Combustion Of Victorian Brown Coal Research Articles

Characterisation of Australian ilmenite oxygen carrier during chemical looping combustion of Victorian brown coal

Chemical looping combustion (CLC) offers a low emission route for fossil fuel-based power generation. In this study, a compact fully looped chemical looping reactor was developed in-house, and operated continuously with a Victorian brown coal as fuel and an Australian ilmenite as oxygen carrier at 940–980 °C and atmospheric pressure for 36 h. The study found that the CLC performance increased with increasing operation time. The CO2/CO molar ratio, used to evaluate the CLC reaction degree, increased from 2.3 to 8.9 over the 36 h of operation. Specifically, this paper provides a comprehensive study and evaluation of the ilmenite characteristics on morphology changes and ash interaction, due to the continuous operation. No noticeable effect by ash interaction on the CLC performance or operation was observed during the continuous operation. The characterisation results revealed a clear segregation of iron species from the iron‑titanium matrix, its migration to and enrichment over the particle surfaces. Hematite (Fe2O3) formation became more pronounced while pores developed internal and external of the ilmenite particles. These changes are believed to improve the carrier reactivity, and so the CLC performance, but also to weaken the mechanical strength of the carrier, promoting particle attrition and leading to its loss by elutriation.

Pilot-scale experimental and CFD modeling investigations of oxy-fuel combustion of Victorian brown coal

This paper presents the first trial in the world for experimental and modeling investigation of oxy-fuel combustion of Victorian brown coal in a 3MWth pilot-scale facility. Two coal samples with different moisture contents, wet (40wt% moisture) and air-dried (24wt% moisture), were tested in air- and oxy-firing modes with oxygen level varying from 27% (vol) to 40% in furnace. The commercial software, FLUENT 13, was employed to interpret the experimental data by taking into account a series of validated refined sub-models for oxy-fuel combustion. The modeling results showed reasonable agreement with the measurements of flue gas temperature profile as well as in-situ flame photographs in furnace. As has been confirmed, a rather identical flue gas temperature profile was achieved between air-firing and oxy-27% O2 for the dried Victorian brown coal. For the wet brown coal with 42wt% moisture, 30% O2 in oxy-fuel mode is however essential to match air to achieve an identical flue gas temperature profile. Even so, under above optimum retrofit conditions, the radiative heat transfer and CO emission profiles were still slightly mismatched between air-firing and oxy-firing cases regardless of the moisture content within coal. The radiative heat transfer was increased in wet coal oxy-firing because of the higher gas emissivity for the abundant CO2 and H2O within the flue gas. The CO emission concentration in oxy-fuel mode was raised, due to the enhanced C–CO2 and C–H2O gasification reactions that occurred readily at relatively low temperatures for Victorian brown coal char. The presence of no less than 30% oxygen in primary gas is also essential to minimise particle ignition delay caused by the large specific heat capacity of CO2 and steam. For flue gas properties, it has been confirmed that, under the optimised oxy-fuel conditions, the typical CO2 purity reached approximately 80% in dried flue gas, in which the excess oxygen remained at ∼4% whereas steam reached 35%, relative to 10% in air-firing flue gas. By shifting coal combustion from air-firing to oxy-fuel mode, the flue gas recirculation ratio varied from 48% to 64% for the use of wet and dried coals, which are considerably lower than the literature results for bituminous coal.

Nitrogen oxides, sulfur trioxide, and mercury emissions during oxy-fuel fluidized bed combustion of Victorian brown coal.

This study investigates, for the first time, the NOx, N2O, SO3, and Hg emissions from combustion of a Victorian brown coal in a 10 kWth fluidized bed unit under oxy-fuel combustion conditions. Compared to air combustion, lower NOx emissions and higher N2O formation were observed in the oxy-fuel atmosphere. These NOx reduction and N2O formations were further enhanced with steam in the combustion environment. The NOx concentration level in the flue gas was within the permissible limit in coal-fired power plants in Victoria. Therefore, an additional NOx removal system will not be required using this coal. In contrast, both SO3 and gaseous mercury concentrations were considerably higher under oxy-fuel combustion compared to that in the air combustion. Around 83% of total gaseous mercury released was Hg(0), with the rest emitted as Hg(2+). Therefore, to control harmful Hg(0), a mercury removal system may need to be considered to avoid corrosion in the boiler and CO2 separation units during the oxy-fuel fluidized-bed combustion using this coal.

