Poplar trees have been extensively cultivated in many countries and several different tech­nologies for using their biomass have been implemented. Using wood obtained from SRP poplar as a fuel has energy, economic and environmental advantages when compared to coal and other fossil fuels. When used for direct combustion in heat and power plants, wood biomass has advantages over herbaceous biomass because of the lower quantity and higher quality ash that, in many cases, can be returned and applied as a soil amendment. The quantity of ash is related to chemical composition and bark content. Therefore, Guidi et al. [58] conducted studies to determine allometric relationships to predict fuel quality of poplar biomass before harvesting was undertaken. They found a significant relationship between bark content and main stem diameter at 130 cm (diameter at breast height, DHB) and pointed out that for DHB classes between 1 and 4 cm there was a rapid reduction in bark content compared to stems with a DHB of less than 1 cm. This indicated that it is more rational to harvest SRP poplar in three or four-year cutting cycles or to use poplar clones that do not produce a high number of low DHB stems.

Poplar wood can also be treated as a feedstock for production of second and third gen­eration biofuels through conversion of lignocellulose into ethanol [59] and other fuels. However, it is important to recognize that lignocellulosic biomass is a complex matrix of hemicelluose, cellulose and lignin and, therefore, pretreatment (sometimes called prehy­drolysis) is required before the biomass can be converted into liquid fuels. Authors studying different methods of Populus nigra biomass pretreatment (steam explosion and hot water pretreatment) have found that the former process gave better cellulose recovery when mea­sured by enzymatic conversion of the biomass into bioethanol [60,61]. In an extensive review, Huang et al. [62] also reported numerous technologies designed to provide the most effective pretreatment of lignocellulosic biomass and conversion into ethanol. They concluded that the best results have been achieved when complex methods (i. e., chemical, physical, and/or biological pretreatments) were combined. For enzymatic hydrolysis and fermentation, the most important and efficient method utilized cellulase produced by the commercially available fungus Trichoderma reesei.

Zhang et al. [63] presented an interesting but challenging approach to utilize the lignin and hemicelluloses in addition to the cellulose components. According to their citations, economically sound and environmentally friendly technologies for processing these com­ponents, once considered waste, have been developed and are being used to produce marketable products. Among them is the potential to replace phenolic compounds from the oil industry with lignin-originated products, while hemicelluloses, because of their less stable nature, can be converted to a mixture of monosaccharides.

Van Acker et al. [64] reported that by using biotechnology, poplar biomass can be converted into liquid biofuels without costly and energy consuming pretreatment. This can be achieved by reducing the amount of lignin in the wood biomass or by changing its composition to obtain forms that are more susceptible to chemical degradation, thus making saccharification more efficient. The key enzyme in the phenylpropanoid pathway for lignin modification is cinnamoylo-CoA reductase (CCR). Trees that have been genetically modified in terms of CCR regulation were originally produced for the pulp industry, but this trait appears to be even more suitable for processing of poplar wood into second generation biofuels, since saccharification was increased by 50%.

Klasnja et al. [65] compared the calorific value of willow and poplar biomass, with special attention given to a comparison between old and young stems of both species. Bark was separated from wood. The higher heating values of oven dry poplar wood (calculated for the whole tree with an adjustment based on the proportion of bark) ranged from 15 787 to 24 275 kJ kg-1 for one and two-year old clones of hybrid I-214, respectively. The authors concluded that the calorific value of wood is more favorable than that of bark, and the highest calorific values refer to two-year-old trees. Their other conclusion was that woodchips from young SRPs harvested biannually could be used as biofuel without the bark separation needed when using older stems.