6.2.1 Site

This study was carried out in February 2003 in two Pinus radiata D. Don planta­tions, aged 13 and 15 years (Table 6.1), in the province of Lugo (northwest Spain) (43° 13′ N, 7°39′ W and 43°07′ N, 749′ W respectively). Shrubs predominated in the understory vegetation (Calluna vulgaris, Cytisus scoparius, Erica umbellata, Halimium lasianthum subsp. alyssoides, Ulex europaeus, etc.).

The average annual precipitation during the study period (2003-2006) was 1,135 mm, and the average monthly minimum and maximum temperatures were

15.1 and 5.0°C, respectively.

6.2.2 Soil

The plots (Table 6.2) were established on different soils developed on migmatites and lutites both classified as Humic Umbrisol (FAO-UNESCO 1998) (Fig. 6.1). The soil in the first plot contained a high percentage of coarse particles (which create a sandy loam soil that favors good drainage), had a low pH (4.7), and had a C-to-N ratio of 19.3. In contrast, the soil in the second plot was of fine texture, with moderate permeability, was low hydraulic conductivity, had a high percentage of organic matter (C-to-N ratio 31.2), and had acid pH (4.8).

In this case it is assumed that the beans are transported to the Netherlands. The beans are stored in the harbor before transport to the cacao processing plant. Beans are broken to nib and the shells are separated. The nib is processed to cacao products. The shells are combusted or used as gardening material. All materials are transported by road or water. An example of cacao shells is given in Fig. 8.3.

For the scenarios it is assumed that the cacao shells are combusted in a circulat­ing fluidized bed combustion plant to produce heat and power. This is a type of boiler for production of energy from biomass on a large scale (so-called bioenergy

flue gas

plants) (Fig. 8.4). The general data of the bioenergy plant that were used in this study are given in Table 8.1. Sand is used as bed material. Some limestone is used to capture sufficient SO2 to meet the Dutch emission requirements.

The predicted chemical composition of the filter ash is presented in Table 8.2. The macroelements are also expressed as oxides (this does not mean that these elements

are present as oxides). The main nutrients in filter ash are potassium, calcium, and phosphorus. Thermodynamic calculations were performed to predict the potential compounds in cacao ash under equilibrium conditions. These were performed using FactSage©. The main compounds will be K2SO4, K2CO3, and Ca5HO13P3. It is assumed that the filter ash contains 5% residual pyrolized cacao shells (carbon).

Earlier studies revealed that Pinus radiata shows a good response to fertilization (Sanchez-Rodnguez et al. 2002; Omil et al. 2005; Solla-Gullon et al. 2008). However, this response varies depending on the age of the trees, on the density, and on the nutritional needs (Zas 2003). The demand for elements such as N and P increases during the first years of life, a maximum of 6-8 years (Turner and Lamber 1986). Ca and Mg, which do not suffer retranslocation before needle fall (Ericsson 1994), may cause problems in forest crops since the demand for these elements by higher plants tends to increase with age (Turner and Lamber 1986).

Analysis of covariance revealed that improvement in soil nutritional status leads to higher tree growth from the second year of treatment onward, confirmingthe findings of other studies (Bonneau 1995; Vesterinen 2003). This is due to the site quality (ecological conditions, soil physical properties, weather, and water avail­ability of the trees): under adverse conditions fertilization does not improve soil nutritional status. Thus, the response was more significant in plots on lutites (of lower site quality) than in plots on migmatites. This confirms the results reported by other authors, mainly for rich soils (Silfverberg and Huikari 1985; Ferm et al. 1992; Emilsson 2006). Production may be improved by application of a higher dose of ash, although there is a risk of leaching contamination and increased heavy metal concentrations.

The application of other types of waste such as slag and dairy sludge was found to increase tree height growth (Virgel-Mentxaka 2002; Omil et al. 2005). The combination of ashes with certain organic wastes has considerably increased the growth of agricultural crops (see Chap. 4, Nieminen 2011). However, the amount of N applied must be taken into account. Prior studies revealed that the tree structure of this species may be affected by N and P stresses (Will 1985). Although height growth is not usually altered, radial growth in both stem and branches was most sensitive to addition of these elements. These changes have important impli­cations such as the loss of apical dominancy in fertilized trees and possible stem distortions, resulting in a decrease in the economic value (Will 1985; Hopmans and Chappell 1994).

