Experimental investigation of microwave-assisted pyrolysis was successfully done using a specially modified domestic microwave oven. At conditions where no parameterised experimental study was ...conducted (except temperature measurement), changing other parameters that influence the microwave heating can be investigated qualitatively in detail by means of simulation work. Therefore, a computer based simulation using Comsol Multiphysics software was applied not only to predict how the electromagnetic field distributes within the cavity but also to study different parameters that influence heating distribution inside the microwave oven. The simulation work was initially performed with verification between experimental and simulation temperature profiles at temperature settings of 500 °C and 800 °C and an agreement was achieved in terms of the temperature profile and heating behaviour of the biomass. The simulation work has proved that the inhomogeneity of temperature of the biomass is reflected by the local occurrence of hot spots and cold spots. The effect of different positions of the waveguide is remarkable where the bottom-fed microwave energy oven was shown to have a poor electric field distribution. However, when simulation was done on combining the effect of having the microwave energy fed from the bottom and the presence of the mode stirrer, the electric field was greatly improved with the heating distribution of the biomass resembling that obtained from the side-fed microwaves energy oven (usually refers to a common home microwave oven). The effect of having a mode stirrer rotating inside the microwave oven is also pronounced where the mode stirrer acts to stir the electric field strength within the cavity so that a more uniform heating within the biomass can be achieved. Interestingly from the simulation, for a specified microwave cavity, an optimum bed size of biomass was found at 50 mm height where maximum microwaves energy absorption takes place. In this sense, more microwaves energy can be converted into heat thereby ultimately helping the biomass to reach the desired pyrolysis temperature in shorter time. The COMSOL modelling on microwave heating therefore has shown to be simple and practical for use as a framework in predicting temperature profile of the biomass and intensity of the electric field.
•Numerical simulation of biomass heating behavior in a microwave oven.•Temperature inhomogeneity of biomass is due to hot spots and cold spots.•Position of waveguide and the presence of mode stirrer highly influence the electric field distribution.•Simulation results are influenced by the dielectric properties of the biomass.
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Dry biomass provides considerable benefits for combustion, such as increased boiler efficiency, lower flue gas emissions and improved boiler operations, compared to fuels with high moisture. Drying ...is however an energy-intensive pre-treatment. Utilising low-grade, waste heat – of which large amounts are available from many process industries – could significantly reduce energy consumption. The integration of a drying process into a power station fuel system was investigated; the results are presented here. Waste heat from a process industry plant (100 MW output) was utilised as the heat source for drying. The biomass, pine chips at 60wt% moisture, was dried and could then be provided as the input fuel for a subsequent 40 MW power plant. The process consisted of a belt conveyor as the dryer and either flue gases or superheated steam (generated from the hot cooling water) as the heat source. Flue gas usage would result in lower capital costs (∼€2.5 m), but environmental issues, such as pollutant emissions must be considered. Superheated steam can combine short drying times, good heat recovery and environmental protection, but would entail greater capital costs (∼€3 m). A 3–4 year return on the initial investment was calculated for both technologies, but profitability was sensitive to fuel price.
▸ Dry biomass greatly enhances combustion compared to high-moisture fuels. ▸ Drying is thus often necessary but can be highly energy-intensive and costly. ▸ Using low-grade/waste heat from process industries can reduce energy consumption. ▸ This process used flue gas or superheated steam as the heat source and a belt dryer. ▸ A 3–4 year return on the initial investment was calculated for both technologies.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Moving bed combustion is commonly used for energy conversion of biomass. Conditions on the moving bed can be conveniently represented by a time dependent fixed bed. The present work experimentally ...investigates the combustion of four biomass materials having different fuel properties in a fixed bed under fuel-rich conditions. Temperature, gas composition and mass loss curves identified two distinct periods as the combustion progresses in the bed: the ignition propagation and char oxidation. The effects of bulk density, particle size and air flow rate on the combustion characteristics during the two periods are interpreted by using the ignition front speed, burning rate, percentage of mass loss, equivalence ratio and temperature gradient. Different channelling of air was observed for small miscanthus pellets and large wood particles due to the fast propagation of the ignition front around a channel. The elemental ash composition was also analysed, which explained the sintered agglomerates of miscanthus ashes in terms of alkali index.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Thermal treatment technologies were compared to determine an appropriate method of recovering energy from two wastes – spent mushroom compost and coal tailings. The raw compost and pellets of these ...wastes were combusted in a fluidised-bed and a packed-bed, and contrasted to pyrolysis and gasification. Quantitative combustion parameters were compared to assess the differences in efficiency between the technologies. Fluidised-bed combustion was more efficient than the packed-bed in both instances and pellet combustion was superior to that of the compost alone. Acid gas emissions (NO
x
, SO
x
and HCl) were minimal for the fluidised-bed, thus little gas cleaning would be required. The fuels’ high ash content (34%) also suggests fluidised-bed combustion would be preferred. The Alkali Index of the ash indicates the possibility of fouling/slagging within the system, caused by the presence of alkali metal oxides. Pyrolysis produced a range of low-calorific value-products, while gasification was not successful.
