logistics of biomass
The 2nd step of value chain is harvest, handling, transport and storage of biomass, which also offers great opportunity to optimize biomass quality. In harvesting of biomass, the time of harvesting plays key role and it depends on the end use of biomass and prevailing weather conditions. However, time of harvesting affects not only biomass quality but also yield. For combustion delayed harvest is preferred, whereas early harvest is suitable for ethanol production. The delayed harvest leads to yield losses through stem damages and leaf fall. Many studies indicate that the delay in harvest time improves the biomass combustion quality but it is on expense of dry matter yield (Clifton-Brown et al., 2001; Lewandowski and Heinz, 2003; Kludze et al., 2012; Bilandzija et al. 2016) and the yield losses can be up to 29% (Stéphanie and Maryse, 2015). For combustion, low leaf content is preferred because leaves carry high N and ash content (Baxter et al. 2012). Therefore to reduce emissions and to increase the overall efficiency of the
process, there is need to use biomass with low leaf proportion. For harvesting time, the content of carbon, hydrogen and oxygen need to be considered because these elements affect the combustion process and play key role in defining the final energy content of biomass. Delay in harvest helps to achieve the right proportion of hydrogen, carbon and oxygen in the harvested biomass to carry out the combustion process efficiently (Bilandzija et al. 2016).
In current study, March is the appropriate harvesting time for combustion depending on prevailing weather conditions. Through delay in harvest for combustion, biomass quality can be improved up to 18% depending on genotype selection. Here, K and Cl are considered as main parameters for combustion. The main aim of the delayed harvest is to provide ample time for relocation of nutrients back to rhizomes and as well as to allow the nutrients to leach down. For leaching of minerals rainfall is required. Therefore if there is sufficient rainfall then biomass can be harvested even earlier in January or February depending on prevailing conditions. The early harvest of biomass can subsequently help to reduce the yield losses. The time of harvesting can be different for each genotype. The genotypes such as M. sinensis which complete the relocation mechanism earlier can be harvested earlier (January- February), whereas for the genotypes (M. x
giganteus and M. sacchariflorus) requiring longer time to complete their growth cycle, delayed
harvest is suitable (e.g. March). In this study, the effect of delayed harvest on different quality characteristics for combustion was quantified through using different statistical models. The results showed that the impact of delayed harvesting in terms of biomass combustion quality improvement for M. sinensis type genotypes was greater than the M. x giganteus and M.
sacchariflorus genotypes (Figure 5)
Figure 5: Decrease in K and Cl content for different miscanthus genotypes when harvesting was delayed from January to March (own data)
The variable response of genotypes to delayed harvesting indicates that genotype selection interacts with harvesting time to affect the biomass quality. Despite quantification of
impact of delayed harvest on specific combustion relevant elements such as K or Cl in this study, the mechanisms behind need more in depth investigations. Therefore further research is needed by taking into account all the relevant factors through more sophisticated modelling tools. It can be done by selecting the peak yield period as baseline and then compare improvement in biomass quality for combustion and yield losses till March.
For ethanol production, early harvest is preferred. The peak yield of miscanthus under these conditions is September-October, during this time period biomass has high moisture content but low lignin content which is suitable for bioconversion for ethanol production. In current study, impact of delayed harvesting was calculated for combustion from January to March, whereas from fiber composition perspective, September-October is more relevant because in January biomass is already lignified. Therefore, in this study the impact of delayed harvest on ethanol production was not quantified. Based on the published data about different miscanthus genotypes (Hodgson et al. 2010), the impact of harvesting time on fiber composition was quantified when harvesting was delayed from autumn to winter. The outcome indicates that early harvest during peak yield period improves the biomass quality up to 12% in comparison to winter harvest. In this case, lignin content was considered as leading parameter. Therefore, it is important to harvest biomass early during peak yield period for ethanol production. However, it affects the relocation mechanism and increases the offtake of nutrients through harvest (Strullu et al. 2011) which lead to high fertilizer inputs for next growth year. One of the studies shows that early harvest could only remobilize 42% of peak nitrogen content back to rhizomes whereas under late harvest regime remobilization of peak N was recorded up to 71% (Strullu et al. 2011). In addition, the leaf fall during late harvest is also good source of soil nutrients, adds up N up to 15.5+3.5 kg N ha-1 whereas this addition is negligible when biomass is harvested early (Karlen, 2014). It shows that early harvest increases the demand of fertilizer inputs especially N fertilization significantly. However, by promoting the recycling of nutrients through application of digestates released at the end of ethanol process the nutrients off take in case of early harvest can be compensated.
