxide. At approximately the same time, zinc oxide was reclassified as ecotoxic, which has severe consequences for bioash. The zinc content in clean wood ash recommended by the Swedish Forest Agency is up to

7,0 mg/kg, i. e. 0.7%, which is larger than 0.25%. Concentrations of approxi­mately 0.4% are not uncommon for bark ash.

One way of tackling this difficulty is to examine whether zinc oxide is a reasonable choice as a reference substance after all, rather than just a conservative one. Three investigations should be mentioned here:

• Van der Sloot at ECN utilised the database LeachXS on leaching properties of MSWI residues and concluded that zinc is present mostly as willemite, a zinc silicate (H. van der Sloot, oral communication).

• Sjoblom (2007) reexamined the chemical thermodynamics of zinc in combus­tion residues, in particular the solubility equilibria, and concluded that zinc is present probably as franklinite, a mixed iron and zinc oxide, rather than willemite.

• Steenari and Noren (2008) studied the binding distances for zinc in different combustion residues using extended X-ray absorption fine structure spectros­copy, and found that the distances agree with zinc being bound in silicates or aluminates. They also found that in fresh ash, some of the more soluble zinc compounds are also present in minor quantities. When ash is wetted, zinc is rapidly redistributed to insoluble compounds.

Thus, zinc does not occur as oxide in combustion residues, but rather in much more stable compounds. Choosing oxide in the ecotoxicity assessment was overly conservative and significantly overestimates the environmental impact of zinc in ash. Settling for a realistic model compound is not straightforward: it seems more

probable that zinc is found as willemite or a silicate, but there is no official assessment of these compounds. On the other hand, less probable franklinite but with similar properties has been assessed as non-ecotoxic.

At the end of the day, when there are doubts, an ecotoxicity test will clinch the discussion. In a study co-financed by the Ash Programme, the Swedish Waste Management Association and the Swedish Environmental Protection Agency, a battery of tests were performed on different combustion residues (Stiernstrom et al. 2009). Care was taken to choose organisms that naturally occur in brackish waters rather than the freshwater organisms that are sensitive to the salt concentration.

At a liquid-to-solid leaching ratio of 10 (waste test condition to be on the conservative side, but for chemical products the liquid-to-solid leaching ratio should be 10,000 according to OECD good practice) almost all residues were ecotoxic. A more detailed examination revealed that the ecotoxicity is due to the high concentrations of potassium and calcium (nutrients) and of aluminium. This is scarcely a reason to consider the residues as ecotoxic.

Results from tests of bottom ash from MSWI are illustrative. Ash which had been stored and allowed to mature for 3 months was somewhat ecotoxic. Ash which had been aged and exposed to the weather (actual liquid-to-solid leaching ratio of 3.4, rainwater as leachant) for 12 years was not ecotoxic at all to the three organisms tested. And yet, its composition had not changed, except for a lower content of chlorides and probably of sulphates, too.

11.3 Conclusion

Projects within the Ash Programme have contributed increased knowledge of the properties of combustion residues as well as demonstrated large-scale uses for residues. The Ash Programme has been active since 2002, and it is quite probable that it will be extended for another 3 years at the end of 2011.

With ash from clean solid biofuels, priority should be given to recycling the ash to forest soils as a compensation for the extraction of mineral nutrients in an intensive harvesting, i. e. whole-tree harvesting. If that is not possible, the residues should be used in civil works, e. g. for building roads or hard surfaces. One should put all such ash to use, instead of rejecting particular streams.

The Ash Programme has striven for a common understanding of the level of risk for human health and the environment when using ash in civil works. Boundaries between low risk and not low risk have been suggested. The Swedish Environmen­tal Protection Agency has developed another boundary, below which one does not even need to pay notice to the environmental authorities.

All reports are public and are available from Varmeforsk’s Web site (http:// www. varmeforsk. se). The language is Swedish, but there is always a summary in English.

The soils under study are acid, as are most forest soils in the region, mainly due to the instability of the parent rock and to the strong leaching of the base cations. Also, the plant growth itself generates soil acidity. This implies a great obstacle to the growth of forest species since it reduces the availability of essential nutrients such as P (Sanchez-Rodriguez et al. 2002; Merino et al. 2005).