3.4 Conclusions

Fertilization with bark ash improved the nutritional status of Pinus radiata planta­tions, and increased the contents of the main macronutrients in the needles. How­ever, some limiting nutrients such as P did not exceed critically low levels, so there is some room for improvement in the fertilization treatments. The effects on the needles were also inconsistent, and delay and the intensity of the effects depended on the time of application. The third application of ash significantly improved the diameter and height (and so the volume) growth in one of the plots (lutites) relative to the control treatment. It may be concluded that only after careful site evaluation ash application is indicated, otherwise the costs may exceed the benefit.

Body fuels, i. e., combustible substances added to the brick feed that are then combusted upon firing the brick, have been in use since the Egyptians added straw to the brick feed. Owing to the remaining organic content, biomass combus­tion ashes might be considered a low calorific value body fuel.

A tunnel kiln (Fig. 9.2) works as a counterflow heat exchanger. In the tunnel kiln packs of bricks set on a car train on rails move through the kiln one after the other. During their journey, the cars move toward, through, and past the stationary firing section at the center of the structure. During its travel, the brick set on the kiln car is slowly, ideally uniformly, heated up to the required firing temperature and then cooled down again (Fig. 9.3). Heat transfer within a tunnel kiln and within the

usually densely packed bricks on the kiln car takes place primarily by forced convection of the turbulent kiln gas flow, and, limited to the firing zone, by radiance of the burner flame to the brick. Further heat transfer by conduction from brick to brick occurs at the contact surfaces of the tightly stacked bricks. At a temperature range of 150-350°C the volatile proportion of any organic addition made to the brick feed is released as low-temperature-carbonization gases. These gases are usually conveyed to the chimney or to an appropriate postcombustion system.

As heating of the bricks set on the kiln car does not take place uniformly across the section of the brick, release of low-temperature-combustion gases at different points of the travel through the kiln can cause problems. The firing curve pictured in Fig. 9.4 shows the delayed reaction of the energy rich additions to the clay body:

Vertically stacked brick Brick stacked in Brick stacked in direction of flow and

direction of air flow

Fig. 9.3 Methods of stacking bricks on kiln cars

The combustion of the energy content in the core of the pile occurs later than that on the perimeter.

Only at a temperature of about 550-600°C do the temperature differences between the lower and the upper layers of the brick stacked on the kiln car diminish considerably.

The problems of using body fuels featuring a high percentage of volatiles is explained here with the example of paper sludge: At a temperature of about 150°C, all water that is still present in the cellulose is evaporated. This evaporation process is concurrent to driving out water still present in the clay body and extends into the first phases of release of crystalline water. At temperatures between 100 and 200°C, volatile substances are dissociated and evaporate, releasing carbon monoxide, hydrogen, and hydrocarbons.

Recent research by the author has shown the effects of concurrent combustion (a substance with an otherwise higher ignition temperature is ignited by the combustion of a substance with a lower ignition temperature) and the impact on the energy balance of a tunnel kiln. Such an example is pictured in Fig. 9.5 for a brick feed with 20% of paper sludge added to the clay in addition to a 1.5% of a bituminous coal featuring a high volatile content.

The above-mentioned differential thermal analysis (DTA) is of particular inter­est when it is compared with the DTA for the same basic mix but with 1.5% of anthracite and less than 2.5% of volatiles instead of the bituminous coal in the previous example. The DTA in Fig. 9.6 shows release of energy at the same temperatures as in the previous case but a second, smaller peak is observed at higher temperatures as well. This second peak has a positive impact on the thermal balance of the kiln.

An effect similar to that observed for the anthracite/paper sludge mix was observed in the tunnel kiln when a biomass ash/paper sludge mix was fired. This effect has to be confirmed in further and longer-lasting tests to prove that, biomass combustion ashes contribute positively to the energy balance of a brick tunnel kiln.