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•Products comparison between slow pyrolysis(SP) and microwave pyrolysis(MP) are studied.•MP produces char and bio-oil products with slightly increased calorific values.•Higher yield of bio-oil is ...obtained from microwave pyrolysis.•No significant improvement of H2 gas produced from MP.•The microwave-pyrolysed oils are better in quality due to the absence of PAH content.
A series of biomass wastes from Malaysia known as Malaysian wood pellets, and rubberwood were employed in the present work. Using these materials as the feedstock, two different heating techniques; external heating by means of conventional slow pyrolysis (SP) and instanteneous volumetric heating by means of microwave pyrolysis (MP) were carried out. Two distinct temperatures; 500°C and 800°C were used. The main objective was to characterise both the microwave-pyrolysed products and slow pyrolysed products together including the influence of temperature so as to compare and contrast in terms of yield, and composition of the high-value fuel gas (H2) or syngas (H2+CO). The research found that the use of the microwave oven system to conduct pyrolysis boosted the production of oil but diminished the total gas yield. Char proportion also reduced when microwave heating method was applied. This research also revealed that the configuration of the microwave oven with mode stirrer and bottom-fed waveguide that produces a constant output power of 1000W at any set temperature has yielded different results when compared to previous studies and so provides a new insight to the microwave pyrolysis community. The results demonstrated that the microwave-pyrolysed chars were slightly porous than slow-pyrolysed chars at 500°C. However at higher temperature of 800°C, lower surface area was obtained from microwave pyrolysis which can be attributed to serious damage of char structure as the consequence of high power supplied into the cavity and high temperature used. SEM microphotographs revealed that microwave pyrolysis at 500°C led to the formation of char with clearly defined pore structure. In the case of gas product, both heating approaches were found to produce a comparable level of H2+CO content except to those produced by MP at higher temperature (800°C). Regarding bio-oil quality, the microwave-pyrolysed oil was found to present compounds with higher aliphatic content and contain less polycyclic aromatic hydrocarbon (PAH) content that is an added quality value as PAH is toxic to the environment.
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6.
Combustion of refuse-derived fuel in a fluidised bed Hernandez-Atonal, Francisco D.; Ryu, Changkook; Sharifi, Vida N. ...
Chemical engineering science,
2007, 2007-01-00, 20070101, Volume:
62, Issue:
1
Journal Article
Peer reviewed
As a medium to maximise the resources recovery from municipal solid waste, refuse-derived fuel (RDF) is considered as a priority solution in industrialised countries. RDF is a value added material ...with a higher calorific value and a homogeneous particle size. The main objective of this study was to investigate the RDF combustion characteristics and the associated pollutant emissions in a fluidised bed combustor. A series of combustion tests was carried out using three different RDF samples in two atmospheric fluidised bed combustors (AFBC). The temperature profiles, gas composition and the properties of fly ash residues were investigated for different RDF fuels and operating conditions. Bed temperatures (ranging from 754 to 906
°C) were correlated with different air and RDF feed rates. It was shown that the overall combustion efficiency of the system improved when secondary air jets were introduced into the system. Only between 2.6% and 4.3% of the potential nitrogen in the fuel was converted to
NO
x
. A slight decrease in the
NO
x
concentration was observed upon injection of secondary air. The capture fractions of N, Cl and trace elements in the fly ash were evaluated from the elemental composition of the fuels and fly ashes. The ratios of
Ca
/
(
S
+
0.5
Cl
)
between 1.6 and 2.2 proved to be very efficient in capturing Cl in the fly ash. The operating conditions of the AFBC system together with the physical characteristics of the ash allowed a full elutriation of the ash residues.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Major challenging issues such as climate change, energy prices and fuel security have focussed the attention of process industries on their energy efficiency and opportunities for improvement. The ...main objective of this research study was to investigate technologies needed to exploit the large amount of low grade heat available from a flue gas condensing system through industrial condensing boilers. The technology and application of industrial condensing boilers in various heating systems were extensively reviewed. As the condensers require site-specific engineering design, a case study was carried out to investigate the feasibility (technically and economically) of applying condensing boilers in a large scale district heating system (40MW). The study showed that by recovering the latent heat of water vapour in the flue gas through condensing boilers, the whole heating system could achieve significantly higher efficiency levels than conventional boilers. In addition to waste heat recovery, condensing boilers can also be