## 2 Lignocellulose Feedstock Biorefineries
### Low-Input Crops: Sources and Availability
#### 2.3.2 Fast Growing Trees: Paulownia, Eucalyptus, and Pinus  
Due to recent efforts in bioreinery development, fast growing trees attract more and more attention for study as an industrial crop. Besides bamboo, poplar, Eastern cottonwood, giant sequoia, and acacia (not discussed here), Eucalyptus, pine, and _Paulownia_ belong to the fast growing lignocellulosic-rich crops that are currently under investigation to be used as potential feedstock for second-generation biorefineries. Compared to conventional trees, the growing cycles (silviculture rotations) of fast-growing trees are below 15 years, thereby offering environmental and/or genetic manipulation [108].  
One prominent example is the fast-growing _Paulownia_ tree, originally cultivated in Asia, mainly in China and other tropical and sub-tropical regions, and characterized by a low demand for water. _Paulownia_ trees grow quickly, reaching 10 to 20 m in height and 30-40 cm in diameter in less than ten years. [PERSON] and colleagues reported a study on _Paulownia tontesoa_, a genotype that reaches 30-40 cm in diameter within five years (Figure 5) [109].  
Figure 4: _Miscanthus_ fields at the Campus Klein-Altendorf, Rheinbach, Germany. **Left**: _Miscanthus x giganteus_ in spring time. **Right**: Color differences resulting from crops planted in different seasons; light colored parcels of dried crops versus darker fields of freshly planted crops. Copyright 2019, [PERSON].  
Paulownia  
samples were cultivated in the Shanxi province in China. The authors used enzymatic hydrolysis for biomass pulping, resulting in a ratio of about 42% cellulose, 20% hemicellulose, and 20% lignin [110,111]. Prior to enzymatic hydrolysis, various pre-treatment methods had been investigated (i.e., using dilute acid, alkali, and alkali supported by ultrasonic pretreatment, with the last one being the most efficient method).  
[PERSON] and colleagues studied Iranian-cultivated _Paulownia fortunei_ L. fibers, with a specific focus on their chemical and morphological characteristics. Results showed that Iranian _Paulownia fortunei_ L. consisted of holocellulose, alpha-cellulose (about 52%), lignin (about 25%), and further extractives (about 15%, isolated from basic media). In addition, the authors determined fiber characteristics (i.e., length, width, cell wall thickness). Of special interest and a focus of scientific investigations is the fibrous parenchyma, a promising raw material for paper of high density, due to the material having a high tensile strength [112].  
[PERSON] et al. studied the polypropylene (PP) filler additives used to reinforce the polymer bulk. The studied samples included canola, paulownia, and nanoclay fillers in varying concentrations (3 and 5 wt%). Compared to canola and nanoclay fillers, _Paulownia_ particles significantly improved the mechanical properties of the studied composites. Transmission electron microscopy and X-ray diffraction were used to specify the final polymer morphology and filler dispersion within the polymer matrix [113].  
Besides _Paulownia_, _Eucalyptus_, and _Pine_ are further examples of fast-growing trees. [PERSON] et al. recently reported a study on thermomechanical densification influenced by process parameters of two different crops: _Eucalyptus grandis_ and _Pinus Elliottii_[114]. Samples of both woods showed comparable densities and mechanical strength. Most obviously, significant differences resulted from differences in crop composition. Thus, the _Eucalyptus_ hemicellulose (in difference to _Pine_) mainly consists of xylose of a higher degree of acetylation, that is more susceptible to degradation. Nevertheless, _Eucalyptus_ samples showed densities close to 1.0 g\(\cdot\)cm\({}^{-3}\) and improved mechanical properties (i.e., bending, hardness, impact resistance) after pre-treatment.