7.3.1 Compost

Compost is a product derived from composting that is high in nutrients and rich in humic acids. Nutrients are usually bound organically and thus released at a moderate speed (Gobat et al. 2003). Mature compost contains a diverse community of microorganisms; addition of compost to soil modifies considerably its biological,

Decomposition of organic residues
and chelation with ash panicles

Release of soluble humic material
and aliphatic organic acids

Reduction in the quantity
of exchangeable Al

Fig. 7.1 A conceptual model of the major processes that lead to the detoxification of soil aluminum and an increase of phosphorous availability when wood ash compost is applied to acid soils

physical, and chemical properties in the short term as well as in the long term (Ryckeboer et al. 2003; Fuchs 2009). The use of compost in agriculture aids in replenishing and maintaining long-term soil fertility by providing optimal condi­tions for soil biological activity and a slow flow of nutrients adapted to the needs of the crop (Gobat et al. 2003).

The large quantities of combustion residues may be swallowed only by a mass market, that of materials for civil works. The largest single use in 2006 was as a

construction material in the closure of landfills (as well as for capping), with approximately 650,000 t; see Table 11.2. The financial incentive for this use is the possibility to waive the tax on waste sent to landfill: the materials are used and replace virgin materials. However, many currently active landfills will be closed within the next 10-15 years. It should be noted that backfilling cavities in Table 11.2 concerns mostly air pollution control (APC) residues from combustion of MSW.

Spreading to forest soils is a small area of use, with approximately 35,000 t/year, but is of vital importance for the sustainability of the production of solid biofuels from biomass harvested from forests. The relevant biomass fraction is the logging residues, and harvesting it on top of the extraction of timber and pulping wood in conventional forestry not only removes the mineral nutrients in the residues that if left in the forest would have been available to the next generation, but also exacerbates the natural acidification of forest soils by conventional forestry.

The study comprised a random experimental design of a wood ash treatment program similar to fertilizer applications that are typically carried out in the study area. In 2003, four experimental plots of size 35 x 35 m2 were established in quadruplicate. The experimental treatments were as follows: control (no treatment); WA [addition of 4.5 Mg dry matter (DM) ash ha-1], and WAP (addition of 4.5 Mg DM ash ha-1 plus 0.1 Mg P2O5 ha-1). In 2004 and 2005, the WA experimental plots were again treated with 4.5 Mg DM ash ha-1 (Fig. 6.2). It is important to

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Table 6.2 Chemical analysis of the horizons in the soils developed on lutites and migmatites Color pH (H2o) pH (KC1) OM (%) C/N (%) S (%) P (mg kg~! Ca ) (mg kg-1) К (mg kg-b Mg 1 (mg kg-1) CECe (cmolc kg-1) Cations (bases %) Lutites Ahl 10YR3/3 4.8 4.1 10.3 31.2 0.08 3.3 38.4 42.8 14.8 21.3 6.4 Ah2 10YR3/4 4.7 4.2 6.9 18.2 0.04 4.6 24.4 29.6 10.6 7.6 9.8 Migmatites Ahl 10YR3/2 4.7 3.8 7.3 19.3 0.02 4.7 39.1 32.1 10.6 10.7 14.8 Ah2 10YR3/2 4.6 3.9 4.4 22.0 0.02 3.8 23.6 22.3 8.5 10.7 14.8 OM organic matter, CECe effective cation exchange capacity

Fig. 6.1 emphasize that the phosphate fertilizer was only applied to the plots in 2003. The phosphate fertilizer (0-29-0) is partially disaggregated, and P release is therefore slower than from commercial fertilizers (e. g., superphosphate). This fertilizer is appropriate for acid soils and enables the amount of P applied to the soils to be monitored over time.

In Cote d’Ivoire the production of dried beans in 2005 was 740 kg/ha (Elzebroek and Wind 2008). According to ADM Cacao (2007 Interview with F. de Kort ADM

Cacao 22 November 2007. Koog aan de Zaan, the Netherlands), 10% of the beans consist of a shell, which means that per hectare 74 kg cacao shells is produced.

