**3. Grass biomass combustion**

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 wood, miscanthus, or peat [31].

Usually, additives are used in addressing the low ash-melting temperatures and the release of critical elements in the flue gas [32]. Using this strategy, slagging is reduced by the introduction of compounds that capture problematic ash components forming higher melting compounds or by diluting the ash with inert, high melting materials [33]. Zeng et al. [34] stated that significant reduction of the slagging risk during combustion of herbaceous fuels can only be achieved for high blending ratios with more than 70 wt% wood.