Use of synthetic oxygen carriers for Chemical Looping Combustion of Victorian brown coal

Chemical Looping Combustion (CLC) is a promising alternative technology for inherent capture of CO2 at a lower cost and energy penalty relative to other Carbon Capture and Storage (CCS) options. This paper details the use of synthetic oxygen carriers composed of 20wt% CuO, Mn2O3 and NiO impregnated on Fe2O3 in CLC with Victorian brown coal as a fuel. The investigation used a variety of experimental equipment including a fluidized bed reactor, Thermogravimetric Analyser (TGA) and Temperature Programmed Reduction (TPR) unit. The oxygen carrier samples were characterized using techniques such as X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Accelerated Surface Area and Porosimetry (ASAP). The characterization also included in situ high temperature XRD using synchrotron radiation to study the changes in the lattice parameter and crystal size of Fe2O3 as a function of temperature in a CLC-like environment. It was observed that the synthetic oxygen carriers demonstrated greater performance and enhanced reactivity over Fe2O3 with the NiO impregnated sample having the highest CO2 conversion efficiency. The synchrotron based in situ XRD study showed that the crystal size of the oxygen carrier increased after one redox cycle. This was also observed in the SEM micrographs of the used samples and is due to coalescence of the grains. Thermochemical modelling was carried out to generate an Ellingham diagram and it was found that the theoretical and experimental values did not conform well and this was attributed to the algorithm used by the software. It was found that all the synthetic oxygen carriers demonstrated superior performance over Fe2O3. However, the viability of using synthetic oxygen carriers in a larger scale will depend on the economics of their production.

Experimental investigation of fluidized bed chemical looping combustion of Victorian brown coal using hematite

An experimental investigation on chemical looping combustion (CLC) of Victorian brown coal is carried out in a fluidized bed of hematite. The aim of this study is to understand the feasibility of Victorian brown coal CLC as very little technical information is currently available on the process using these coals. The in situ CLC experiments are first performed using a thermogravimetric analyzer (TGA). The TGA results show good performance of hematite as oxygen carrier over five multiple re-dox cycles under CO2 gasification environment. Therefore, further investigation is performed using a bench-scale fluidized bed that operates in a batch mode cyclically with reduction in CO2 environment, and oxidation in air. Several tests have been conducted to assess the impact of different temperatures, particle size of hematite and CO2 concentration in a flowing fluidizing gas medium. It is observed that the hematite particles of 100–150μm performed best with respect to carbon conversions that show an increasing trend with increasing temperature and CO2 concentration in the fluidizing gas.

Oxy-fuel fluidized bed combustion using Victorian brown coal: An experimental investigation

Oxy-fuel combustion is a promising power generation technology due to its potential for easier CO2 separation and capture. This when combined with fluidized bed combustion offers additional advantages like fuel flexibility, uniform temperature distribution, low NOx emissions and in-bed SO2 capture via sorbent addition. Although few studies have addressed the oxy-fuel fluidized bed combustion of lignites, its applicability using Victorian brown coal has remained unexplored. From experiments in a bench-scale fluidized bed under oxy-fuel combustion condition using Victorian brown coal, this study finds that the flue gas CO2 concentration increases with oxygen concentration and steam in the feed gas while the effect of temperature is modest between 850°C and 900°C. The addition of steam lowers the trace element retention in ash; this may indirectly affect their partitioning behavior. These results are important for selection of operating conditions for oxy-fuel fluidized bed combustion of Victorian brown coal.

Numerical Modelling of Oxy-Fuel Combustion in a Full-Scale Tangentially-Fired Pulverised Coal Boiler

This paper presents a computational fluid dynamics (CFD) modelling study to investigate Victorian brown coal combustion in a 550 MW utility boiler under the air-fired (standard) and three oxy-fuel-fired cases. The standard case was modelled based on the real operating conditions of Loy Yang A power plant located in the state of Victoria, Australia. A level of confidence of the present CFD model was achieved validating four parameters of the standard combustion case, as well as the previous preliminary CFD studies which were conducted on a lab-scale (100 kW) unit firing lignite and propane under oxy-fuel-fired scenarios. The oxy-fuel combustion cases are known as OF25 (25vol. % O2 concentration), OF27 (27vol. % O2 concentration), and OF29 (29vol. % O2 concentration). The predictions of OF29 combustion case were considerably similar to the standard firing results in terms of gas temperature levels and radiative heat transfer compared with OF25 and OF27 combustion scenarios. This similarity was because of increasing the residence time of pulverised coal (PC) in the combustion zone and O2 concentration in feed oxidizer gases. Furthermore, a significant increase in the CO2 concentrations and a noticeable decrease in the nitric oxides (NOx) formation were noted under all oxy-fuel combustion conditions. This numerical study of oxy-fuel combustion in a full-scale tangentially-fired PC boiler is important prior to its execution in real-life power plants.

