Bioenergy Production Potential Of Decaying Hardwoods

As the quality of unharvested hardwoods fibre is low and their outlets in conventional wood transformation industries are absent, unharvested hardwoods are abundant in eastern Canada. The objective of this study was to assess the biochemical and thermochemical energy conversion potential of decaying hardwoods and compare their relationships with external and internal indicators of tree degradation. By Éloïse Dupuis, Evelyne Thiffault, Laval University, Julie Barrette, Direction of Forest Research, Kokou Adjallé, Université de Québec, Christine Martineau, Canada Laurentian Forestry Centre

Unharvested hardwoods are plenty in eastern Canada, due to the low quality of their fibre and the absence of outlets in conventional wood transformation industries. 

Thus, we need assess the biochemical and thermochemical energy conversion potential of decaying hardwoods and compare their relationships with external and internal indicators of tree degradation. 

We characterised how wood-decay processes altered the physical and chemical properties of these woods and affected their digestibility yield and their performance according to indexes of stability and efficiency of combustion. 

DNA analysis on wood samples was also performed to determine the relative abundance of white-rot fungi compared to that of other saprotrophs. All properties stayed within the range of variations allowing the wood to remain suitable for conversion into bioenergy, even with increased decay. 

We found no significant differences in the physical and chemical properties that are crucial for energy production between wood from externally-assessed live and decayed trees. 

However, the proportion of wood area affected by rot was significantly associated with increased digestibility yield, and with decreased combustion reactivity. We could not detect any specific effect associated with increased relative abundance of white-rot fungi. 

These results suggest that the utilisation of biomass from decayed hardwoods instead of live trees for bioenergy production should not alter the conversion efficiency and even potentially increase the performance of biochemical pathways, and hence, support their use as feedstock for bioenergy production.


Importance And Conversion Of Forest Biomass

The Intergovernmental Panel on Climate Change (IPCC) has identified forest biomass as an important source of renewable energy in the context of GHG mitigation goals. 

The use of whole trees for energy generation is limited by ecological and carbon balance constraints. 

However, pulp-quality logs that have no other market have been considered as part of sustainable biomass feedstock for bioenergy. 

Moreover, the IPCC recognises that biomass from trees affected by natural disturbances can contribute to the overall technical potential of forest biomass. 

On the other hand, recent work has shown that the harvesting operations of degraded trees in naturally disturbed stands for wood pellet production were not economically profitable, and suggested that the production of high-value bioenergetic products could raise operational profitability.

The conversion of forest biomass to high-value bioenergetic products can follow the biochemical or the thermochemical pathway. The biochemical conversion pathway includes processes that involve natural degradation (that can be induced artificially) through the action of enzymes or bacteria, such as bioethanol production via enzymatic hydrolysis and fermentation or anaerobic digestion. 

The main obstacle to biochemical conversion success is biomass recalcitrance, which is defined as the natural resistance of the plant to enzymatic and microbial degradation. 

Biomass recalcitrance resides in the plant’s physical structure (pore size and volume, specific surface area, cellulose crystallinity) and chemical composition (high lignin concentration and acetyl group). 

Overcoming biomass recalcitrance is possible by the application of pre-treatments that alter the physical structure and chemical composition of biomass and enhance the accessibility of cellulose and hemicelluloses to enzymatic digestibility. 

Pre-treatments are responsible for a significant proportion of the conversion costs, and it is difficult to achieve good bioethanol yield without high energy inputs. 

The high cost of the conversion process restricts bioethanol production on an industrial scale. Some pre-treatment technologies also lead to the denaturation of sugars that will inhibit the fermentation process and therefore lower the conversion efficiency. 

Furthermore, some pre-treatment technologies produce acidic or alkaline wastewater that needs to be treated before being released into the environment.

Fungal and microbial treatments have been suggested as an environmentally friendly and cost-competitive alternative to traditional pre-treatments in order to enhance enzymatic hydrolysis. 

