Ash-forming elements in four Scandinavian wood species. Part 1: Summer harvest

Abstract

Woody biomass in Finland and Sweden comprises mainly four wood species: spruce, pine, birch and aspen. To study the ash, which may cause problems for the combustion device, one tree of each species were cut down and prepared for comparisons with fuel samples. Well-defined samples of wood, bark and foliage were analyzed on 11 ash-forming elements: Si, Al, Fe, Ca, Mg, Mn, Na, K, P, S and Cl. The ash content in the wood tissues (0.2–0.7%) was low compared to the ash content in the bark tissues (1.9–6.4%) and the foliage (2.4–7.7%). The woods’ content of ash-forming elements was consequently low; the highest contents were of Ca (410–1340 ppm) and K (200–1310), followed by Mg (70–290), Mn (15–240) and P (0–350). Present in the wood was also Si (50–190), S (50–200) and Cl (30–110). The bark tissues showed much higher element contents; Ca (4800–19,100 ppm) and K (1600–6400) were the dominating elements, followed by Mg (210–2400), P (210–1200), Mn (110–1100) and S (310–750), but the Cl contents (40–330) were only moderately higher in the bark than in the wood. The young foliage (shoots and deciduous leaves) had the highest K (7100–25,000 ppm), P (1600–5300) and S (1100–2600) contents of all tissues, while the shoots of spruce had the highest Cl contents (820–1360) and its needles the highest Si content (5000–11,300). This paper presented a new approach in fuel characterization: the method excludes the presence of impurities, and focus on different categories of plant tissues. This made it possible to discuss the contents of ash element in a wide spectrum of fuel-types, which are of large importance for the energy production in Finland and Sweden.

Introduction

Woody biomass accounts for 20% (78 TWh) of Finland's and 14% (89 TWh) of Sweden's total energy supply [1], [2]. The solid fuels for large-scale energy production comprise mainly three categories: wood-fuels, bark-fuels and forest residue. In wood-fuels, i.e. sawdust and wood-chips, the main constituent is the wood from the stem of a tree, mainly of domestic species. The bark-fuels are debarking residues from the wood- and paper-industries, and contain mixes of the stem-bark of Scandinavian species. Forest residue is the logging debris collected after harvesting of timber. It contains the remaining parts of the tree, which includes the branches, twigs and foliage, often in a mix of more than one species.

The fuel's incombustible leftover—the ash—may cause problems for the combustion device. It may adhere to the heat transfer surfaces and this way cause fouling and corrosion. In fluidized bed combustion, the ash renders the sand particles sticky and cause bed agglomeration problems. The ash originates in the fuel, and sudden and unknown variations in fuel quality increase the risk for ash-related problems. For biomass fuels, particularly the levels of K, Na, S and Cl are in focus. In combustion, these elements may form molten phases that make the ash-particles sticky and cause adhesion to the heat transfer surfaces [1], [2].

The contents of ash-forming elements in woody biomass fuels vary greatly, both from fuel to fuel and within a certain fuel-type. In a compilation of fuel data, of which some are published earlier [3], [4], [5], particularly strong variations were noticed for the concentrations of Si (20–19,200 ppm), Al (4–2100) and Fe (2–1600), as shown in Table 1. The variations reflect the presence of various plant tissues and the presence of foreign matter, such as sand and soil that accompany the fuels.

The causes for variations in fuel quality are not well known. The variation caused by the sand and soil impurities could be excluded by analyzing uncontaminated plant tissues. Such analyses would also recognize differences between fuel components, i.e. various plant tissues of different species, which would further allow fuel-suppliers, power-plant operators and boiler manufacturers to specify or even influence the fuel quality before firing, or to specify the quality of the fly ash for recycling purposes.

The presence of ash-forming elements in trees is the result of biochemical processes, the mineral uptake from soil and the element transport within the trees. Some of the main ash-forming elements are essential for plant growth [6]. These are divided in macronutrients (K, Ca, Mg, P and S) and micronutrients (Fe, Mn, and Cl) after their level of occurrence [7]. Si, Al and Na are non-essential for plant growth [6].

The elements enter the tree in aqueous solution. The metals enter as cations, Fe³⁺, Ca²⁺, Mg²⁺, Mn²⁺, K⁺ and Na⁺, and the non-metals enter as anions [8], H₂PO₄⁻, SO₄²⁻ [9], Cl⁻, or non-charged as H₄SiO_4(aq) [10], [11]. After uptake, the elements are transported by the fluid in the wood (xylem sap) and by the fluid in the inner part of bark (phloem sap) [6].

The element content in the xylem and phloem sap solutions [7], [12], [13] does not correspond to the total contents in any part of the tree. Instead, the elements are trapped by different means, either by forming a part of the solid tissues, such as becoming organically associated [14], [15] or precipitating as inorganic salts [14], [15], [16]. Some elements remain in aqueous solution and accumulate in a third type of sap [17], the cell sap, which is present inside the living cells.

This article reports on the natural concentrations of 11 ash-forming elements in four Scandinavian wood species: Norway spruce, Scots pine, Downy birch and Aspen. The wood, bark and foliage of these species constitute the vast majority of the woody biomass in Fenno-Scandinavia. This includes various wood- and bark-fuels, forest residue and even black liquor, although black liquor is quite different in terms of composition and will not be discussed here.

The objective of this study was to map the main ash elements in four Scandinavian wood species in order to: generate reference data for comparisons with industrial fuel samples; explain the natural variations in fuel quality; and to render possible to improve the fuel quality by pre-treatment. The wood, bark and foliage from four full-grown trees were collected and analyzed using two parallel analytical procedures, of which one used in-house equipment and the other required external equipment. The results were compared and validated with data from the literature. This rendered a good overview and a sound basis for the interpretation of the results.