The grate furnace design is the most common in Sweden up to approximately 100 MW fuel, and the only one for capacities below 20 MW fuel (approximately 6 MW electricity)2. The ash has been through high temperatures, but not as high as in PF furnaces. The major part of the residues is bottom ash, and various fly ashes are a minor part.

In a PF furnace, the quantities of fly ash are so much larger than those of bottom ash that the composition of the fly ash corresponds very closely to that of the ashed fuel. With a grate furnace, however, the elements in the ash will be fractionated into several streams of residues. With a grate at more than 1,000°C, volatile elements and their compounds will concentrate in the fly ashes. If heavy metals are an issue, as in the regulations on recycling ash to forest soils, this may pose a problem with fly ash.

The bottom ash is a good road building material, and it is even better if the ash has been burned out and sintered. Even if its water absorption in laboratory tests is quite high, the road will be of good quality and will withstand freeze-thaw cycles. Because of their binding properties, the fly ashes are good building materials for roads. Tests have shown that up to 50% of the cement in stope mine filling may be replaced with these ashes.

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.

Depending on the amount, the type, the interval of application, and especially the characteristics of the soil, compost will improve soil structure and aggregate stability, hydraulic conductivity, infiltration, resilience against erosion, water hold­ing capacity, air balance, and soil temperature (Gerzabek et al. 1995; Hartmann 2003). Stabilization of soil aggregates by organic matter occurs in three main ways: (1) Application of organic matter maintains the microbial activity and thereby the production of metabolic products with cementing properties by microbial degra­dation, mature compost performing better than immature compost; (2) application of organic matter supports the activity of the soil microfauna and mesofauna, e. g., earthworms. Excrement aggregates have positive effects on the soil structure and influence the formation of wide macropores and generally enhance microbial acti­vity; (3) application of high molecular weight humic acids improves the long-term stability of microaggregates (Hartmann 2003; Fuchs et al. 2004). Annual applica­tions of small amounts of compost are more effective in stabilizing aggregates and pores size distribution than any singular application of high amounts (Lamp 1996).

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.

Beatriz Omil, Federico Sanchez-Rodriguez, and Agustin Merino

Abstract The aim of this study was to evaluate the effectiveness of multiple applications of biomass ash to acid soils. The study was carried out in two stands of Pinus radiata D. Don, aged 13 and 15 years, in the province of Lugo (northwest Spain). The soils in the stands were developed on lutites and migmatites. Experi­mental plots (each 1,225 m2) were established, and the experimental treatments were as follows: control (untreated), ash (addition of 4.5 Mg dry matter ha-1 year-1 in 2003, 2004, and 2005) and ash plus P (addition of ash plus phosphate fertilizer in 2003).

The ash was generated in a moving grate furnace, and had the following characteristics: pH 8.9 -13.5, high concentrations of K, Ca, Mg, and P, and low N content and low concentration of heavy metals.

The responses of the forest stands, evaluated as the effects on forest nutrition and tree growth, were measured in 2005, 3 years after the initial treatment. The results showed that continuous fertilization with ash improved the nutritional status and growth of Pinus radiata D. Don stands, and resulted in increased contents of the main macronutrients in needles and soil.

6.1 Introduction

The Galician timber industry makes an important contribution to the regional economy. Forest land covers more than 60% of the total area, and the annual timber harvest is approximately 7,000,000 m3. The wood is used in sawmills and to prod­uce paper, particle board, and fiberboard. Lignocellulosic by-products generated in the latter industries are used in biomass incineration plants to meet increasing energy

B. Omil (H), F. Sanchez-Rodriguez, and A. Merino

Forestry Faculty, Escuela Politecnica Superior, University of Santiago de Compostela, 27002 Lugo, Spain

e-mail: beatriz. omil@usc. es

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

DOI 10.1007/978-3-642-19354-5_6, © Springer-Verlag Berlin Heidelberg 2011 needs and to reduce waste production. The by-products mainly consist of bark (from pine, eucalyptus, and a small amount of birch, depending on the type of manufacturing) and to a lesser extent sand, dust, and panel fragments. The volume of these by-products created annually is approximately 900,000 m3 and that of other by-products is approximately 40,000 m3.

This source of energy is considered neutral from an environmental point of view, as it releases the same amount of CO2 to the atmosphere as trees have removed. Combustion does not affect global warming or the greenhouse effect and has several advantages, such as a reduction in the use of fossil waste and the reuse of waste that has no value other than being a source of energy.