Apart from harvesting time, type of harvesting system can affect the yield as well as biomass quality. Currently, two types of harvesting systems are being used for miscanthus; a) harvesting of biomass and chipping it simultaneously; b) mow and bale system (Meehan et al. 2013). Based on the net energy yield in terms of harvested biomass, harvesting of biomass and chipping is more efficient compared with mow and bale system (Meehan et al. 2013). It indicates that in case of harvesting and chipping the biomass simultaneously, the yield losses are low compared to other system but it can vary with transportation distance. The adoption of harvesting system affects yield, quality, cost of production and environmental performance of whole production chain. Currently, there are no miscanthus specific harvesters. However, development of large miscanthus specific harvesting machinery as in case of straw collection will help to control the yield losses, deliver stable quality by avoiding contamination such as dirt (affects ash content) and will improve overall performance of whole production chain. Among the above
discussed forage based harvesting systems, cutting of biomass and chipping it simultaneously can be recommended for miscanthus because of low yield losses and low cost (Smeets et al. 2009).
After harvesting, handling and storage of biomass also affects both biomass quality and yield. Inappropriate handling during harvesting and storage of biomass leads to contamination such as soil contamination, which subsequently affects the processing of biomass both for combustion and ethanol production. At field level soil contamination can lead to increase in ash content, therefore it is important to avoid any dirt. For every bioconversion route, low ash content is preferred.
Handling also involves the pre-treatment of biomass at field level especially with the aim to improve the biomass quality before transporting to processing unit. This is done mainly for combustion. The pre-treatment could be cutting the biomass and leaving it on ground to let it dry and allow the minerals to leach down. However, it is relevant only for combustion quality characteristics especially for Cl because Cl is main challenge during processing of biomass for combustion and it is not easy to get rid of Cl during combustion process. Research shows that leaving the biomass in field after harvesting improves the biomass quality significantly with low Cl and ash content (Meehan et al. 2014). This is especially performed in UK, where the moisture content in harvested biomass is still too high even when harvested in March.
The other important component of biomass logistics is storage. The storage method depends on end use of biomass, potential losses of biomass and cost. There are different options to store biomass; a) open air storage either covered with plastic sheet/organic material; b) storage in farm buildings (Smeets et al. 2009); c) ensiling (Oleskowicz-Popiel et al. 2011). The simplest and cost effective way of biomass storage is open air storage with plastic sheeting (Smeets et al. 2009). However, this method can lead to high moisture and ash content, loss of biomass and decrease in heating value (Yue et al. 2014). The high moisture and ash content directly affects the combustion quality but it can be controlled through better storage facilities such as storing biomass in warehouses with drying capability. This method has the potential to minimize the yield losses (Cundiff et al. 1997). However, building such warehouses will require high investments.
For ethanol production, storage method also plays very important role. Ensiling of biomass is very established method for forages which can be applied for miscanthus as well. This method offers opportunity to store biomass with high moisture content. In addition, it minimizes the yield losses and improves the enzymatic digestibility of biomass. The combination of ensiling and low temperature pre-treatment of biomass had improved the efficiency of overall process for ethanol production and decreased the cost and energy input (Chen et al. 2007). In grass variety Festulolium Hykor, ensiling had improved the cellulose convertibility, which subsequently led to high ethanol yields (Ambye-Jensen et al. 2013). Therefore, in miscanthus
this method can be adopted. This will not only offer a good solution for biomass storage but also improve the subsequent processing of biomass. The ideal characteristics of ensiling are presence of high soluble carbohydrates, with high moisture content (50-70%) and chopped biomass which can easily be compacted (FAO, 2002). Miscanthus does not exactly fit to the criteria defined for ideal ensiling crop, however fine chopping and use of additives can improve the quality of miscanthus biomass ensiling. The other option to improve the biomass quality for miscanthus ensiling is to combine with maize because maize can supply the easily soluble sugars for microorganisms which subsequently will help in fermentation of miscanthus biomass. The adoption of ensiling as a storage process will reduce the chemical and energy input required during pre-treatment of biomass, which will improve the overall performance of production chain in economic and environmental terms.