For the environmental impact of transportation many data are available. In this study the Eclipse data were used. Eclipse (Environmental and Ecological Life Cycle Inventories for Present and Future Power Systems in Europe) was funded by the EU and was carried out in 2002-2003. One of the objectives of Eclipse was to provide a harmonized set of public, coherent, transparent, and updated data on new and decentralized power systems for life cycle analyses.

The transportation by road in Europe is carried out by means of a heavy lorry trailer (401). The transportation by sea is carried out by means of a bulk carrier. The transportation by road in Cote d’Ivoire is carried out with a medium lorry. In Table 8.3, an overview is given of the emissions per ton kilometer of these vehicles. An overview of the assumed distances used in this study is given in Table 8.10. Transport of cacao beans to the Netherlands and transport of shells to bioenergy plants are not included, as these are present in both scenarios.

Blaise Pascal Bougnom, Brigitte Amalia Knapp, Francois-Xavier Etoa, and Heribert Insam

Abstract Infertility of acid soils is a major limitation to crop production on highly weathered and leached soils throughout the world. The main characteristics of these soils are their low pH, low levels of organic matter, Ca, Mg, P, or Mo deficiency, Al or Mn toxicity, or both, and very low mineralization and nitrification rates. Lime is generally recommended to correct soil acidity, but lime is unaffordable for resource-poor farmers in the tropics. Many alternatives have been proposed, and among them products from organic waste materials, e. g., composts, have proven to be an efficient alternative to the use of lime. Wood ash is a potential source of trace elements, nutrients, and lime. Wood ash could be used as an additive to fertilizer, and wood ash admixture to organic wastes prior to composting is known to improve compost quality and may reduce the amount of compost required to raise the pH to suitable levels. Wood ash compost as a liming agent as a replacement for lime could potentially aid in remediating acidity and base deficiency as well as boosting the soil microbial pool in tropical agricultural soils.

7.1 Introduction

Agricultural primary production is essential for maintaining human life. Sustaining the productivity of soils is important for future generations, but the way to maintain productivity is often disputed. Intensive agriculture is based on the use of large quantities of pesticides and other chemical substances aiming at increasing yields, but the price to pay is the deterioration of soil quality and the environment, water pollution, the emergence of new pathogens that are more and more resistant to pesti­cides, and the threat to human health from the consumption of pesticide residues

B. P. Bougnom (H) and F.-X. Etoa

Laboratory of Microbiology, Department of Biochemistry, University of Yaounde I, P. O. Box 812,

Yaounde;, Cameroon

e-mail: bpbougnom@uy1.uninet. cm

B. A. Knapp and H. Insam

Institute of Microbiology, University of Innsbruck, TechnikerstraBe 25 d, 6020 Innsbruck, Austria

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

DOI 10.1007/978-3-642-19354-5_7, © Springer-Verlag Berlin Heidelberg 2011

entering the food chain as well as inhalation of toxic gases. These practices, however, are beyond the reach of resource-poor farmers in the tropics, because of their high cost. On the other hand, agricultural and forestry products are exported from tropical to western countries; together with nutrients like nitrogen, phosphorus and potassium, present in this biomass. Sustainability issues are thus becoming more important, reflecting the need for long-term fertility and environmental protection. A sustained agricultural system is one in which the sum of income extracted every year is sustained over years without altering the natural resource levels (Yunlong and Smit 1994). Organic farming principles and objectives are achieving good crop yields by using techniques which minimize the human impact on the environment (Rigby and Caceres 2001). Organic farming can allow resolution of the problem of disposal of organic wastes that human beings have to get rid of and which are a valuable source of nutrients for plants, and serve the purpose of soil conditioners. That allows the carbon cycle to be closed and thus greenhouse gas emissions to be reduced.