The biological pre-treatments employ microorganisms such as white-rot fungi to degrade lignin and hemicelluloses extensively, but leave cellulose mostly untouched. 

The downsides of this method are the long residence period and the loss of carbohydrates (hemicelluloses and, to a lesser extent, cellulose) that reduce the overall conversion efficiency. 

However, evidence has shown that natural wood degradation processes associated with fungi, insects or fire have the potential to reduce recalcitrance and enhance enzymatic digestibility.

The thermochemical conversion pathway includes processes that use heat as a vector of decomposition of the raw material. The most studied conversion technologies are combustion, pyrolysis and gasification. 

The energy content per mass unit, expressed by the calorific value (or higher heating content, HHV), is the most important parameter that influences the quality of thermochemical fuels. 

Since lignin has a higher calorific value (23.26 to 25.58 MJ/kg) than the other wood components such as cellulose and hemicelluloses (18.60 MJ/kg), the HHV of biomass is strongly correlated to the lignin content of the biomass. 

The presence of lignin-decomposing fungi such as white-rot could negatively affect the HHV of decaying biomass, whereas degradation caused by other types of saprotrophs (whose actions affect other wood components) could have a neutral or a positive impact.

Unharvested volumes of hardwoods are abundant in Quebec (Canada). Although they are part of the available annual cut (i.e., the sustainable rate of forest harvest), these volumes are often not harvested because of the low quality of their fibre induced by past unsustainable management techniques (e.g., stand high-grading in which only trees of superior quality were harvested) and the attack of fungus and insects. 

Bioenergy production from unharvested low-quality woods could potentially contribute to meet renewable energy goals as well as the restoration of degraded forests.

The main objective of this study was to assess the biochemical and thermochemical energy conversion potential of decaying and unharvested hardwoods.


Analyses On Sampling And Laboratory

Five different species, indigenous to the province of Quebec, including sugar maple (Acer saccharum Marshall), yellow birch (Betula alleghaniensis Britt.), white birch (Betula papyrifera Marshall), American beech (Fagus grandifolia Ehrh.) and trembling aspen (Populus tremuloides Michx.), were sampled in different study areas across Quebec’s temperate hardwood forest and the southern limit of the boreal forest. 

Sampling sites were spread across the following bioclimatic domains: sugar maple-basswood, sugar maple-yellow birch and balsam fir-white birch. 

Trees with a diameter at breast height (1.3 m) ranging from 10 to 40 cm were selected to represent different stages of degradation according to Hunter’s visual classification. 

We modified Hunter’s classification and grouped the stages into three primary decay levels: live (Hunter’s classes 1–2), dead (Hunter’s classes 3–4) and advanced rot (Hunter’s classes 5–6–7). 

These decay levels are therefore based on external visual assessment of tree degradation. The number of sampled trees within each combination of species and the level of decay are specified.

Each wood disc was scanned, and the scans analysed with the ImageJ software; portions of the disc affected by rot were delineated, and their areas were measured, along with the total area of the disc, so that the proportion of wood area affected by rot could be calculated.

The physical properties of biomass were assessed by the determination of moisture content and basic density, which are relevant to both conversion pathways. 

Generally, moisture content varies widely because it depends on harvesting and storage methods; moisture content can also be easily manipulated with proper pre-treatment and conditioning practices. 

Basic density is influenced by other factors such as environmental conditions, growth rate, wood defects, and compression wood. We applied the standard methods from ASTM International to determine moisture content and basic density.

In order to assess the bioenergy conversion potential for biochemical and thermochemical pathways, we used specific chemical characteristics and conversion tests. 

As an indicator of performance for the biochemical pathway, we tested sugar production by enzymatic hydrolysis. For the thermochemical pathway, we chose to test the combustion process with thermogravimetric analysis.