In this study, the application of ash slightly and temporarily increased the pH of the solid soil fraction, resulting in higher availability of elements such as K, Ca, and Mg. The availability was proportional to the quantity of ash applied. The concen­tration of Ca increased greatly after the second application. Thus, a direct relation­ship between the application and the availability of the element was observed.

Nevertheless, the increases were less than those reported for soil treated with fly ash (Ohno and Erich 1990; Saarsalmi et al. 2001; Hytonen 2003; Solla-Gullon et al. 2006).

The behavior of P was different, and a low response to the different treatments was observed. This may be due to the low availability of P in the ash, as well as to the chemical and biological reactions that this element undergoes in the soil. In acid soils, phosphate ions tend to precipitate with Fe and Al, forming insoluble com­pounds and thus reducing the concentrations of H2PO4 and H2PO4 taken up by plants (Garcia-Rodeja and Gil-Sostres 1997). It is also possible that the availability of P decreases owing to microbial immobilization, a process that is accelerated by the application of organic waste to the soil (Salas et al. 2003).

The brick production process in itself has remained unchanged for a very long time: a shapeable mass is formed into a brick, assumed to be a small, regularly shaped unit that a bricklayer can grasp with one hand while picking up mortar with the other. A typical modern production process is shown in Fig. 9.1.

Additions to the brick feed can take place either at the beginning of the process or in later steps.

In the trial runs explained later, biomass ashes were introduced as a substitution displacing quarried raw materials. They were mixed with the other raw materials right at the beginning of the process.

Quarried raw materials Body fuels

La La ih

Packaging

Transport to final point of use

Fig. 9.1 Brick manufacturing process

The mixture of MBM and BWA gave the highest barley yield (Table 3.5), which was significantly higher than for MBM alone. The barley yield of the MBM and different combinations of MBM and crushed rock was at the same level as for mineral NPK. All treatments with MBM or mineral NPK gave significantly higher yields of both barley and wheat compared with the unfertilized control (treatment 1). The yield of wheat was at the same level for mineral NPK and MBM with or without K addition (Table 3.5), but there was a significantly lower yield for MBM pellets (treatment 4) and MBM plus Altagro plus Olivin compared with MBM powder alone (treatment 2).

Different additions of K did not influence the concentrations of K in the wheat grain (Table 3.6). The unfertilized control had a significantly lower concentration of N and a higher concentration of P in wheat grain than obtained for treatment with MBM plus K, Mg and S, and mineral NPK gave a significantly lower P concentra­tion than the unfertilized control. The concentrations of plant nutrients taken up in barley grain were not analyzed.

The efficiency of the N applied was high and at the same level for the treatments with MBM or mineral NPK, whereas relatively small amounts of the P and K applied were taken up in the wheat grain (Table 3.6). On the basis of the nutrient uptake in wheat, the unfertilized control had a negative balance for N, P and K, whereas addition of MBM alone caused a negative K balance (Table 3.6). All the other treatments had a positive balance of P and K. The MBM treatment lowered the amount of readily available K in the soil, but the difference from the other treatments was not statistically significant. There was no significant change in the level of nonexchangeable K (KHNO3) for any of the treatments compared with the unfertilized control. MBM plus BWA gave a significantly increased amount of readily available P compared with use of MBM pellets plus K, Mg and S and the

control. MBM plus BWA also increased the amount of readily available Ca in the soil significantly compared with the treatments with MBM, MBM pellets plus K, Mg and S and mineral NPK. The only treatment that significantly increased soil pH was the MBM plus BWA treatment. The pH increase was around 0.5 (Table 3.7).

The BWA used in this experiment had a high concentration of Ca relative to K (Ca-to-K ratio, 8.6; Table 3.3). To find ash with a stronger effect as a K fertilizer than liming material, analyses of the chemical properties of bottom ash from other plants were performed (Table 3.8). BWA Akershus (Table 3.8) had a high concen­tration of K and a low concentration of Ca (Ca-to-K ratio, 1.8) and low concen­trations of heavy metals. The wood used originated from a timber terminal at Gardermoen, where bioenergy wood from a large district in eastern Norway is collected. The ash of cereal waste had high concentrations of P and K and low concentrations of heavy metals. The analyses indicated differences in the Ca-to-K ratio between ash of spruce and ash of pine (Table 3.8). The wood used at Reinsvoll was dominated by spruce and the ash had properties similar to those of the ash of pure spruce wood.

