Influence of Phosphorus on Alkali Distribution during Combustion of Logging Residues and Wheat Straw in a Bench-Scale Fluidized Bed

Abstract

The influence of phosphorus on the alkali distribution in fluidized (quartz) bed combustion using two different typical biomasses (logging residues and wheat straw) was studied. Phosphoric acid (H 3 PO 4 ) was used as an additive. The produced ash fractions were analyzed for morphology and elemental composition by scanning electron microscopy-energy-dispersive spectroscopy (SEM-EDS), and crystalline phases by powder X-ray diffraction (P-XRD). For both fuel assortments tested, a reduction of volatilized deposit and fine particle-forming matter, containing mainly KCl, was achieved by adding phosphorus. For the wheat straw, this effect was considerable at medium and high phosphorus addition. As a consequence, an increased amount of potassium was found in the coarse ash particle fractions, principally as CaKPO 4 , KMgPO 4 , and CaK 2 P 2 O 7 , at the same time that the levels of HCl and SO 2 in the flue gases increased. Generally, the addition of phosphorus to the studied biomasses changed the alkali distribution from being dominated by amorphous K-silicate coarse ash fractions and fine particulate KCl, to a system dominated by crystalline coarse ash of K−Ca/Mg-phosphates and fine particulate K 2 SO 4 . This implies that the fouling and hightemperature corrosion observed in industrial-scale combustion of problematic biofuels can possibly be reduced by employing additives rich in reactive phosphorus, on the condition that the higher concentrations of acidic gases can be tolerated. In order to achieve these effects, the relationship between alkali and alkaline-earth metals (i.e., (K + Na)/(Ca + Mg)) in the overall fuel ash must be considered. With respect to this, the formation of low-temperature-melting alkali-rich phosphates should not be promoted, to avoid potential increases in bed agglomeration tendencies and phosphorus release from the bed.

A Comparative Study of Fouling and Bottom Ash from Woody Biomass Combustion in a Fixed-Bed Small-Scale Boiler and Evaluation of the Analytical Techniques Used

“…Major elements were found in larger amounts using SEM-EDS, which is possibly derived from the precision and accuracy of the techniques. Infrared absorption spectroscopy is used primarily to identify organic compounds, especially gases [3,4,19,22]. FTIR is not typically used for the determination of inorganic compounds, as these components vibrate at wavenumbers less than 400 cm −1…”

“…However, biomass, as a solid fuel, generates a significant amount of ash. Some of the ash is retained in the boiler grate as bottom ash, some is released in the flue gas as fly ash and, the remaining amount forms deposits in the internal parts of the boiler, causing problems such as fouling, slagging and corrosion among others [2][3][4][5][6]. Fouling is defined as deposits that form in areas of the boiler that are subjected to convective heat exchange.…”

“…[6,7,9,12,[14][15][16][17][18]. Many researches have focused on minimizing or removing these problems using methods such as co-firing with other types of fuel, such as different coals [9][10][11]13,[19][20][21] or blends of various biomass species [14,22], and the use of additives such as phosphoric acid [3], ammonium sulfate [19], kaolin [14], halloysite [23], titanium oxide [24], Ca-based additives [25] and others [26][27][28]. Therefore, the knowledge on the composition of the ash and fouling material is crucial to understand the mechanisms behind fouling formation and to provide a method to combat it.…”

Thermodynamic Equilibrium Model Applied To Predict the Fouling Tendency in a Commercial Fluidized-Bed Boiler, Combusting Solid Waste

“…Subsequently, the rate of alkali chloride sulfation increased, and more HCl was released in the flue gas. Grimm et al 48 showed that the addition of phosphorus to biomass changed the system from being dominated by amorphous alkali-silicate ash fractions and fine particulate alkali chlorides to a system dominated by crystalline coarse ash of alkali-Ca/Mg-phosphates and fine particulates of alkali sulfates.…”

Fuel design in co-combustion of demolition wood chips and municipal sewage sludge

“…Co-combustion of MSS with other types of waste streams or biomass may increase the solubility of the phosphates formed and thereby increase the potential for phosphorus recovery from the ash produced in combustion. 5,13,[18][19][20][21] The impact on alkali capture from the increased sulphur content in the fuel blends can be seen in table 3. Both arkanite (K 2 SO 4 ) and aphtitalite (K 3 Na(SO 4 ) 2 ) were observed in considerable amounts for the cocombustion cases whereas the alkali chloride content was reduced.…”