Biomass ash is produced as a result of this process (on average, combustion of wood produces 6-10% ash; Gaskin and Risse 2002). Ash is considered a non­hazardous waste (ERL codes 100101 and 100103, bottom ash and fly ash, respectively) and is therefore stocked at dumping sites. However, to reduce these stocks, alternative uses of the waste are being investigated, e. g., for production of enamel and glass (Xirokostas et al. 2001), as a building material for tracks or rural paths, for amation of coal mine (Gil-Bueno and Monterroso 1998; Seoane and Leiros 2001), as an absorbent for the removal of dichlorodi — phenyldichloroethane (DDD) and dichlorodiphenyldichloroethylene (DDE) ori­ginating from pesticides (Gupta and Ali 2001), and as an additive in cement production (Van Der Sloot and Cnubben 2000). The latter use is not recom­mended because of the high level of C in the ashes. In this case, wood ash is usually used as a pozzolan (a siliceous and aluminous material). Despite not having binding properties, pozzolan reacts with finely divided calcium hydroxide in the presence of water to form compounds with cementing properties at room temperature (ASTM 1994).

Nevertheless, wood ash contains nutrients such as P, K, Ca, and Mg, which are present in relatively soluble forms (the NPK content is typically 0-1-5). Apart from these macronutrients, the waste contains oxides, hydroxides, and carbonates. The waste is therefore highly alkaline and contains low amounts of heavy metals (Erich and Ohno 1992; Korpilahti et al. 1998; Demeyer et al. 2001; Miller et al. 2002; Solla-GullcSn et al. 2006). For all these reasons, application of wood ash to forest soils may be of interest as regards the environmental management of such waste, improvement of the nutritional status of forest plantations, and completing the CO2 cycle (Torre-Minguela and Giraldo 2006). Fertilizer is added in an attempt to replenish the nutrients exported as a consequence of the extraction of biomass after final harvesting.

However, despite the high productivity of Galician forests, most Pinus radiata D. Don plantations are deficient in nutrients such as P, Mg, and Ca (Sanchez — Rodnguez et al. 2001; Zas 2003), which can be attributed to the strongly acidic soils and to the extraction of nutrients as a result of the management of the plantation in medium rotations (less than 40 years). Fertilization with wood ash would also contribute to the sustainability of the stands, intensive exploitation of which results in large losses of nutrients.

To estimate the amount of nutrients needed for fertilization of the soil, an estimate of the nutrient balance is needed. The plant density and the climate are important (IFA 2008). Nutrient stocks have been restricted to the upper 30 cm, as most feeding roots of cacao are concentrated at that depth. Removal of nutrients from cacao ecosystems is caused by yield (beans and husks), immobilization in stem and branches, and leaching of nutrients below the rooting zone (Hartemink 2005). Most nutrients in cacao ecosystems are lost by the harvest of beans and husks. In Table 8.4, an overview is given of the nutrient removal caused by the crop of 1 ha (740 kg dry cacao beans and 1.0 t cacao husks).

For the nutrient demand there are several recommendations for cacao. The recommendations for the nutrient demand for 1 ha of cacao plantation differ hugely, as shown in Table 8.5. We decided to use the statistical approach: all improbable data have been removed.

For the mineral balance it is assumed that the husks are returned to the soil and that all the available nutrients are reused. This assumption is assessed in the sensitivity analysis. In Table 8.6, an overview is given of the mineral balance. The husks are supposed to be returned to the soil, so these nutrients are directly

recycled to the plantation. The nutrient demand minus the natural addition of nutrients minus the husks gives the amount to be added by fertilization. Further, some natural addition occurs by deposition and transfer. Data from plantations in Cameroon (Hartemink 2005) are used, as data for Cote d’Ivoire were not found in the literature.

The preferred nutrient sources for the nitrogen, phosphorous, and potassium fertilizers for a cacao plantation are urea, triple superphosphate (with 48% P2O5), and potassium chloride (with 49% K2O), respectively (IFA 2008). It is assumed that the fertilizer is produced in Cote d’Ivoire. Data for production in Europe were used, as other data were not available (Table 8.7).