Samples were oven-dried at 60 deg C for 72 h, debarked, and a section of the discs was milled into 0.5 mm particles and stored in hermetic bags at room temperature until further analysis. The relative content of cellulose, hemicelluloses, lignin and extractives were measured with the method presented in Van Soest, et al..

Ash content was determined by weighting samples in porcelain crucibles, before and after spending 6 h at 600 deg C, following the standard procedure of. 

We used the potassium concentration in ash as a modified alkali index. The concentration of alkali metals in ash indicates the risk of slagging and fouling in the reactor, with a higher index or a higher K concentration representing greater risk. 

The remaining ashes were then mixed with chloric acid, heated to allow dissolution, and filtered. The resulting liquid was transferred for analysis of major elements using inductively coupled plasma optical emission spectrometry IPC-OES (Agilent 5110).

The higher heating value (HHV), defined as the energy content per mass unit (MJ/kg), was measured with a bomb calorimeter (Parr 6400), which was calibrated with benzoic acid. 

Approximately 0.7 g of ground wood sample was pressed into a tablet before being burnt in the presence of oxygen. Combustion in the bomb calorimeter was induced by a cotton thread attached to the platinum ignition wire in contact with the sample tablet.


Thermogravimetric Analysis

Thermogravimetric analysis (TGA) was used to assess the thermochemical conversion potential. This type of analysis makes it possible to determine the thermal degradation pattern of biomass. The ignition (Ti) and burnout (Te) temperatures, along with the ignition index (Di), combustion characteristic index (S) and the flammability index (C) of biomass, can be deduced from the thermogravimetric results. 

The comparison of the different parameters expressed by thermal degradation patterns makes it possible to determine which type of biomass has the highest heat transfer and combustion efficiency. TGA analysis was performed on a Mettler Toledo TGA.

The crucible was heated to red before and between the analysis of each sample. Approximately 0.15 g of ground material was placed in the crucible. Air was used as a carrier gas, with a flow rate of 50 mL/min. The samples at room temperature were heated to 105 deg C, at a rate of 30 deg C/min and kept at 105 deg C for 10 min to remove moisture entirely from the samples. 

In the third heating segment, samples were then heated from 105 to 800 deg C, at a rate of 30 deg C/min. Thermogram (TGA) curve and its first derivative (DTG) curve, presented in the relative weight scale, were used to determine ignition and burnout temperature using the intersection method (IM) described by Lu, J.-J. The ignition temperature corresponds to the minimum temperature at which the wood sample ignites spontaneously in the absence of an external source of ignition. 

For its part, the burnout temperature is an indicator of the reaction degree of the fuel: it corresponds to the temperature at which the wood is almost entirely consumed, or at which 99 percent of the conversion is completed.


Effects Of Wood Decay On Physical And Chemical Properties

Results from the ANOVA suggested that the effect of the interaction of species x decay level.

The standard deviation of relative wood area affected by rot (an internal indicator of tree degradation), moisture content and basic density showed a considerable variation within each combination of species x decay level. 

The percentage of wood affected by rot and moisture content were significantly affected by the interaction of species x decay level (p-value > 0.001). 

For trembling aspen, the percentage of wood area affected by rot and moisture content showed somewhat clear distinctions between trees with advanced rot and trees from the same species at lower decay levels (i.e., live and dead trees, based on an external indicator of tree degradation), the former displaying higher percent of rot and lower moisture content. 

American beech trees at an advanced rot decay level also had higher percent of wood affected by rot relative to live and dead trees. 

On the other hand, the effect of species and decay level was not significant for basic density (p-value: 0.390).

Relative lignin content was significantly affected by the interaction of species and externally assessed decay level (p-value: 0.004). However, no clear pattern of the effect of decay level on lignin was detected. 

The relative carbohydrate content (cellulose + hemicelluloses) was also significantly influenced by species and decay levels (p-value < 0.001). Trees in the live and dead stages had a higher carbohydrate content than the ones in the advanced rot stage. However, the effect was not detectable on the white birch sample in the advanced rot stage.

