• Energy Research

Abstract The ignition temperatures of a Loy Yang brown coal and a Datong bituminous coal were investigated in a wire-mesh reactor where the secondary reactions of the evolved volatiles were minimised. An increase in the average particle ignition temperature of 21 °C was observed for the brown coal when air (21% O 2 + 79% N 2 ) was replaced with a mixture of 21% O 2 + 79% CO 2 . Combustion was also carried out in the mixtures of 21% O 2 + 79% argon and 21%O 2 + 79% helium in order to determine the effects of heat transfer on the observed particle ignition temperature. It is concluded that the thermal conductivity of gas atmosphere surrounding the particles greatly influences the observed particle ignition temperature while the effects of the heat capacity of the gas atmosphere was very minor under our experimental conditions. The structure of char and the reactions involving the char (char-O 2 and char-CO 2 ) can greatly affect the observed particle ignition temperature. In particular, the char-CO 2 reactions were largely responsible for the observed difference in particle ignition temperature in air and in 21% O 2 + 79% CO 2 . Alkali and alkaline earth metallic (AAEM) species in the brown coal also significantly affect the observed particle ignition temperature.

Combustion of a low-volatile bituminous coal in air versus two O2/CO2 mixtures (21/79 and 27/73, v/v) was conducted at two furnace temperatures of 800 and 1000 °C in a lab-scale drop tube furnace (DTF). Through in situ photographic observation and measurement of overall coal burnout rate, CO emission profile, and unburnt char properties, a variety of distinct phenomena relating to oxy-fuel combustion has been revealed. Consistent with the literature, the significant thermal effect of CO2 due to its large product of Cpρ (specific heat capacity and density) relative to that of N2 retarded volatile ignition in the two O2/CO2 mixtures. As a result, the volatiles released in O2/CO2 remained as a thick protective sheath on char surface for a relatively long duration, which mainly converted into CO through partial oxidation in 21% O2/79% CO2. Increasing the O2 fraction to 27% in CO2 triggered the ignition/oxidation of the unburnt volatiles once their concentrations were critically accumulated on char surface in ...

Lignin is the second most abundant natural polymer after cellulose, and valorisation of lignin-rich streams has attracted increasing attention recently. This paper presents a novel and sustainable method to recover lignin from Cocoa Bean Shells (CBS) using Deep Eutectic Solvents (DES) and microwaves. A DES containing p-toluenesulfonic acid, choline chloride and glycerol (2:1:1 M ratio) was selected based on its dielectric properties. Under 200 W microwave power, the optimum yield of 95.5 % lignin was achieved at 130 °C and 30 min. DES-extracted lignin exhibited unique structural characteristics including larger particle sizes (242.5 µm D50 size), structural diversity (410.4 µm D90-D10 size) and H/G sub-unit ratio (71.9 %) compared with commercial Kraft lignin (77.2 µm, 157.9 µm and 0.1 % respectively), indicating the potential of DES in the modification and upgrading of lignin for novel value-added products.

This study has demonstrated, for the first time, a simple, fast and flexible microwave processing method for the simultaneous preparation of bio-products (bio-oil, bio-gas and biochar) using a methodology that avoids any form of catalyst or chemical activation. The dielectric properties of biomass and physicochemical characterisation such as TGA, elemental and proximate analysis, XRD, SEM/EDX and textural properties, showed that 8 kJ g-1 of microwave energy can produce superior biochars for applications in CO2 capture. The maximum CO2 uptake capacity for biochar produced was 2.5 mmol g-1 and 2.0 mmol g-1 at 0 and 25 °C and 1 bar, which and also exhibited high gas selectivity compared with N2, fast kinetics of adsorption (95%) after 20 cycles. GC-MS analysis of generated bio-oil products revealed that higher microwave energies (>8 kJ g-1) significantly enhanced the amount of bio-oil produced (39%) and specifically the formation of levoglucosan, furfural and phenolics compounds, and bio-gas analysis identified trace levels of H2 and CH4. The results from this study confirm a green, inexpensive and efficient approach for biomass valorisation which can easily be embedded within bio-refinery process, and also demonstrates the potential of biochars for post-combustion CO2 uptake.

Microwave pyrolysis of pecan nutshell (Carya illinoinensis) biomass was used to produce carbon-based solid products with potential application in contaminated water treatment.A range of analytical techniques were applied to characterize the intermediate products of microwave pyrolysis in order to monitor the physio-chemical effects of the interacting energy on the biomass.The performance of the carbon-based products was tested through evaluation of lead ion removal capacity from solution. Further analyses demonstrated that ion-exchange by calcium ions on the material surface was the main mechanism involved in lead removal. Calcium compound development was directly correlated to the interaction of the electromagnetic waves with the biomass.Through monitoring the physio-chemical effects of biomass-microwave interactions during microwave pyrolysis, we have shown for the first time that the intermediate products differ from those of conventional pyrolysis. We hypothesise that selective heating leads to the (hemi)cellulosic and lignin degradation processes occurring simultaneously, whereas they are largely sequential in conventional pyrolysis.This work provides optimization parameters essential for the large scale design of microwave processes for this application as well as an understanding of how the operating parameters impact on functionality of the resulting carbon-based materials.

Abstract Experimental investigation of the combustion of an air-dried Victorian brown coal in O2/N2 and O2/CO2 mixtures was conducted in a lab-scale drop-tube furnace (DTF). In situ diagnostics of coal burning transient phenomena were carried out with the use of high-speed camera and two-colour pyrometer for photographic observation and particle temperature measurement, respectively. The results indicate that the use of CO2 in place of N2 affected brown coal combustion behaviour through both its physical influence and chemical interaction with char. Distinct changes in coal pyrolysis behaviour, ignition extent, and the temperatures of volatile flame and burning char particles were observed. The large specific heat capacity of CO2 relative to N2 is the principal factor affecting brown coal combustion, which greatly quenched the ignition of individual coal particles. As a result, a high O2 fraction of at least 30% in CO2 is required to match air. Moreover, due to the accumulation of unburnt volatiles in the coal particle vicinity, coal ignition in O2/CO2 occurred as a form of volatile cloud rather than individual particles that occurred in air. The temperatures of volatile flame and char particles were reduced by CO2 quenching throughout coal oxidation. Nevertheless, this negative factor was greatly offset by char-CO2 gasification reaction which even occurred rapidly during coal pyrolysis. Up to 25% of the nascent char may undergo gasification to yield extra CO to improve the reactivity of local fuel/O2 mixture. The subsequent homogeneous oxidation of CO released extra heat for the oxidation of both volatiles and char. As a result, the optical intensity of volatile flame in ∼27% O2 in CO2 was raised to a level twice that in air at the furnace temperature of 1273 K. Similar temperatures were achieved for burning char particles in 27% O2/73% CO2 and air. As this O2/CO2 ratio is lower than that for bituminous coal, 30–35%, a low consumption of O2 is desirable for the oxy-firing of Victorian brown coal. Nevertheless, the distinct emission of volatile cloud and formation of strong reducing gas environment on char surface may affect radiative heat transfer and ash formation, which should be cautioned during the oxy-fuel combustion of Victorian brown coal.

An advanced high-speed camera with a spatial resolution of ∼20 μm and time scale of 1−2 ms was employed to observe coal particle combustion in a laboratory-scale drop-tube furnace (DTF). Dynamic in...