• Energy Research

Abstract Most of the advanced coal combustion and gasification processes operate under pressurised conditions. Current knowledge of the in situ effect of pressure on coal devolatilisation and swelling, however, is limited, but is essentially required for optimisation of these technologies and to ensure future developments. During heating, fluidity is induced by breaking the coal covalent bonds and forming a plastic state where nucleation occurs, volatiles evolve as bubbles and they flow, diffuse, coalesce and rupture in a complex combination of events that lead to the transient structural evolution of the heated coal particle. The effect of pressure on swelling of individual coal particles is the subject of this work. Density fractions of particles were prepared using a sink–float technique to achieve homogeneous particle properties. Groups of particles from each density fraction were heated in a pressurised single particle reactor at pressures ranging from 0.1 to 5 MPa. The thermal behaviour of each sample was recorded using a long distance microscope attached to a CCD camera. Pressure was found to have parallel and competitive effects on the particle fluidity and transient swelling, resulting in a maximum for both transient and ultimate particle swelling at pressures of 1 MPa. For pressures of over 2 MPa, the observed particle swelling was lower than at higher pressures. In most cases, post-swelling particle contraction was observed with the largest contractions occurring under atmospheric pressure conditions as a result of the major bubble rupture and consequent mass loss. The contraction showed a minimum at 2 MPa and a slight increase at the pressure of 5 MPa thought to be due to an increase of the time the particle remained fluid, enabling the high pressure to further deform the particle. Particles from the lower density group showed larger transient swelling and particle oscillations, while the transient swelling decreased rapidly with increases in the particle density.

The combustion behavior of coal depends upon not only the coal characteristics but also the combustion conditions. The current understanding about the combustion of coal char during the extreme con...

A new method for the prediction of the viscosity of coal ash slags, in the Newtonian region, is presented. The technique is modeled on experimental viscosity data less than 1000 Pa s and hence is most reliable in that region. The capability of the model in predicting the viscosity of slags from coal ash was found to be superior to a number of the most commonly used empirical models found in the literature, which are based on simplified oxide melts or British coal ash slags. The method also provides an indication of the relative fluxing strength of the basic oxides usually found in coal ash slags. It was found that the fluxing strength is related to the inverse of the cation radius.

This paper presents a comparison of techniques that can be used to obtain the sintering temperature of coal ash. These techniques are thermal conductivity analysis (TCA), thermomechanical analysis ...

Measurements are presented of the flow pattern and mixing within the quarl of a 1/9th-scaleair model of a brown coal burner. For flow conditions within the range used in operating burners, two flow types were observed: one, previously reported, where the primary (coal-laden) flow is deflected gradually around a central, swirl-induced, reverse-flow zone; the other, where the primary flow partially penetrates the reverse-flow zone before being deflected rapidly in the radial direction. The combustion behaviour of the coal in each type of flow is predicted by a computationalmodel, and the results compared with limited measurements on an industrial burner. The second type of flow is shown to mix the coal-laden primary flow with the internal reverse flow before mixing with the secondary air flow, resulting in a higher evolution of volatiles and the ignition of a greater proportion of the char mass. The temperature field within the quarl is shown to be dominated by the temperature of the reverse flow.

Effects of pyrolysis pressure on char reactivity remain a poorly understood aspect of the coal gasification process. In an attempt to address this problem, effects of pyrolysis pressure on char structure and reactivity are being investigated. In this paper, chemical reactivities to O2, CO2, and H2O of chars made from three Australian black coals were measured in a pressurized thermogravimetric analyzer, under conditions where chemical processes alone controlled reaction rates. These chars were prepared at a range of pyrolysis pressures, using pressurized flow reactors and an atmospheric pressure tube furnace, to ascertain any effects of devolatilization pressure and heating rate on the chemical reactivity of the resultant coal chars. It was found that while the apparent (as measured) reaction rate can be affected by pyrolysis pressure, the rate normalized to the char surface area (intrinsic rate) is much less affected, because of large effects of pyrolysis pressure on char micropore surface area. This fin...

