The time-dependent layer-formation process of the agglomerates for three common agricultural residues in Australia with different ash-forming elements, together with quartz sand as the bed material, were investigated in a lab-scale, fixed-bed reactor under combustion (5% v/v O 2 ) and steam-gasification (50% v/v steam) atmospheres at 900 °C. The impact of the atmosphere on the ash−bed material interaction was studied from the elemental composition and the morphology of the agglomerates, which were characterized with scanning electron microscopy in combination with energy-dispersive X-ray spectroscopy. The ash−bed material interaction mechanisms for the three feedstock were identified as part of the alkali metals react to form ash particles, which, for wheat straw and cotton stalks, consist of Na, Mg, Si, P, K, and Ca and, for grape marc, is composed mostly of KCaPO 4 ; the remaining alkali metals react with either Si from the quartz sand (for grape marc and cotton stalk) or reactive Si from the fuel (for wheat straw) to form a low-melting-point alkali silicate coating layer; Ca dissolves or diffuses into the coating layer (for wheat straw and cotton stalk); and the ash particles formed in the first step then deposit on, and progressively embed in, the coating layer. The elemental composition of the coating layer is relatively independent of both the reaction time and the gas atmosphere. The coating layer increases in thickness with an increase in the reaction time. The addition of steam results in the production of more liquid alkali silicates, which augment the agglomeration. Any residual S may form sulfate particles with K, Ca, or Na in a combustion atmosphere, while in a steam-gasification atmosphere, the S is released to the gas phase so that more alkali metal may remain to form the low-melting-point alkali silicate.

The interactions between quartz sand and wood, which was doped with either K or Na salts, were investigated in a lab-scale, fixed-bed reactor under either a steam gasification or a combustion atmosphere at 900 °C. For the cases of the potassium/sodium phosphate salts, the interaction was found to be melting induced, while for the cases of the other salts, it was found to be coating induced. Large agglomerates were found to have already been formed during the early stages of steam gasification for the wood samples doped with potassium/sodium carbonate and acetate. However, the agglomerates obtained under the combustion atmosphere were found to be insignificant for the wood samples doped with potassium/sodium chloride. Both the reaction atmosphere and the types of salts were found to affect the formation of potassium/sodium silicates in the agglomerates and the interactions between the quartz sand with gaseous K or Na significantly.

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Effects of Ca and P on the interactions between quartz sand as the bed material and wood loaded with K2CO3, K2SO4, or KCl salts were studied in a lab-scale, fixed bed reactor at 900 °C under a combustion (5%, v/v, O2) and steam gasification (50%, v/v, steam) atmosphere. The addition of calcite to these salt-loaded wood samples decreases the K retention in agglomerates and reduces the size of agglomerates. The extent of these effects depends upon the salt species. Steam increases the K retention in agglomerates. For wood loaded with salt mixtures of K2CO3/KH2PO4, K2SO4/KH2PO4, or KCl/KH2PO4, agglomerates dominated by K silicates are formed when the content of P in the wood samples is high, while agglomerates dominated by K phosphates are formed when the content of P in the wood samples is low. Both the addition of calcite at 1 wt % Ca to the wood samples loaded with the K salt mixtures and the addition of Ca­(PO3)2 at either 0.5 or 1 wt % Ca to the wood samples loaded with K2CO3, K2SO4, or KCl salts result in the formation of K–Ca phosphates (KCaPO4, K2CaP2O7, or other phases) within the silicate coating layer. Further increasing the Ca­(PO3)2 concentration to 3 wt % Ca leads to the formation of agglomerates dominated by partially molten K–Ca phosphates (K2CaP2O7 or other phases), which is inhibited by steam. With the addition of Ca­(PO3)2, the K retention in agglomerates is increased by the formation of K–Ca phosphates while decreased by the inhibition of K silicates. The domination of the two opposite effects depends upon the concentration of Ca­(PO3)2 and the types of K salts.

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Aluminum fluoride (AlF3)-modified HZSM-5 obtained by simple mechanical mixing and calcination was used to enhance the conversion of the lignite pyrolysis volatiles to light aromatics. The results revealed that the appropriate amount of AlF3 modification can simultaneously dealuminize and aluminize to enhance the catalytic performance, in which dealumination considerably enlarged the pores while realumination generated more midstrength acid sites to compensate for the loss of acid sites caused by dealumination. HZ-2 (HZSM-5 mixed with 2 wt % AlF3) obtained the maximum light aromatics yield of 31.0 mg/g as a result of its abundant mesopores and suitable midstrength acid sites, especially the yield of benzene increased by 6.3 mg/g in comparison with that of the original HZSM-5. However, mixing with only a small amount of AlF3 generated the most serious carbon depositions because of the acidity enhancement by the critical role of alumination. Moreover, excessive AlF3 etched more framework Al that dramatically reduced the midstrength acid sites, inhibiting the formation of light aromatics, although the enlargement of pore size facilitated mass transfer. This one-step modification combining the expansion of the pore size and the optimization of acid sites provides a promising strategy for upgrading the lignite pyrolysis volatiles.

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