Ash Characterization and Transformation Behavior of the Fixed-Bed Combustion of Novel Crops: Poplar, Brassica, and Cassava Fuels

Abstract

New biofuel raw materials for energy pellet production are now being studied as potential energy sources for the heating market. Because of the complexity of the chemical and physical properties of novel fuels, such as some agricultural residues and energy crops, the study of their ash-related aspects is crucial for the sustainable development of this potential energy sector. Ash fractions formed during fixed-bed combustion of different pelletized novel crops; i.e., two Mediterranean crops (one herbaceous, brassica, and one woody species, poplar) and three Chinese cassava stems (cassava species from three different Chinese regions), and three Chinese cassava stems (cassava species from three different Chinese regions), were characterized, and their formation paths assessed in this study. Special emphasis was placed on elucidating the role of major ash-forming elements in the fractionation and transformation behavior, leading to the formation of bottom ash, deposits, and particulate emissions (fine and coarse ash particle fractions) on the basis of experimental data. In the Mediterranean fuels, the predominant ash fraction obtained was bottom ash, mainly characterized by silicates. Phosphates were found to be the main crystalline phases in the Chinese fuels. The slagging tendency was low for all of the fuels, although more significant for the cassava species under the studied conditions. Further, combustion of the studied Chinese energy crops resulted in a considerably finer particle fraction compared to the Mediterranean fuels. Deposits and particulate matter were dominated by K-sulfates as well as K-chloride in all fuels (except poplar), with the occurrence of K-phosphates for cassava pellets. Overall, this study showed fundamental differences in ash transformation behavior during combustion of P-rich fuels (i.e., cassava mixtures) compared to Si-rich fuels (i.e., poplar and brassica mixtures). Of major importance is the experimental verification of the higher thermodynamic stability of phosphates in relation to silicates. Furthermore, in P-rich fuels at high (K + Na)/(Ca + Mg) ratios, a significant degree of alkali metal volatilization occurs, which forms larger amounts of particulate matter, whereas this ratio has no/low effect in Si-rich fuels at high alkali metal ratios.

Multivariate modelling on biomass properties of cassava stems based on an experimental design

“…In general, the results obtained from the fuel analyses (Table 1) are comparable to those reported by Díaz-Ramírez et al [7]. Compared with wood fuel, the cassava stems had much higher contents of ash and even of ashforming elements; approximately 10 to 100 times greater (>10 times for ash, 110 times for K, 15 times for S, 16 times for Cl).…”

“…Biomass properties include such physical characteristics as energy value and chemical composition, i.e., content of carbon (C), hydrogen (H), oxygen (O), nitrogen (N), sulfur (S), chlorine (Cl), ash, and the major ash-forming elements phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), silicon (Si), and sodium (Na) [5,6]. These properties may vary substantially with species, genotype [7,8], environment [9,10], harvest time or age [9,11], and management [12]. Few studies, if any, have investigated the effect of biotic and abiotic factors on variation in biomass, which is important not only for the management and prediction of biomass production but also for the development of specialised biorefinery feedstock.…”

“…stems, the residue remaining after harvesting starchy roots, with an annual production of approximately 32 Tg dry mass worldwide and associated with the livelihood of approximately 0.5-1 billion people [14,15], is one of the plant species whose biomass properties need to be characterised. Approximately 20 % of cassava stems are used for propagation, mushroom substrates, or recycling to maintain soil fertility; the rest are basically waste, but could possibly be used for biofuels [7,[16][17][18][19][20][21].…”

“…standard deviation; CV, coefficient of variation = SD/Mean × 100 % b Data from Díaz-Ramírez et al (2012)[7] …”

Corrosive components of nutshells and their chars

“…Cl/S ratio [18,19], Biomass slagging evaluation index [20] and Alkali index [16]. More on index calculations can be read elsewhere [21,22]. Given the importance of biomass ash composition, a complex analysis needs to ensure biomass fuel quality.…”