Mario Berra, Giancarlo De Casa, Marcello Dell’Orso, Luigi Galeotti, Teresa Mangialardi, Antonio Evangelista Paolini, and Luigi Piga

Abstract The feasibility of using woody biomass fly ash (WBFA) as a mineral admixture in cement-based materials was investigated. This fly ash was character­ized for chemical composition and used to prepare a cement blend with 70 wt% Portland cement and 30 wt% WBFA. Cubic specimens were cast from a blended cement paste (water-to-binder ratio 0.50) and, after 28 days of curing at 20°C and 100% relative humidity, these specimens were tested for heavy metal leachability through the use of a sequential leaching protocol, at a constant pH of leachant (deionized water; pH 6.0). It was found that, except for the chloride content, the WBFA is able to meet the European chemical requirements established for reuse of coal fly ash in cement-based materials. Although the WBFA is characterized by a significant content of heavy metals of particular environmental concern (Cd, Cr, Cu, Ni, Pb, Zn), the results of the monolith leaching test have shown a good immobiliza­tion capacity of such metals by the cementitious matrix and, consequently, a good environmental quality of the blended cement investigated.

10.1 Introduction

In March 2007, the European Commission undertook an approach to climate and energy policy in order to fight climate change and increase EU energy security while strengthening its competitiveness. The European Commission committed itself to transform Europe into a highly energy efficient, low-carbon economy.

M. Berra

ERSE S. p.A., Milan, Italy

G. De Casa, L. Galeotti, T. Mangialardi, A. E. Paolini, and L. Piga (H)

Faculty of Engineering, Sapienza University of Roma, Rome, Italy e-mail: luigi. piga@uniroma1.it

M. Dell’Orso

Chemical Laboratory of DGERM-Economic Development Ministry, Rome, Italy

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

DOI 10.1007/978-3-642-19354-5_10, © Springer-Verlag Berlin Heidelberg 2011

To achieve this goal, the European Commission aimed to carry out, by 2020, what is known as the 20:20:20 project, namely:

— A reduction in EU greenhouse gas emissions of at least 20% below 1990 levels

— Twenty percent of EU energy to be produced from renewable resources

— A 20% reduction in primary energy use to be achieved by improving energy

efficiency

In this context, the use of biomasses in place of traditional fuels represents a suitable way of reducing greenhouse gas emissions. In fact, the biomasses may be regarded as clean and renewable energy resources with no net CO2 production, since the amount of CO2 produced from biomass combustion is approximately equivalent to that taken up from the environment during biomass growth.

The most important biomasses are the residues from woodworking or forest activities, the wastes from farms and agrobusiness, the organic fraction of munici­pal solid wastes, and the plants deliberately grown for energy production purposes. In Italy, the most utilized biomasses for burning in power plants are chipped wood, and, to a minor extent, rice-husk and olive residues (GSE 2009).

Although the use of biomass in Italy is less than the European average, the high potential of burnable biomass along with the fast increase in the number of biomass-based thermal plants calls for a solution to the disposal problems asso­ciated with ash production. Both the quality and the quantity of ash depend on the type of biomass used as a fuel. The amount of ash produced per unit weight of original biomass can vary from about 2% (w/w) (chipped wood) to 15% (w/w) (rice husk) (Lokare et al. 2006).

Irrespective of the type of biomass used, two kinds of ashes are produced: fly ash and bottom ash. Fly ash is generally trapped by electrostatic precipitators or sleeve filters located downstream of the combustion process, before the gas and the very fine particles are released to the environment. Bottom ash is collected in the bottom of the boiler. The relative amount of fly ash and bottom ash depends on the type of boiler. Powder boilers produce more bottom ash than fly ash; fluidized bed boilers produce more fly ash than bottom ash. Grate boilers produce about the same quantity of both ashes.

According to the European waste catalog and hazardous residues list (Commis­sion of the European Communities 2000), both fly ash and bottom ash originating from combustion of untreated wood are classified as nonhazardous wastes and are listed with codes 10.01.03 and 10.01.01, respectively. The former code also includes fly ash from peat; the latter also includes slag and boiler dust.

Woody biomass ash, being a waste, has to be disposed of in authorized landfills. Alternatively, this waste may be reused as a fertilizer or for building purposes, provided it passes the tests prescribed by the environmental laws. Bottom ash may be used directly as a building material to replace granular material in geotechnical works, such as road foundations. Fly ash may be reused as a filler in cementitious mixes. However, the high content of alkalies and chlorides could prevent the reuse of fly ash in cementitious mixes.