Brigitte Amalia Knapp and Heribert Insam

Abstract Biomass ash is a final by-product from biomass incineration and is being produced in increasing amounts. Ash contains a variety of macronutrients and micronutrients and thus requires an appropriate recycling strategy. This chapter addresses various recycling strategies and technologies, with a particular focus on a smart combination of wastes from different sources for optimising recycling efficiency.

1.1 Introduction

Biomass ash is the solid residue accumulating from the thermal combustion of plant biomass for heat and electricity production, containing a variety of macronutrients and micronutrients resistant to incineration. As combustion of biomass is among the dominant bioenergy applications worldwide, increasing numbers of biomass-based power plants are being built and thus vast quantities of ashes are produced. Despite the value of the various elements contained in the ashes, their disposal in landfills is still common practice, generating considerable costs for biomass plant operators and negating the recycling potential of ashes. A prerequisite for sustainable use of ashes in agriculture and forestry, however, is their quality in terms of heavy metal contents and organic pollutants. Appropriate combustion and separation techniques for the different ash fractions are thus highly needed.

To bring together knowledge and ideas on the reutilisation of biomass ashes, a conference entitled “Recycling of Biomass Ashes” was held in Innsbruck in March 2010, focusing on various recycling technologies for biomass ashes. The confer­ence sessions were targeted at the use of ashes as fertiliser or a supplement for organic and inorganic fertilisers as well as their combination with compost and anaerobic sludges. Further, ash amendments to forest soils were a major topic, as was the use of ashes for geotechnical constructions and industrial processes.

B. A. Knapp (H) and H. Insam

University of Innsbruck, Institute of Microbiology, TechnikerstraBe 25, 6020 Innsbruck, Austria e-mail: b. knapp@uibk. ac. at

H. Insam and B. A. Knapp (eds.), Recycling of Biomass Ashes,

DOI 10.1007/978-3-642-19354-5_1, © Springer-Verlag Berlin Heidelberg 2011

Moreover, national and international policies regulating the application of ash were presented and joint programmes for advancing knowledge in the field of ash recycling were discussed.

In the following chapters an overview of different fields of application for biomass ashes is presented, summarising current knowledge on the reutilisation of biomass ashes and highlighting future research needs. As most investigations on ash recycling are based on wood ash, this chapter will focus on ash produced from wood combustion.

Only high-quality ashes are suitable for the uses in agriculture and forestry explained in the previous sections. In particular for ashes that are characterised by elevated heavy metal contents, other uses are suggested, such as construction of roads, a surface layer in landfills, and as an additive in industrial processes such as concrete, brick, glass and cement production (van Alkemade et al. 1999; Ribbing 2007; Obernberger and Supanic 2009). Specific information on the use of biomass ashes in brick making is presented in Chap. 9 (Modinger 2011), and in Chap. 10 Berra et al. (2011) focus on the reuse of woody biomass fly ash in cement-based materials. The use of ashes for civil works such as road construction or as a surface layer for landfills in Sweden is illustrated by Ribbing and Bjurstroom (see Chap. 11, Ribbing and Bjurstrom 2011).

Different K sources combined with MBM gave an increased yield ofbarley compared with the use of MBM alone, whereas supply of K in addition to MBM did not significantly influence the yield of wheat. The soils used in this experiment had higher concentrations of readily available K than planned. The soils used in this experiment were within the group of sandy soils with a low content of acid-soluble K (acid-soluble K minus readily available K, 8-24 mg 100 g-1) as described by 0gaard

et al. (2002). 0gaard et al. (2002) found no yield response to applied K in perennial grass leys when the amount of readily available K exceeded 10 mg (100 cm3)-1 (8 mg 100 g-1 and bulk density 1.25 mg m3). In the present experiment the readily available K level was not significantly lowered owing to only a small negative K balance in the MBM treatment (Table 3.6), and was clearly above the expected minimum readily available K level based on texture (0gaard et al. 2002). Sufficient K in the soil is therefore the most probable reason for not finding a significant effect of K supply on K uptake in wheat grain. Using sand of the same origin as the Elverum sand in this study, Haraldsen et al. (2010) found that application of 80 and 160 kg N ha-1 in MBM alone and other organic N fertilizers with low K content gave signifi­cant K deficiency and reduced barley yield compared with the use of the same amounts of mineral NPK and a liquid anaerobic digestate based on source-separated household waste. That batch of sand had a considerably lower content of readily available K than the sand used in this study, and had a readily available K level close to the expected minimum level as described by 0gaard et al. (2002).

