Maritime pine (Pinus pinaster Aiton.
) is the second species in terms of occupied area in Portugal, accounting for around 23% of forest area, occurring mostly in the north and central regions of Portugal [1
]. An estimation made in 2010 for NUTS II (Nomenclature of Territorial Units for Statistics) reported a yearly average of 579.9 thousand dry tons, ranging between 400.96 thousand dry tons per year and 673.54 thousand dry tons per year [2
In 2017, forest fires were particularly severe, with a forest area of about 500,000 ha burned in the country and, presently, a large amount of biomass supply is available for conversion [3
The energy acquired from biomass is distinguished and remarkable in that it can be obtained without difficulty and is also renewable [6
]. It promotes the protection of the environment, since biomass is abundant, natural, and reusable [7
]. The chemical composition and physical properties of biomass ashes and biomass components, wherein the latter is strictly linked to its anatomic polymeric structure, are determinant for the performance of thermochemical conversion of the feedstocks [8
]. Biomass shows highly variable chemical composition and properties regarding moisture, structural and inorganic components, which is related to the myriad plant growth processes and growing conditions [13
]. The main and less important elements of biomass, in diminishing order of quantity, are normally the following: C
, in addition to Mn
and other trace elements. Typical proximate analysis of maritime pine components shows values ranging between 65% to 72% for volatiles, 12.5% to 21% for fixed carbon, and 0.2% to 2.6% for ash content. Typical values for ultimate analysis of C
, and O
range between 46% and 56%, 5% and 6%, 0.1% and 0.9%, and 31% and 37%, respectively. Sulphur amounts are negligible. Finally, the High Heating Value (HHV) ranges from 18 MJ·kg−1
to 17 MJ·kg−1
As a comparison, for coal, the carbon content is, on average, about 80% or higher, the oxygen about 8% or lower, and the N
are about 1.8% and 0.8% or higher, respectively. Fixed carbon and volatile amounts for coal are of the order of 50% and 44%, respectively. HHV and LHV for coal are of 35 MJ·kg−1
and 33 MJ·kg−1
, respectively. This chemical profile fits the overall picture that woody biomasses posit advantages over fossil fuels such as coal related to carbon neutrality and reduced emissions of SO2
]. However, handicaps of woody biomass, such as lower energy density and calorific values, hygroscopic properties, and high moisture content leading to degradation and self-heating, make the handling and transportation costlier and more complex.
Torrefaction of the feedstock is an option for minimising these drawbacks and making the chemical profile of torrefied biomass closer to that of coal. Biomass torrefaction is a thermochemical pre-treatment, carried out in the absence of oxygen at temperature ranges of about 220 °C to 320 °C, delivering a product with lower O/C and H/C ratios, closer to typical coal ratios [15
]. The torrefied products are more homogeneous and show higher energy density and grindability [16
]. Also, the similarity of the torrefied pulverised product to coal powder makes possible its co-firing, at amounts as high as 40%, with coal in power plants [17
]. Experimental evidence with torrefaction of maritime pine carried out at a lab scale made it possible to obtain a torrefied product with a LHV of 22.6 MJ·kg−1
, corresponding to a gain of about 20%, a C amount of 65% (15% gain), a fixed carbon amount of 30% (35% gain) and 29% for oxygen amount, corresponding to a decrease of 21% from raw biomass [18
]. Thereby, the ratio of O/C decreased by 32% for torrefied maritime pine biomass. The ratio of torrefied pine biomass also decreased by 18%. The LHV in conifers is on average 2% higher than in hardwoods. This difference occurs due to a higher lignin content, and sometimes due to the higher resin, oil and wax content present in conifers [18
]. Indeed, while LHV for cellulose ranges between 17.2 MJ·kg−1
and 17.5 MJ·kg−1
and is about 16 MJ·kg−1
for hemicelluloses, in lignin, LHV is higher, ranging between 26 MJ·kg−1
and 27 MJ·kg−1
. A slight variability in the calorific value can be witnessed and occurs due to certain variability in the elementary H
content. It also occurs as a result of a much larger variability in ash contents [19
Low heating value is a variable which allows evaluating the enthalpy released in forest fires through the combustion of fuels available in the field surface, thereby providing additional information about the easiness of fire propagation. The LHV of the different forest canopy components is thereby one valuable indicator of the energetic status of forest biomass which helps to optimise the management of the energetic forest resources in the field. In [20
], data is presented for the seasonal variability of biomass flammability and the heating value of maritime pine woody biomass components in NW Spain.