Besides organic wastes, ashes from biomass incineration are also worth consid­ering for agricultural recycling. Wood energy production is classified as a form of green energy production because it is both carbon neutral and renewable (Kumar 2009). In many African regions, fuel wood constitutes 61-86% of primary energy consumption and generates large amounts of wood ash, which are just discarded to the natural environment without any control, causing serious environmental pro­blems (Samir Amous 1999). On the other hand, in Europe and North America, the increased use of wood to produce bioenergy generates huge quantities of wood ash that are currently deposited at high cost. That ash, known to be rich in nutrients and lime, could be returned to depleted soils as a supplement to organic fertilizers by suitable management practices (Bougnom et al. 2009). Addition of wood ash to compost is among the available possibilities, and the compost produced could be used for forest fertilization as well as for agricultural purposes such as replenishing depleted and/or acid soils (Bougnom et al. 2009, 2010; Kuba et al. 2008). In this chapter, we explore the possibility of using both compost and wood ash as an addi­tive to remediate tropical acid soils. The problem of soil acidity is also explained to inform the reader about the origin of this phenomenon and its consequence for soil fertility as well as the mechanisms by which organic wastes could alleviate soil acidity.

The data were obtained in industrial testing with manufacturing batches of about 1,750 t of brick. The daily production capacity of the plant where the tests were performed is, depending on the format and characteristics of the brick manufac­tured, approximately 750 t/day. Such large test runs are necessary to assess with sufficient accuracy, considering the latency of the tunnel kiln, the effects of addi­tions or substitutions on the firing process, emissions into air, and the energy consumption.

The use of ashes as an addition to the brick feed must take into account that in the clay free calcium ions are available for a pozzolanic reaction with some compo­nents of the biomass ash. This reaction is not immediate but rather slow and certainly accelerated by temperature. This reaction can cause problems in extru­sion: stiffening associated with a loss of plasticity and hence denting of the extruded strand. It is hence necessary to hinder or delay this behavior either by adding the ashes, as done for polystyrene, for example, directly prior to extrusion or by hydrophobizing the ashes at least partially with a suitable agent. In brick making
the two most suitable agents are waste glycerine from the production of biodiesel and waste antifreeze from the car industry. So far, no industrial tests with waste antifreeze compounds have been carried out. The trials were carried out with glycerine-treated ash.

The fruit and the wood combustion ashes were mixed in a 30:70 ratio and then subjected to hydrophobization with waste glycerine. The ashes were added to the raw material feed prior to its storage in the silo for souring. The raw material characteristics are indicated in Table 9.2. The mix was made up of 20% by volume of ashes and 80% by volume of clay. The use of ashes resulted in a noticeable reduction of the density and hence substantial improvement of the thermal perfor­mance of the finished product, as summarized in Table 9.3.

The reduction in unit weight is tied to a lower setting density. In the tests carried out, a reduction of about 8% of fuel was observed. A certain percentage can certainly be attributed to the energy content, body fuel, of the biomass ash.

Extrusion data were collected automatically and show that the power require­ment of the extruder was lowered by 8%, whereas the density achieved is lower. The overall savings in production costs, considering that the ash is delivered free of charge to the brick plant, is estimated to be about 7.5%.

Reactive swelling was not observed, at least at the percentages used in the tests. The bricks manufactured with the addition of the biomass ashes are of good quality. The analytical data for the raw ashes used and the brick manufactured are given in Table 9.4. No significant changes to the leaching values were found between a standard brick and a brick formed from the addition of biomass combustion ashes.

9.2 Conclusion

When hydrophobized biomass ashes were not used, the extrusion power require­ments increased rapidly to a point where the tests had to be stopped because of excessive power requirement.

The results of the tests with hydrophobized ashes are as follows:

• Lower power requirement of the extruder at almost the same cutting frequency but at the same time a higher extrusion pressure

• Reduction of the water content of the brick by 2.5-3.0% (wt)

• Reduction of drying cracks

• Reduction of fired sulfate visible on the surface of the brick

• Substantial modifications to the firing curve

These results are positive. A problem, at least with the ashes used in the test, is that they are not delivered in a way that allows the brickyard to accept them, without any major and costly modifications to the plant. Most are delivered in big bags. This requires extra effort and expense on the part of the brickyard. If the ashes were delivered in trucks, an appropriate silo, similar to one used for coal or pet coke, could be installed. Another unresolved problem is the seasonality of the ashes. A storage bunker to overcome this problem usually cannot be accommodated in a brickyard. Once the producer of the ash has resolved these problems, a brickyard can certainly become an ideal recycler for this type of waste.