optimised for emission abatement, especially for particle removal. Two technical barriers for the condensing boiler application are corrosion and return water temperatures. Highly corrosion-resistant material is required for condensing boiler manufacture. The thermal design of a “case study” single pass shell-and-tube condensing heat exchanger/condenser showed that a considerable amount of thermal resistance was on the shell-side. Based on the case study calculations, approximately 4900m2 of total heat transfer area was required, if stainless steel was used as a construction material. If the heat transfer area was made of carbon steel, then polypropylene could be used as the corrosion-resistant coating material outside the tubes. The addition of polypropylene coating increased the tube wall thermal resistance, hence the required heat transfer area was approximately 5800m2. Net Present Value (NPV) calculations showed that the choice of a carbon steel condenser ensured cash return in a relatively shorter period of time (i.e. 2years) when compared to a stainless steel condenser (i.e. 5–7years). Moreover, the NPV for the stainless steel was more sensitive to the change of the interest rate.
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Char is one of the major products of solid-fuel pyrolysis. It is lightweight and yet retains a substantial proportion of the original thermal energy. Gasification of char provides cleaner energy ...compared to its original fuel for both domestic heating and electricity production. The effect of the thermal as well as chemical parameters on the char gasification process, however, has not yet been fully investigated. In this paper, mathematical models are employed to simulate the char gasification process in a fixed bed and model parameters are varied to assess the subsequent effect on the characteristics of fixed-bed char gasification. A series of benchtop experiments were carried out to validate the theoretical simulation. It is found that the gasification processes is affected by not only the reaction kinetics but also the heat and mass transfer between the gas and solid phases and correct model parameters are critical for the prediction of gasification performance, especially the gas compositions.
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As one of the most easily accessible renewable energy resources, straw can be burned to provide electricity and heat to local communities. In this paper, mathematical modelling methods have been ...employed to simulate the operation of a 38
MWe straw-burning power plant to obtain detailed information on the flow and combustion characteristics in the furnace and to predict the effect on plant performance of variation in operating conditions. The predicted data are compared to measurements in terms of burning time, furnace temperature, flue gas emissions (including NO
x
), carbon content in the ash and overall combustion efficiency. It is concluded that straw burning on the grate is locally sub-stoichiometric and most of the NO is formed in the downstream combustion chamber and radiation shaft; auxiliary gas burners are responsible for the uneven distribution of temperature and gas flow at the furnace exit; and fuel moisture content is limited to below 25% to prevent excessive CO emission without compromising the plant performance. The current work greatly helps to understand the operating characteristics of large-scale straw-burning plants.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
•Bio-char from conventional and microwave pyrolysis have been compared.•Char characteristics and char reactivity have been determined.•Microwave bio-chars are more reactive for CO2 ...gasification.•Lower char reactivity is linked to the presence of secondary chars.
Thermal–chemical processing of biomass is expected to provide renewable and clean energy and fuels in the future. Due to the nature of endothermic reactions, microwave and conventional heating have been applied to this technology. However, more studies need to be carried out to clarify the difference between these two heating technologies. In this work, we investigated two bio-char samples produced from conventional pyrolysis of wood biomass (yield of bio-char: 38.48 and 59.70wt.%, respectively) and one bio-char produced from microwave pyrolysis with a yield of 45.16wt.% from the same biomass sample at different process conditions. Various methodologies have been used to characterise the bio-chars. CO2 gasification of bio-char has also been studied using a thermogravimetric analyser (TGA) and a fixed-bed reaction system. The results show that volatile and carbon contents of the bio-char derived from microwave pyrolysis were between the two conventional bio-chars. However, the microwave bio-char is more reactive for CO2 gasification, as more CO was released during TGA experiments, and the CO release peak was narrower compared with the CO2 gasification of the conventional bio-chars. It is suggested that the conventional bio-char is less reactive due to the presence of more secondary chars which are produced from secondary reactions of volatiles during the conventional biomass pyrolysis. While the microwave pyrolysis generates more uniform bio-chars with less secondary char, and therefore, has advantages of producing bio-char for downstream char gasification.
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