Prediction of distribution of trace elements under Oxy-fuel combustion condition using Victorian brown coals

In reducing CO2 emissions from the energy sector, power generation using Oxy-fuel combustion has become a promising technology. The issue of trace element emissions during coal combustion, however, has important implication on the operation of Oxy-fuel combustors using different types of coals, as the inclusion of excessive amounts of these elements in the gas is harmful to the health and environment with additional implications for CO2 transport and storage. Though a number of studies have been carried out to investigate the distribution of trace elements in air combustion, limited data are available on Oxy-fuel combustion for different grades of coals. In this study, thermodynamic equilibrium calculations, using FactSage software, were performed to predict the trace elements (Cr, As, Se and Hg) speciation during Oxy-fuel combustion of Victorian brown coal at different temperatures (800–1400°C). These predicted trace element emissions from Oxy-fuel combustion were also compared with that from air combustion. It was found that the amount of toxic hexavalent chromium (Cr6+) species, such as CrO2(OH)2, in the vapour phase was greater for Oxy-fuel combustion than that for air combustion. This is due to the presence of water vapour in the Oxy-fuel atmosphere. At low temperature the distributions of toxic arsenic and selenium species in Oxy-fuel combustion showed almost the same behaviour as in air combustion. The model also indicates that the mercury emissions were marginally greater for Oxy-fuel combustion than for air combustion. These results are important for the selection of operating conditions to the emitted trace elements within permissible limits.

CFD modelling of air-fired and oxy-fuel combustion in a large-scale furnace at Loy Yang A brown coal power station

Oxy-fuel combustion technique is a viable option to reduce several types of greenhouse gases (GHGs) emissions from the pulverized coal (PC) combustion systems. In this paper, a computational fluid dynamics (CFD) modelling study has been developed in order to investigate the Victorian brown coal combustion in a 550MW utility boiler under the air-fired (reference case) and three oxy-fuel-fired scenarios. The reference firing case was modelled based on the operating conditions of Loy Yang A power plant located in the state of Victoria, Australia. While Chalmers’ oxy-fuel combustion approach was selected for the present oxy-fuel combustion simulations, which referred to as OF25 (25vol.% O2), OF27 (27vol.% O2), and OF29 (29vol.% O2). User-defined functions (UDFs) were written and incorporated into the CFD code to calculate the following mathematical models: the PC devolatilization, char burnout, multi-step chemical reactions, mass and heat transfer, carbon in fly-ash, and NOx formation/destruction. A level of confidence of the CFD model was achieved validating four different parameters of the conventional combustion case, as well as the previous preliminary CFD studies that conducted on a 100kW unit firing propane and lignite under oxy-fuel combustion environments. The numerical results of OF29 combustion condition were considerably similar to the reference firing results in terms of gas temperature levels and radiative heat transfer relative to the OF25 and OF27 combustion cases. This similarity was due to increasing the residence time of PC in the combustion zone and O2-enriched in feed oxidizer gases. A significant increase in the CO2 concentrations and a noticeable decrease in the NOx formation were observed under all oxy-fuel combustion scenarios. The combustion chemistry was adopted in these investigations in order to capture the effects of O2 concentrations and gas temperatures on the CO/CO2 production rate and equilibrium between H2 and H2O in the combustion zone. Also, the use of O2-enriched atmospheres during oxy-fuel-fired cases was slightly enhanced the carbon burnout rate. These predicted results were reasonably consistent with the experimental investigations and numerical modelling found in the literature. This study of Victorian brown coal oxy-fuel combustion in a large-scale tangentially-fired boiler is important prior to its implementation in real-life.

Chemical looping combustion of Victorian brown coal using NiO oxygen carrier

This study is part of a program assessing the suitability of chemical looping for direct combustion of Victorian brown coal. The performance of NiO as an oxygen carrier in presence of a dried Victorian brown coal was assessed during five alternating cycles of reduction and oxidation in a CO 2 environment using a TGA. The experiments indicate a 4.4–7.5% weight loss of the oxygen carrier per cycle. Preliminary SEM-EDX and FACTSAGE predictions also indicate weight loss, but not to the same extent. The percentage of combustion of coal achieved at the 5th cycle was approximately 67%. Cycle 2 showed maximum reactivity (during reduction) with a decreasing trend during the subsequent cycles. These initial experiments did not reveal much agglomeration between ash and NiO although longer duration experiments are required to explore this issue further.