From the environmental point of view, reuse of biomass fly ash in concrete would be very profitable as partial replacement of Portland cement. This may (1) solve the problem of fly ash disposal, (2) reduce the CO2 emissions involved in the industrial production of cement from traditional raw materials, namely, limestone and clay (0.83 t CO2 is emitted for each ton of Portland cement produced), and (3) preserve the natural resources involved in cement production, with further benefi­cial effects on the environment.

A recent study (Rajamma et al. 2009) has shown that the replacement of Portland cement with woody biomass fly ash (WBFA) up to 20% (w/w) of cement does not negatively affect the development of the mechanical properties of cementitious mixes. The practical inference of such fly ash reuse would be a 20% reduction of CO2 emission related to cement production, and this would be an innovation in line with what is expected by the European Commission.

However, the reuse of biomass fly ashes in cement-based materials is strongly related to their chemical and environmental characteristics. Generally, fly ashes originating from traditional and innovative fuels may contain significant amounts of heavy metals that pose severe limitations for their disposal in landfills or for their reuse in agricultural/industrial applications. In particular, cadmium appears to be the most problematic heavy metal in biomass fly ashes, and chromium, mainly in the Cr(VI) state, may be problematic in many ash stabilization processes because of its mobility at high pH values (Lima et al. 2008). Presently, little it is known about the environmental compatibility of blended cements made with Portland cement and biomass fly ash.

In this study, the leaching behavior of a mixture of Portland cement and WBFA was investigated in view of the possible reuse of this kind of fly ash as a mineral admixture in the formulation of blended cements. The eco-compatibility of such a mixture was assessed through the use of a monolith leaching test on hardened cement pastes under constant pH conditions (pH 6.0).

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).

Soil acidity can be considered as the capacity of soils to manifest properties of acids or proton donors (Vorob’eva and Avdon’kin 2006). It occurs when acidity-generating processes outweigh acidity-consuming processes (Ulrich 1994). A soil is defined as acid when its pH is lower than 7. Soil acidification has many causes that are natural and unnatural. Although soil acidification is a slow natural process, it can be acce­lerated by plants, animals, and human activities or slowed down or reversed by care­ful management practices (Bolan et al. 1994; Poss et al. 1995).

7.2.1 Some Causes of Soil Acidity

7.2.1.1 Rainfall and Leaching

In climates where rainfall exceeds evapotranspiration, soils with low buffer capac­ity tend to acidify. Excess water infiltrating the soil enhances leaching of basic ions such as calcium (Ca2+) and magnesium (Mg2+) from the exchange complex of soil (clay minerals, humus) and their substitution by protons (H+) and aluminum ions (Al3+) (Mayer 1998). This way, neutral clay may be converted into a hydrogen clay or acid clay, which gradually accumulates and intensifies under increasing amounts of rainfall.

The two types of fluidised bed furnaces commonly distinguished are the circulating fluidised bed furnace and the bubbling fluidised bed furnace. The bed material, usually sand, is finer for the circulating fluidised bed furnace than for the bubbling fluidised bed furnace, 0.3 and 2 mm respectively. Part of the bed is bled out to keep an acceptable chemistry in the bed: alkalis tend to dissolve in the sand and reduce

There are other types with special designs, e. g. cyclone furnaces, but they are not common.

the temperature at which the particles sinter together. The combustion temperature is low, approximately 850°C.

The largest part of the residue is fly ash. The proportion of bottom ash depends on how much material is bled from the bottom of the bed. Both streams of ash contain not only ash from the fuel but also bed material, oversized material in the bottom ash and fine particles from attrition in the fly ash.

The combustion temperature is too low to allow the Portland cement reactions in the fly ash (these require at least 1,400°C). However, they still have binding properties through other reactions which are not fully known. Because of the low combustion temperatures, the particles have irregular shapes. The compressive strength is good even in the green stage if they have been well packed. Among other properties, this makes them very good materials for road building. Mixed in equal proportions with digested sewage sludge, they also provide dense sealing layers to landfills.

The bottom ash contains mostly bed material. It is often too fine and has too narrow a size distribution to be useful in construction, other than as low-quality fill material. However, under certain circumstances, e. g. absence of binding properties, it could be used as backfill material in trenches for piping.