**4. Densification of grasses**

The grass biomass should be harvested once per year, for which standard hay production equipment can be used. Grasses cut in the fall and left to overwinter produce less biomass, but have the advantage of leaching potassium and chlorine, two minerals that may create issues

The combustion of grasses normally produces more ashes than the combustion of wood. The range in total ash content of grasses can be very wide, from 2% to greater than 20% [14]. Ash values higher than 10% in mature grasses are generally the result of excessive surface soil contamination. The issue of primary concern when burning grass is mineral composition that determines the melting point of ash and the potential for corrosion [15] and also elevated gaseous and particulate emission levels contributing to deposit formation or high-temperature corrosion as well as operational problems resulting from low ash-melting temperatures. High ash content or low ash-melting temperature poses technical issues through deposition, sintering, fouling, slagging, and corrosion. The latter can damage boilers and increase maintenance costs and can cause severe operation problems usually above 850–1000°C [14, 16]. Several indicators affect the ash-melting temperature such as nitrogen fertilizer used on the crop, meteorological conditions, and chemical composition [17]. The ash-forming elements potassium (K), phosphorus (P), chloride (Cl), silicon (Si), calcium (Ca), and sulfur (S) contribute to the abovementioned ash-related mechanical problems [18–20]. Silica is the major component of ash and is found in much higher concentrations in the leaf and inflorescence, compared to the grass stem [21], and the silicon content of the biomass ash may sum up to more than 90 wt% [18, 22, 23]. Silica can combine with alkali metals to form silicates that melt at lower temperatures [16]. K and Cl are the most problematic minerals, and both are consumed in high concentrations by the grasses. K is the most abundant alkali metal in grass biomass [24, 25]. This mineral reduces the melting temperature of the fuel and also contributes significantly to corrosion potential. Chlorine is a particularly undesirable component of grass biomass, as it acts as a catalyst for corrosion reactions and also increases the potential of chlorinated hydrocarbon emissions [26]. Sulfur reacts with alkali metals and forms deposits on heat transfer surfaces, and nitrogen content directly increases NOx emissions. Therefore, reduced concentration of all the abovementioned minerals in grass biomass is highly convenient. To enable and facilitate the utilization of a wide range of grasses in combustion systems, several strategies to mitigate the ash- and emission-related problems have been employed [25]. Appropriate harvesting time and fertilization application can all contribute significantly toward improvement of ash-melting behavior [27]. Potassium and chlorine can be reduced by controlling fertilization of these elements or by leaching them out of grass biomass [28, 29]. The content of some critical elements in fresh grass can be substantially reduced by mechanic dewatering [30]. Nitrogen concentration can be reduced by harvesting mature or overwintered forage. On the other hand, silica can be minimized by using warm-season grasses or by growing grass biomass on a sandy soil. Reduction of ash content and relative amount of critical elements can also be achieved by blending with less problematic biomass fuels such as

during combustion [13].

4 Advances in Biofuels and Bioenergy

**3. Grass biomass combustion**

wood, miscanthus, or peat [31].

Grasses have low energy density (MJ m−3) and low yield per unit area (dry tons ha−1). Volumetric energy content of grasses used for biofuels is considerably lower than traditional fossil fuel sources, and this low energy density is due to low bulk densities of biomass materials [8]. Often, long distances have to be bridged between the biomass place of origin and the place of its utilization, resulting in expensive handling and transportation. Transportation costs of low-density grasses which increase the total cost of biomass processing are an important limitation to their use as an energy source [35]. To increase the bulk density of grasses, they can be densified into pellets using a mechanical process [35, 36]. Therefore, the densification of grasses is an important issue to improve the transport, storage, and handling capabilities of this lignocellulosic material. Densified biomass, especially pellets, has drawn attention due to its superiority over raw biomass in terms of its physical and combustion characteristics. With the international quality standard [37] for nonwoody biomass pellets, the foundation for an increasing commercial utilization of a wide range of biomass such as grasses was laid in 2014. Pellets have multiple end-use applications which range from smaller scale combustion for residential heating to an industrial scale where grass pellets could be co-fired with coal at power plants [38]. The increased demand of pelleted fuel sources in Europe and North America could allow for more nonwoody biomass resources such as perennial grasses to be used for pelletization. One of the most important variables in pellet production is moisture content, since this property will finally determine the durability and density of pellets [36, 39]. A less-expensive method of densification method (higher yield per hour) is by forming the grass into larger briquettes, also called tablets or cubes, which allows to manipulate and store the material easily, and they can also be transported economically and burned efficiently.