The Fuelwood Value Index (FVI), expressed in MJ·m−3
and defined as the ratio between the products between the calorific power and density and between the ash and moisture contents, is another relevant variable for ranking the fuel aptitude of woody biomasses for different species [21
Energy density (Ea), expressed in GJ·m−3, defined as the product between LHV and bulk density, is another variable that can be assessed in order to identify the potential and to choose a fuel for use in small-scale heating plants and households. The energy density is relevant for woody fuels, because its storage and transport could be cheaper and more efficient at a higher energy density.
Another factor that could be taken into consideration is the evaluation of biomass ashes, concerning either its chemical composition as the mechanisms and quantities produced in the thermo-chemical conversions. Biomass ashes are made up of Cl and S, with major elements (Al, Ca, Fe, K, Mg, Na, P, Ti, Si) and minor or trace elements (As, Ba, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Sb, Pb, Tl, V, Zn). In comparison with coal, biomass usually shows higher amounts, in decreasing order, of Mn, K, P, Cl, Ca, Mg, Na, O and H, and lower amounts of ash, Al, C, Fe, N, S, Si, and Ti.
Elements included in biomass ashes can be divided in two classes: intrinsic or inherent chemicals bonded to a carbon structure, and entrained elements that can come with biomass as mineral soil particles that have been incorporated into biomass during plant growth, or taken away during harvesting and transport [22
]. These elements are relevant for aerosol emissions, ash melting, deposit formation, fly ash and corrosion, and also for the utilisation/disposal of the ashes.
During combustion, some volatilisation of ash-forming compounds occurs, with the volatised fraction depending on factors such as the technology employed, chemical composition of the fuel, operative temperature, and surrounding gas atmosphere. Indeed, volatilisation of heavy metals such as Pb
can happen under high operative temperatures and low oxidising atmospheres. Toxicity due to heavy metals (Cu
) can cause serious consequences in soils located in sites corresponding to former agricultural areas with excessive use of fertilisers, fungicides and insecticides. Dangerous trace elements in the form of salts are highly mobile on the ashes, but issues related to their accessibility and availability still lack some fundamental knowledge. For example, these trace elements may be water soluble or bound into glass, which is formed from the fusion of inorganic material in biomass [14
Some toxic elements such as As
, and Zn
are described in the literature with concentrations as high as 243 ppm, 657 ppm, 0.17%, 5%, 0.24%, 4.7%, 25 ppm, 0.5 ppm and 16.4% in fly ashes, respectively (e.g., [23
]). Plant growth and biomass can thereby be drastically affected by release of these elements.
Typical ranges in ash element composition of biomass tree components (wood, bark, leaves, tops or branches) for Fe
, and Ni
are 3000–40,000, 4700–74,000, 3–60, 0–25, 15–650, <1–20, 15–400, 10–250, 1000–30,000, 15–4400 and 6–200 mg·kg−1
, respectively [14
]. In industrial areas, nickel toxicity is particularly acute, at high soil concentrations above a threshold of 200 ppm with drastic physiological consequences. Excess Cd
exerts similar effects in plants, and damage to root cells has been reported on grey poplar. Plant growth, biomass production, shoots and root lengths has been ascribed to arsenic contamination in soils as a result of the widespread use of arsenical pesticides. Thereby, heavy metals in biomass ash, e.g., Zn
above certain limits, are surely toxic to plants, precluding their use as fertiliser, insofar as its application to soil can constitute a potential source of contamination for aquatic and terrestrial ecosystems [24
Under this negative environmental context, there are strict regulations in some countries, e.g., in North and Central Europe, specifying thresholds on amounts of elements such as Co
in biomass fuels and ashes, considering all possible forms of conversion [26
The assessment of ash use in forestry and agriculture differs between European states, to some extent due to different conditions [28
]. While in some countries, mainly the Nordic countries of Europe, the use of ashes is a key factor for the replenishment of nutrients to acidic and poor soils; in other countries, such problems are not so deep and are therefore less worrisome. On the other hand, the logging of forest residues for energy production is made in a superior extent, extracting a large amount of nutrients in biomass. For instance, in Sweden, the recycling of ash is deemed to be a significant measure for forestry sustainability, while in Finland, the ash is considered to be a fertiliser and is used to increase the growth of forests growing on peatlands. As another example, in Denmark, ash recycling is deemed to be a method to make up for the loss of phosphorous and potassium [29
]. As a consequence, several producers of wood in Northern America and Europe are beginning to recycle ash on an operational scale. For instance, in Finland, over 10% of wood and bark ash created by forest industry returns to the forest [30
], as the wood ash is dispersed at high rates in many forests (in Finland 3–5 t·ha−1
and in Sweden 1–3 t·ha−1
The present work, under all of the above context, aims to evaluate the chemical composition and physical properties of the maritime pine biomass, considering proximate, ultimate and ash analysis in the tree components stem, tops, branches and needles, sampled from 16 plots in the Northern Portuguese county of Viseu.