**1. Introduction**

The search for an alternative fuel due to environmental concerns and depletion of fossil fuels has raised interest in sustainable energy systems. The utilization of biomass as renewable energy source is becoming increasingly important in the light of its potential for lowering global warming effects and sustainably securing fuel supply [1]. The main challenge in utilization of biomass as fuels would be a stable supply of raw materials [2]. In Europe, wood fuels (e.g., log wood, wood chips, and wood pellets) are the predominant biomass fuels for small-scale heating. However, in several regions, the rapid increase in wood pellet production resulted in shortage of raw materials [3, 4]. Wood assortments are also considered as promising raw materials for the growing biorefinery sector; therefore, this competition is expected to significantly increase in the future, resulting in an increase in raw material costs [5]. Thus, to fulfill the anticipated growth of biomass utilization, expected worldwide, a wider assortment of raw materials will be required including low-quality wood fuels (e.g., logging residues, short rotation coppice) and nonwoody biomasses [6]. Within the available biomass sources, there has been an increasing interest in the use of perennial grasses as energy crops. In order to achieve a positive energy balance, the condition for a plant species to be a potential energy crop is that its bioenergy yield must be produced with a low level of inputs that require minimal energy for their own production and utilization [7]. In this context, perennial rhizomatous grasses display several positive attributes as suitable energy crops. The characteristics which make perennial grasses attractive for biomass production are their high-yield potential and the high contents of lignin and cellulose of their biomass. The biomass of perennial grasses has higher lignin and cellulose contents than the biomass of annual crops [8]. These characteristics are desirable when used as solid biofuels, mainly because they have a high heating value associated with the high carbon content in lignin and, also, strongly lignified crops have the advantage of remaining stand upright with low water content. Therefore, its biomass has lower water content and a late harvest is possible to improve the quality of the biomass. From the point of view of crop management, high yields of biomass from perennial grasses are possible, but the quality of combustion is lower than that of wood products. Compared to stem wood, all these materials are usually characterized by higher ash content and a large variation in the composition of ash-forming elements. Therefore, the use of perennial grasses as fuel usually requires a greater maintenance of the boiler due to the particular characteristics of this type of biomass [9]. The chemical composition of the biomass is highly influenced by the date of harvest as well as by the procedure to make the bales, the condition of the soil, and the population of the plant. High ash content in the raw material will increase slagging tendency during combustion and also will cause high abrasions during the processes of grinding and densification. High contents of N, Cl, and S are mainly related to technical problems during the combustion process and to the increase of polluting emissions [9].

because of their more efficient photosynthetic pathway [10]. Furthermore, the need for soil tillage in perennial grasses is limited to the year in which the crops are established, which is an advantage over annual crops. The advantages of the long periods without tilling are reduced risk of soil erosion and a likely increase in soil organic matter content. In addition, due to nutrient recycling by their rhizome systems, perennial grasses have a low nutrient demand [11]. Since they are affected by few natural pests, they may also be produced with little or no pesticide use. Furthermore, there are many environmental benefits expected from the production and use of perennial grasses. The substitution of fossil fuels by biomass is an

Bioenergy from Perennial Grasses

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emissions.

Perennial grasses have many benefits as an energy crop. They are easy to grow, harvest, and process. Grasses is a "traditional agricultural crop" that does not need any special equipment, and the same could be used as for hay production. Perennial grasses are long-lived and thus do not need to be planted each year. In addition, it is not necessary to plow the soil every year, which leads to less soil disturbance. Grasses have several advantages as raw materials for fuels, since they conveniently occur throughout the world in a wide range of climates, geographies, and types of soils, and additionally, they sequester and store large amounts of carbon in the root systems and in the soil. Grasses can be grown on marginal lands unsuitable for continuous crop production or on open rural lands that currently are abandoned or underutilized. They yield more biomass per hectare and require much fewer inputs compared to annual crops that require more fertilizers, pesticides, and fuels. Perennial grasses are being used as a solid fuel in co-fired coal power plants and are also selected as the raw material for advanced biofuels such as cellulosic ethanol. The dry biomass of perennial grasses can also be densified and transformed into pellets and briquettes, which have uses as heating fuel to replace or supplement fuels made of wood fibers. The inclusion of a thermal component in the use of solid biomass for energy increases the efficiency of the combustion

In general, grasses grown as energy crop are managed for biomass yield rather than forage or nutritive quality. Grass biofuel requires minimum management expertise. It is as well suited to small farms as it is to large farming operations, and also works for all levels of management intensity. In fact, lower levels of nutrients such as N, S, K, and Cl may improve fuel quality and reduce emissions. The growth and yield of the grass crop depends significantly on several factors such as soil conditions, fertility, moisture, weed as well as pest control, and the timing of harvest. During the growing season of the grasses, the moderate use of fertilizers may be necessary to maintain soil fertility and to improve crop biomass production [13].

Good weed control in the first year of an establishment is critical to achieve a successful establishment. For example, switchgrass (*Panicum virgatum* L.) seedlings are slow to establish and are susceptible to competition from weeds. Emergence can take several weeks, depending on soil temperatures and moisture. It is critical that perennial weeds are eliminated from the fields prior to planting. To prevent competition from these species, it is important that cultural or chemical weed control is performed to ensure that the field is free of weeds. Nitrogen fertilizer is not recommended in the first year to reduce grass weed competition. Manure nutrients can be applied in the spring or anytime following grass harvest, as long as the grass

important contribution to reduce CO2

system more than three times [12].

is still actively growing.