Ash content significantly increased with increasing externally assessed decay level (p-value: 0.003). The concentration of K in ash was not affected by decay level but differed between species (p-value: 0.009): Tukey contrast showed that American beech presented a significantly higher concentration of K than white birch. 

Higher heating values (HHV) were significantly affected by the interaction effect of species x decay level (p-value: 0.023), although the variations were maintained within a narrow interval (19.18 for American beech in the advanced rot stage and 20.08 for trembling aspen in advanced rot).

Results from linear regression also showed the significant effects of wood decay when expressed as the proportion of wood area affected by rot, on ash (positive effect; p-value < 0.001) and carbohydrate (negative effect; p-value < 0.001) contents. 

Moreover, this internally assessed indicator displayed a significant relationship with basic density (negative effect; p-value = 009) and lignin content (positive effect; p-value = 0.048).


Decaying Fungal Organisms And Effect On The Wood Composition

Results from DNA analysis suggested that fungal communities (beta diversity) were different between decay levels, but not between tree species. The number of OTUs (alpha diversity) detected in each tree increased with the external signs of decay. 

The number of different OTUs also differed between tree species. Using ANOVA and Tukey contrasts, we observed a significant difference between the alpha diversity of yellow birch and American beech (p-value: 0.002).

A total of 286 different taxonomic units (OTUs) were identified among the 91 samples tested. Over 20 percent of the overall fungal diversity (in terms of the total number of OTUs) was represented by organisms belonging to the Agaricomycetes class, which are thought to be the most efficient decaying fungi. 

Agaricomycetes exhibit two main modes of plant cell wall decomposition: (i) white-rot, which will digest lignin first and; (ii) brown-rot, which will digest carbohydrates first. 

Our results showed that the relative abundance of Agaricomycetes (based on the relative abundance of DNA sequences associated with this fungi class) was higher within advanced rot samples than within dead and live samples. 

The relative abundance of white-rot fungi (also based on the relative abundance of DNA sequences associated with these fungi) was also significantly higher in advanced rot samples than in live and dead samples (p-value< 0.001).


Decaying Hardwoods Impact On Bioenergy Conversion

In order to assess the biochemical and thermochemical energy conversion potential of decaying and unharvested hardwoods, firstly, we characterised the impact of wood degradation and fungus type on the expression of the wood’s physical and chemical properties. 

Then, the potential conversion of decaying hardwoods to energy via the biochemical and thermochemical pathways was evaluated using enzymatic hydrolysis and thermogravimetric analysis, respectively. 

Two indicators were used to describe wood degradation: an external visual indicator of tree degradation based on Hunter’s decay classification, which could be useful during forest inventory and during tree marking for selection cutting, and an internal indicator of wood decay based on the relative wood area affected by rot.

Through the study, it gives an overview of the decay process of hardwoods in the eastern Canadian context, and its impact on bioenergy conversion. It explored the effect of decay processes on the bioenergy conversion of hardwoods, by analysing the impact on physical and chemical properties on the biochemical and thermochemical responses of decaying biomass.

We found out that biochemical conversion can be improved by a higher proportion of wood rot. Nonetheless, natural wood decay, even by white-rot organisms, is apparently not sufficient to act as a biomass pre-treatment, as enzymatic hydrolysis of decaying hardwoods yielded overall poor digestibility. 

Results however showed that indicators of combustion performance drastically dropped with the advance of decay. Our study also suggests that tree decay, as visually assessed with Hunter classification on standing trees during forest inventories, is not precise enough to serve as an indicator of wood suitability for bioenergy conversion. 

However, the proportion of wood area affected by rot, which can be assessed on cut logs, can be a relevant index for bioenergy conversion potential. As such, this index could be used during log sorting at the roadside or in a mill yard to identify adequate feedstocks for conventional wood products (lumber, pulp) and bioenergy.

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