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• Iron existed in the form of excluded minerals in Zhundong coal was found. • The spherical particles in the raw coal were the main source of the coal ash. • The slagging mechanism of high iron Zhundong coal on water-wall was proposed. Due to the wide region of Zhundong mining area, the coal properties of different mines are quite different. In this paper, a Zhundong coal containing red and cyan spherical particles (named as RP and CP) which can be observed by naked eye was studied. To better understand the characteristics of coal and spherical particles, the raw coal with spherical particles was directly ground for density classification, and then ashed. Besides, the raw coal was also separated according to the macroscopic appearance, and then the separated samples were ground and then burned at different temperature. The results showed that 91.57% of the coal mass was contained in classes with density less than 1.50 g/cm3, but the ash mass only occupied 46.74% of the total ash mass. On the contrary, the mass ratio of pulverized coal which density greater than 1.50 g/cm3 was occupied 8.43%, but its ash content was more than half of the total ash content. Iron was the main element in CP and RP ashes, but there was still a high iron content (reached 6.58%) in the ash of the coal without CP and RP. Finally, the slagging mechanism of high iron Zhundong coal on water-wall during combustion process was proposed. [ABSTRACT FROM AUTHOR]

• The melting tendency of biomass ash would be reduced by increasing reacting pressure. • The ash weight loss ratio at 1000 °C reduced from 18.1 % at 0.1 MPa to 4.3 % at 4 MPa. • Amount of KCl crystals were detected in ash which burned at 1000 °C and 2 MPa. The transformation characteristics of inorganic elements in biomass ash are crucial for the efficient and continuous operation of gasifiers. Cotton straw ashes produced initially at 500 °C were heated under different temperatures and pressures, and then the resulting ash samples were analyzed by XRF, XRD, and SEM-EDS. The results indicated that increasing reacting temperature aggravated ash melting and the ash loss rose linearly, reaching up to 18.1 % at 1000 °C and 0.1 MPa. Reversely, high pressure effectively declined ash loss and melting degree. Under 2 MPa and 4 MPa, the weight loss at 1000 °C was only 8.0 % and 4.3 %, respectively. High pressure decreased ash loss by inhibiting the decomposition and vaporization of low-temperature minerals (K 2 SO 4 , Na 8 Al 6 Si 6 O 24 SO 4 , CaCl 2, and CaSi 2) and promoting the reactions among minerals in ash. At 1000 °C, KCl was present in ash, and its content rose with increasing pressure at the same temperature. The SEM-EDS images revealed that the structure of ash varied from melted to adhesion with increasing pressure at 1000 °C. KCl particles were present in large ash particles at 1000 °C and 2 MPa. As the temperature increased from 700 °C to 1000 °C, K 2 O content decreased more than 5.0 %, while that of CaO increased 6.0 % at 0.1 MPa. At 4 MPa, the K 2 O and CaO increased and reduced 0.6 %, respectively. Compared to atmospheric pressure, pressurization significantly reduced the release of inorganic elements from ash. [ABSTRACT FROM AUTHOR]