In-situ observation of the combustion of air-dried and wet Victorian brown coal

This work investigates the fundamental and practical implications of the application of drying technologies to Victorian brown coal combustion. The base case of 60% moisture content coal preheated prior to combustion is compared with partially dried coal (with or without pre-heating) and coal dried to equilibrium moisture content (10–15%). Pulverised coal was combusted in a drop tube furnace and in-situ observations of combustion phenomena, particle temperature and gas temperature were made. An ignition delay was found to occur when partially dried coal was combusted without pre-heating. Flame stability was also decreased when wet coal was combusted without pre-heating. No ignition delay was observed when the water in coal was heated prior to entering the furnace, as in current boilers. The peak particle temperature was found to be higher than the wall temperature by around 130 °C for dried coal, 80 °C for preheated wet coal and 40 °C for non-preheated partially dried coal. The gas temperature profile in the furnace was measured and found to lag behind the particle temperature peak. It was concluded that the evolution and evaporation of water in the wet case lead to an ignition delay, cooler peak particle temperatures and prolonged char combustion. The difference in particle temperatures between preheated wet coal and dried coal and the gas temperature behaviour was attributed to the steam gasification reaction, although studies to elucidate reasons for the differences are ongoing. The quantified results on ignition delay and particle temperatures have important implications for the design of new technologies, in particular the boilers and feed size preparation, for power generation from high-moisture brown coals.

Numerical Modeling of Victorian Brown Coal Combustion in a Tangentially Fired Furnace

A computational fluid dynamics (CFD) model of a 375 MW brown-coal-fired furnace in the Latrobe Valley, Australia, has been developed using ANSYS CFX 12.0. To improve the model predictions, a coal combustion model that takes into consideration carbon monoxide reactions has been utilized in ANSYS CFX 12.0. A level of confidence in the current CFD model has been established by carrying out a mesh independence test and validation against the furnace gas exit temperature (FGET), concentration of flue gas components, total boiler heat supply, and the wall incident heat fluxes measured in the power plant. The validated CFD model is then applied to investigate the effects of several operating conditions at full load, such as different out-of-service firing groups and different combustion air distributions on the coal flame. It is found that the selection of out-of-service firing groups has a considerable effect on coal combustion in terms of high-temperature zone shape and location and distribution of incident ra...

In situ diagnostics of Victorian brown coal combustion in O 2/N 2 and O 2/CO 2 mixtures in drop-tube furnace

Experimental investigation of the combustion of an air-dried Victorian brown coal in O 2/N 2 and O 2/CO 2 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 CO 2 in place of N 2 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 CO 2 relative to N 2 is the principal factor affecting brown coal combustion, which greatly quenched the ignition of individual coal particles. As a result, a high O 2 fraction of at least 30% in CO 2 is required to match air. Moreover, due to the accumulation of unburnt volatiles in the coal particle vicinity, coal ignition in O 2/CO 2 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 CO 2 quenching throughout coal oxidation. Nevertheless, this negative factor was greatly offset by char-CO 2 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/O 2 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% O 2 in CO 2 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% O 2/73% CO 2 and air. As this O 2/CO 2 ratio is lower than that for bituminous coal, 30–35%, a low consumption of O 2 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.

Sources, winning, properties

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.

00/02334 Screening of potential mineral additives for use as fouling preventatives in Victorian brown coal combustion

00/02334 Screening of potential mineral additives for use as fouling preventatives in Victorian brown coal combustion

00/02807 Screening of potential mineral additives for use as fouling preventatives in Victorian brown coal combustion

00/02807 Screening of potential mineral additives for use as fouling preventatives in Victorian brown coal combustion

00/00012 Screening of potential mineral additives for use as fouling preventatives in Victorian brown coal combustion

00/00012 Screening of potential mineral additives for use as fouling preventatives in Victorian brown coal combustion

Screening of potential mineral additives for use as fouling preventatives in Victorian brown coal combustion

Laboratory tests have been undertaken on a suite of 12 mineral samples to ascertain their effectiveness as fouling alleviating additives to coal during combustion. Two samples of Victorian coal from the Loy Yang and Yallourn coal mines were used in the investigation. The laboratory tests include measuring the total chemisorption capacity of the minerals for different Na compounds (NaCl, NaOH and Na 2SO 4) using a simple thermogravimetric technique. The second method involves ashing mixtures of the minerals and coal in air. Chemical analysis of the Na content within the mixtures (before and after ashing) is used to determine the effectiveness of the different additives, with a high Na retention indicating good potential for fouling mitigation. Minerals containing both alumina and silica were found to be the best sorbents for NaCl in the chemisorption experiments. They are also found to be the best additives for retaining sodium within the coal ash (for both coal types) as well. In contrast, minerals exhibiting high NaOH and Na 2SO 4 retention were not necessarily good candidates for use as coal additives. This indicates that the sodium released from the coal during combustion is primarily in the form of NaCl, agreeing with thermodynamic modelling of the combustion process. The best additive candidates are kaolin, bentonite, miclay (micaceous clay), diatomite, pumice, pyrophyllite and overburden. On a cost basis kaolin mineral types and overburdens (provided there are sufficient reserves) appear to be the best candidates for use as fouling preventative additives during the combustion of Victorian low-rank coal.