Development of energy and emission parameters for densified form of lignocellulosic biomass
Highlights
► This study estimates the net energy ratio and greenhouse gas emissions in production and utilization of agri-pellets. ► The life cycle approach has been used in this study. ► Several scenarios were developed to study the impacts of variations in unit operations on overall net energy ratio and emissions. ► Agri-pellet has the potential to offset substantial amount of GHG emission compared to other fuel sources including wood pellets.
Introduction
Environmental concern and unstable fossil fuel market are main drivers for use of biomass based pellet as an energy fuel. Governmental obligation on use of biomass fuel is another predominant basis for increasing use of pellet in European countries (8 million tonnes in 2008) [1], which is not common in North America. A significant amount of pellets produced today in North America are exported to European countries [2], [3]. The conversion of biomass to pellet form upgrades it’s physical and chemical properties especially in terms of calorific value. In addition to the environmental advantages, biomass based pellets have other value-added opportunities, such as, increased energy density, higher bulk density, and higher heating value.
A number of studies have been performed on the life cycle analysis (LCA) of biofuels especially on ethanol from straw which have shown positive energy balance and reduced greenhouse gases [4], [5], [6], [7]. Most of the LCA analyses were done on transportation fuels, such as, bioethanol, biodiesel, hydrogen [8], [9], [10], [11], [12], [13]. Both the emission and the energy use of wood pellet have been analyzed in previous studies [1], [14], [15], [16], [17], [18]. Mani [14] analyzed streamlined life cycle analysis approach to quantify emissions of wood pellet production. Raymer [15] quantified the amount of GHG emissions for six forms of woody biofuels including wood pellet. Hagberg et al. [16] calculated life cycle energy and emission analysis of wood pellet production in Swedish settings by considering different assumptions and methodological choices. Magelli et al. [17] mainly dealt with life cycle analysis of wood pellet production and transportation from Canada to Europe. Zhang et al. [18] investigated a life cycle analysis of wood pellet with co-firing options and compared with coal and hypothetical natural gas combined cycle. The aforementioned studies focused on pellets from woody biomasses. The life cycle analysis of pellet made of agricultural biomass (i.e., straw) is non-existent. The aim of this paper is to analyze pellet production from agricultural residue, especially from wheat straw with regard to its energy input and emission throughout its life cycle. This study uses data on Western Canada (Prairie Provinces) for life cycle analysis of pellets. The selected geographic region is endowed by large agricultural land area and large energy demand.
Canada is the sixth largest producer of wheat in the world and most of which is produced in Prairie provinces, e.g., Saskatchewan, Alberta and Manitoba. Agricultural residues are available in significant quantities in areas where growth of grain crops are concentrated [19], [20]. Agricultural activities of Western Canada produce 37 million of tonnes of biomass each year [21]. The potential of recovering agricultural residues (i.e., straw from wheat, barley and oats) after accounting for current use is about 6.2 million tonnes per annum [19]. Most of these biomass resources are wasted or underutilized. This biomass potential could be used as a feedstock for bioenergy development.
The objective of the current study was to develop a data intensive model to estimate of the energy use and GHG emission associated with production and use of agricultural biomass based pellets (or agri-pellets). The scope of the paper includes the life cycle analysis of agricultural pellet starting from wheat farming to the distribution of pellets to users taking into account all the input and output flows of energy and emission occurring along the pellet life cycle. This is a standard approach and has been applied to life cycle analysis of other biofuels from herbaceous residues. A number of scenarios have been examined to study the impacts of changing tillage system, taking organic farming option, omitting farming activities, modes of transport and drying options. The analysis also takes into account land use change aspect, i.e., effect of crop residue removal on soil organic carbon and N2O emission.
Section snippets
Methodology
This study followed four steps to a life cycle analysis: goal definition and scoping; inventory assessment; impact assessment; and interpretation. In this paper a detailed model was developed to determine energy consumption and emission over the life cycle of biomass pellet using agricultural residues. Direct and indirect energy consumptions and emissions at each stage of life cycle of pellet production were considered in the model. Key stages of energy consumption and emission estimation
Impact assessment and interpretation – results and discussion
The energy requirements and associated emissions for base case are shown in Table 9. In the base case energy and emission are allocated between wheat grain and straw on mass basis. The nutrient replacement was also considered in the base case to compensate for the nutrients removed due to straw harvesting. The energy and emission due to nutrient replacement is completely assigned to straw. It is seen from Table 9 that the total energy use for the base case (after allocation to wheat and straw)
Conclusions
The life cycle analysis of the energy use and associated emission of agri-pellets were carried out by considering field operations, transport of straw to pellet plants, operations in pellet plants and transport of pellets to the user. The energy use and emissions are the highest in field activities. Nitrogen-based fertilizer production, transportation and application are the highest contributors among the field activities. Large reductions of energy use (64%) and emission (65%) are possible if
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