Abstract Oxy-fuel combustion is a promising CCS technology which is being demonstrated prior to commercialisation. While the flue gas in oxy-fuel combustion is concentrated in CO 2 , it contains impurities such as SO 2 . The elimination of SO 2 can provide a clean CO 2 stream ready for storage. This paper is to understand the absorption of SO 2 in scrubbing relevant to those used in oxy-fuel technology. Steady state experiments were conducted in a continuous well stirred reactor to understand the absorption rate of SO 2 /CO 2 into a total concentration of 0.28 M of mixtures of NaHSO 3 and NaHCO 3 simulating liquids formed by scrubbers using NaOH as the reagent at solution pH values from 4 to 7 with the exiting gas concentrations of SO 2 from 19 ppm to 1500 ppm and a constant CO 2 concentration of 70%. Online measurement included gas phase SO 2 and liquid pH, and offline measurement included CO 2 (aq), HCO 3 − , S (IV), SO 3 2− and S (VI) after each experiment. Three aspects investigated were the impacts of pH on the solution chemistry, the significance of solution pH and the concentration of gas phase SO 2 on the absorption rate of SO 2 . The total sulphur concentration in liquid was found to be related to the effectiveness of Na + . The effective ratio of Na + can be defined as the total sulphur to Na + ratio and this effectiveness ratio of Na + is pH dependent. At pH + is 99% effective. It reduces dramatically from 99% at a pH 5 to less than 15% at pH above 7. With regard to carbon based species also absorbed, super saturation of CO 2 (aq) was observed at pH > 5.5. The concentration of HCO 3 − increases dramatically above pH 6 and below this pH, the concentration of HCO 3 − is negligible. The absorption rate of SO 2 was found to increase with pH with some increase with the concentration of SO 2 . The operational pH window for scrubbing may be defined by an upper limit pH where the absorption rate of SO 2 starts to decreases from the maximum absorption rate of SO 2 and the lower limit pH where the absorption rate of SO 2 reduces to half of the maximum absorption rate of SO 2 . Both the upper limit and the lower limit decrease initially and stay stable with the concentration of SO 2 . This decrease is caused by the reversible reaction of the hydrolysis of SO 2 and confirmed by equilibrium experiments of SO 2 and sodium solutions. Operation within region 2 (pH 5–6) is recommended, depending on the scrubber design. The operation exit pH of the produced liquid can be varied within the region. The absorption rates of SO 2 obtained in the steady state experiments were predicted by a model based on the instantaneous reaction assumption. This model generally overestimates the absorption rates of SO 2 at pH values below 6 indicating a kinetic limitation of SO 2 and water reaction at low pH values. The analysis on the controlling regions indicates that the gas side mass transfer resistance decreases with the concentration of SO 2 . Liquid side resistance becomes more important at a lower pH and a higher concentration of SO 2 .

Pyrolysis and combustion characteristics of an Indonesian low-rank coal are studied under oxy-fuel (O2/CO2) and air (O2/N2) conditions using a drop tube furnace (DTF) and a thermogravimetric analyzer (TGA). Raw coal, dried coal, and binderless briquette samples of the same coal were used in the experiments, and the effects of drying and binderless briquetting on the reactivity of the coal under different conditions were investigated. Chars were prepared in the DTF in both N2 and CO2 atmospheres in the temperature range of 800−1400 °C. The reactivity of chars under oxy-fuel and air conditions was analyzed in the TGA. The coal reactivity under oxy-fuel conditions differed from that under air combustion conditions. The temperature at which significant gasification of the coal and char took place in the concentrated CO2 gas stream was also identified. Characteristics of chars from different conditions were compared. Drying and briquetting had some noticeable influences on the reactivity of the coal under oxy-...

Low-temperature pyrolysis offers a potential way of upgrading lignite and producing chars to replace thermal or pulverized coal injection (PCI) coals in combustion or being used as inert components in a blend for coking. In this study, the characteristics of chars from low-temperature pyrolysis of two lignite coals have been investigated. The changes in char morphology and chemical structures were investigated using scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The combustion reactivity of chars was analyzed in a thermogravimetric analyer (TGA) using non-isothermal techniques. The results show that chars from low-temperature pyrolysis of lignite coal below 450 °C were more reactive than higher temperature chars. Higher reactivity of low-temperature chars was attributed to the higher concentration of active sites and lower degree of structural order compared to that of high-temperature chars. Indonesian (YN) lignite showed a higher weight loss rate compared to Hulun...