Although the effect of K supply was not large in this study, the mixture of MBM and BWA gave at least the same yield as mineral NPK or supply of K, Mg and S in addition to MBM. The concentrations of potential plant-available K in the crushed rock powder types was too low for making commercial fertilizer products, and the amounts used in this experiment did not influence K uptake and did not cause any change in the level of readily available K or nonexchangeable K in the soils. Combining N-rich waste (human urine) and wood ash gave more biomass of red beets than mineral fertilizer (Pradhan et al. 2010), and Kuba et al. (2008) as well as Bougnom et al. (2010, 2011; see Chap. 7) found positive effects on growth of mixing wood ash with compost. These examples indicate a potential of combining

N-rich and K-rich waste streams as fertilizer or soil amendments. Especially for use in organic cropping there is demand for recycled NPK fertilizers, which have predictable effects.

The Ca-to-K ratio of the BWA used in the present experiment was not optimal in the mixture with MBM, and the ash also had a higher concentration of Ni than allowed in materials that can be used as fertilizers for agricultural crops in Norway (maximum quality class II according to the Norwegian Ministry of Food and Agriculture 2003). BWA Akershus (Table 3.8) has a low Ca-to-K ratio and can be categorized as quality class I according to the Norwegian Ministry of Food and Agriculture (2003). Because a smaller amount of BWA Akershus than of the ash used in this study is needed to obtain the same NPK ratio, it is expected that a mixture of MBM and BWA Akershus will cause a smaller pH increase than the mixture of MBM and BWA in this study. The pH increase of 0.5 after a single application of 1,200 kg BWA ha-1 represented an estimated addition of about 700 kg CaO equiv ha-1, and caused a significant increase in the amount of readily available Ca in the soil (Table 3.7). According to Franzefoss (2007) natural acidification (by leaching and acid precipitation) represents an annual demand of lime of 100-200 kg CaO ha-1 in eastern Norway and 200-400 kg CaO ha-1 in coastal areas in western and central Norway. A liming effect of 200-300 kg CaO equiv ha-1 will be suitable for a fertilizer that is to be used annually for cereals. The ash of cereal waste from a milling plant had interesting properties as a PK fertilizer, as the levels of heavy metals were lower than the limits for use on agricultural land (Norwegian Ministry of Food and Agriculture 2003). For further development of organic mixtures based on MBM plus BWA, ash of similar quality as BWA Akershus (Table 3.8) will be selected. The challenge is to find sufficient quantities of BWA with similar properties, in order to establish commercial production of organic NPK fertilizer by combination of waste streams.

2.3 Conclusions

A mixture of MBM and BWA with a low Ca-to-K ratio and lower concentrations of heavy metals than the limits in the governmental regulations for organic fertilizers may give a NPK fertilizer with reliable effects of all three major plant nutrients. Such a fertilizer, based on combining waste streams from society, will be more complete fertilizer product than the raw materials represent and is of special relevance and interest for organic cropping. The BWA used in the mixture with MBM in the present experiment did not have optimal chemical properties, and gave too high an increase in pH for annual use for cereals. Further investigations on optimization of mixtures of MBM and BWA are needed before such fertilizer is ready for commercial production.

Acknowledgements The pot experiment was a part of a project supported by the programme “ORIO-Organic Waste Products and Recycling of Resources” and Norsk Protein AS (grant no.

403). Ellen Zakariassen is thanked for skilful technical assistance and experimental work. Additional analyses of ash and further work on the fertilizer concept were carried out as a part of WP 1.4 “Residues upgrading and use” in CenBio, Bioenergy Innovation Centre (http://www. cenbio. no), which is supported by the Research Council of Norway, Norsk Protein AS and Akershus Energi AS.