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).

Bundling different residues into one use in Table 11.2 does not contradict our attention to detail in Table 11.1. The properties of individual combustion residues need to be taken into account even when considering the same use. All quantitative information on the properties of ash that has been determined in the Ash Programme as well as all information from Swedish R&D projects on ash is stored in a database, Allaska. It is available in Swedish and English at http://www. askprogrammet. com.1

11.2.4.1 Pulverised Fuel Furnaces

Pulverised fuel (PF) furnaces are often very large furnaces converted from furnaces for coal or oil to biomass. The fuel must be ground finely for it to be injected into the 1rThe Web site of the Ash Programme will soon be incorporated into that of Varmeforsk (http:// www. varmeforsk. se), but visitors will be redirected to the Varmeforsk Web site.

burners. This is not a very common type, as one usually prefers to retrofit such a furnace with a grate or to convert it to a fluidised bed furnace. However, when there are severe constraints on space in an existing combustion plant, a new PF furnace may be attractive.

The major part of the residues is fly ash. Having passed through high tempera­tures, 1,200°C and more, fly ash consists of small glassy particles that can yield pozzolanic reactions as in Portland cement. Biomass fly ash is not as good a binding material as coal fly ash, but it is very suitable for road building.

In converted coal or oil furnaces, the bottom ash usually has high contents of unburned carbon and it may be used as fuel in fluidised bed furnaces. It is a poor road building material because of its high water uptake. However, it may be used as low-quality filling material.

The chemical composition of biomass ash varies depending on the combustion technique used, the type of material used in the combustion (tree species and

original material, bark, stem, braches, etc.), the storage conditions, etc. (Vance 1996; Blander 1997; Obernberger et al. 1997; Larsson and Westling 1998). The material used in this study was bottom ash generated in moving grate furnaces in three biomass plants in Spain (FINSA, Financiera Maderera). Chemically, this type of ash is less reactive than ash generated in fluidized bed combustors (equivalent neutralizing value 18% CaCO3) since it contains a high percentage of unburned materials. The composition and the quantity of elements provided by such ash are shown in Table 6.3. The concentration of heavy metals is very low (bottom ash), especially in comparison with other waste such as sewage sludge and coal ash (below the limits established by the EU-European Community (1986) and the US Environmental Protection Agency).

However, there are some drawbacks associated with bottom ash which may hinder the spreading process, such as the high humidity (55%) and the high

proportion of coarse elements (more than 20% larger than 4 mm) such as slug, unburned wood swath, and metals.

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).

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.

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).

Composts (e. g., from green and kitchen waste) and soil differ in their compo­sition and properties in almost every parameter. The amount of organic carbon, the amount of organic nitrogen, pH, electrical conductivity (EC), cation exchange capacity (CEC), salinity, chloride concentration, and sulfate concentration are higher in compost. Therefore, depending on the quality and quantity of compost, climate, and soil characteristics, their application will result in a modification of the soil organic matter composition (Fuchs et al. 2004). Compost application increases soil organic carbon and total nitrogen content at the upper horizon in both sandy and clay soils, and that increase can be observed even more than 10 years after application (Melero et al. 2007; Delschen 1999). Generally, an increase in soil pH after compost use is observed because most composts are basic and have a considerable buffering capacity. The mineralization of carbon and the subsequent production of OH~ ions by ligand exchange as well as the introduction of basic cations such as K+, Ca2+, and Mg2+ leads to an increase of soil pH (Mkhabela and Warman 2005). Compost application can ameliorate soil acidity by increasing soil pH and allows large quantities of lime required for these soils to be saved (Haynes and Mokolobate 2001). Application of municipal, mixed green and animal waste compost has been reported to proportionally increase the EC and salt effects of soils (Stamatiadis et al. 1999; Walter et al. 2006). Agricultural soil EC levels ranged from 0 to 4 dS m-1, whereas soil EC levels of municipal solid waste composts ranged from 3.69 to 7.49 dS m-1 (Brady and Weil 1996); therefore, the increase of EC could be of concern. In some cases, soil EC levels were excessive and inhibited plant growth; nevertheless, Zhang et al. (2006) reported that the increased soil EC values decline over time because of nutrient removal by crops and leaching, but the long-term soil biological activity could be negatively affected (Iglesias-Jimenez and Alvarez 1993). Incorporation of compost into soil, especially at high doses, increases the CEC (Bengtson and Cornette 1973); the rise is generally linked to an increase in the level of organic material, the pH, and in base saturation (Ca, K, Na) (Fuchs et al. 2004). Nitrate leaching is one of the concerns regarding compost utilization. NO3~ is highly mobile in soils and is susceptible to leaching through the soil profile and into the groundwater by the infiltrating water. When compared with organic and mineral fertilizers, the nitrate leaching potential of composts is very low (Insam and Merschak 1997). Composts for agricultural use should come from source-separated organic waste and green waste only; then heavy metals are not of concern, as they would be if municipal solid waste compost were used (Epstein et al. 1992; Sharma et al. 1997).

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.

In each experimental plot, samples were collected from each soil layer, at a depth of 0-40 cm, from the center of each plot, following a zigzag route. Soil samples were mixed to ensure homogeneity, dried at 40°C, and sieved through a 2-mm screen; microoelements and macroelements were extracted by the Mehlich 3 procedure (Mehlich and Mehlich 1984). Soil pH was measured in H2O and KCl (0.1 M). For elemental analysis (CNS), the soil samples were ground in a mortar to obtain a fine powder.

Sampling was carried out in spring and autumn. Needle samples were also collected, approximately 25% from the floor of the plot (Ballard and Carter 1986), approximately 25% from the upper third, approximately 25% from the growth of the year, and approximately 25% from the sunniest branches (Will 1985). The needle samples were dried at 60°C to constant weight and were ground for CNS analysis by inductively coupled plasma optical emission spectrometry.

The environmental impact analysis is focused on emissions, especially those of CO2, NOx, and SO2. The emissions related to ash transport and fertilizer production and transport need to be calculated to obtain the total environmental impact.

8.4.1 Combustion of the Cacao Shells in the Bioenergy Plant

Per hectare cacao plantation in Cote d’Ivoire, 74 kg cacao shells are produced. These shells are combusted in a bioenergy plant. In Table 8.8, an overview is given of the main characteristics of the circulating fluidized bed combustion plant for the combustion of 74 kg cacao shells. The combustion of 74 cacao shells produces

5.3 kg filter ashes. Only the filter ash is suitable as a fertilizer. The bed ash consists of approximately 69% bed material (sand) and additive, whereas the filter ash contains only 13% bed material and additive. It is assumed that all potassium is present as K2CO3.