Effect of Sludge-Based Additive on Ash Characteristic and Potassium Fixation during the Rice Straw Combustion Process

Abstract

The effects of a sludge-based additive (mainly composed of calcium phosphate) on the retention of potassium in bottom ash and ash melting characteristics during rice straw (RS) combustion were studied with a bench-scale tube furnace reactor at the reaction temperatures of 600, 700, 800, and 900 °C. The potassium content in bottom ash and ash composition characteristics were analyzed using powder X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS). The results showed that the addition of sludge-based additive into rice straw at a mass fraction of 2.5–7.5% could effectively improve the potassium fixation rate and retain potassium in the bottom ash during combustion process by the formation of potassium aluminosilicates and K–Ca–P compounds. The ash melting characteristic was enhanced and the sintering of bottom ash was prevented by the addition of sludge-based additive. For instance, the soft temperature (ST) of RS was enhanced by 238 °C for the case of addition of 5.0% sludge-based additive into RS. In general, addition of sludge-based additive into RS with high potassium and chlorine contents was predicted to significantly increase the retention capacity of potassium as well as improve the melting characteristics of bottom ash.

Review of Phosphorus Chemistry in the Thermal Conversion of Biomass: Progress and Perspectives

“…Crystalline phosphorus phases reported to be present in residues from different types of biomass, including biomass blends (left) (refs , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , – , , , – , , , , – ) grain- and seed-based biomass (upper middle) (refs , , , , , , , , ) animal biomass (upper right) (refs , , , , , – ) sewage sludge (lower middle), (refs , , , , , , , , , , , , , , , , …”

“…The effect on potassium release was quantified for several d i ff e r e n t a g r i c u l t u r e r e s i d u e s i n c l u d i n g w h e a t s t r a w , 2 3 , 1 3 3 , 1 3 8 , 6 2 0 , 6 7 0 m a i z e s t r a w , 2 1 , 2 2 , 6 6 2 r i c e straw, 19,627,664,665,671 corn stalk, 21,22,672 and mixtures of wood and straw, 673 using additives such as NH 4 H 2 PO 4 , 138,670 (NH 4 ) 2 HPO 4 , 21 Ca(H 2 PO 4 ) 2 , 22,138,662,672 CaHPO 4 , 672,673 Ca 3 (PO 4 ) 2 , 23,138,672 and sewage sludge. 23,133,138,620,627 The potassium release quantified in these studies is summarized in Figure 52.…”

“…Normally, XRD analysis is only performed on the final biomass residues, and does therefore not provide a lot of information on the transformation pathways, but it gives an indication of the fate of phosphorus in thermal conversion of biomass. In an attempt to investigate what crystalline phosphates are most commonly found in biomass residues, the reported phases detected in 384 samples, from 65 references (refs , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , – , , , – , , , , – ) were compiled in this work. The result of the compilation is shown in Figure .…”

“…Using phosphorus as an additive has also proved to reduce the release of potassium to the gas phase. The effect on potassium release was quantified for several different agriculture residues including wheat straw, ,,,, maize straw, ,, rice straw, ,,,, corn stalk, ,, and mixtures of wood and straw, using additives such as NH 4 H 2 PO 4 , , (NH 4 ) 2 HPO 4 , Ca­(H 2 PO 4 ) 2 , ,,, CaHPO 4 , , Ca 3 (PO 4 ) 2 , ,, and sewage sludge. ,,,, The potassium release quantified in these studies is summarized in Figure .…”

Review of Biomass Agglomeration for Fluidized-Bed Gasification or Combustion Processes with a Focus on the Effect of Alkali Salts

“…There are several countermeasures proposed to reduce or eliminate bed agglomeration. For example, several recent studies focus on the addition of sludge-based additives, , to reduce ash-related problems. In addition, biomass co-gasification or co-combustion with different types of fuels and the utilization of alternative bed materials are also common countermeasures.…”