Realization of oxyfuel combustion for near zero emission power generation

Oxyfuel combustion is one of the promising carbon capture and storage (CCS) technologies for coal-fired boilers. In oxyfuel combustion, combustion gas is oxygen and recirculating flue gas (FGR) and main component of combustion gas is O₂, CO₂ and H₂O rather than O₂, N₂ in air combustion. Fundamental researches showed that flame temperature and flame propagation velocity of pulverized cloud in oxyfuel combustion are lower than that in air with the same O₂ concentration due to higher heat capacity of CO₂. IHI pilot combustion test showed that stable burner combustion was obtained over 30% O₂ in secondary combustion gas and the same furnace heat transfer as that of air firing at 27% O₂ in overall combustion gas. Compared to emissions in air combustion, NOx emission per unit combustion energy decreased to 1/3 due to reducing NOx in the FGR, and SO_x emission was 30% lower. However SO_x concentration in the furnace for the oxyfuel mode was three to four times greater than for the air mode due to lower flow rate of exhaust gas. The higher SO₃ concentration results that the sulphuric acid dew point increases 15–20 °C compared to the air combustion. These results confirmed the oxyfuel pulverized coal combustion is reliable and promising technology for coal firing power plant for CCS.In 2008, based on R&D and a feasibility study of commercial plants, the Callide Oxyfuel Project was started in order to demonstrate entire oxyfuel CCS power plant system for the first time in the world. The general scope and progress of the project are introduced here. Finally, challenges for present and next generation oxyfuel combustion power plant technologies are addressed.     

A reactive molecular dynamics study of NO removal by nitrogen-containing species in coal pyrolysis gas

Coal splitting and staging is a promising technology to reduce nitrogen oxides (NOx) emissions from coal combustion through transforming nitrogenous pollutants into environmentally friendly gasses such as nitrogen (N₂). During this process, the nitrogenous species in pyrolysis gas play a dominant role in NOx reduction. In this research, a series of reactive force field (ReaxFF) molecular dynamics (MD) simulations are conducted to investigate the fundamental reaction mechanisms of NO removal by nitrogen-containing species (HCN and NH₃) in coal pyrolysis gas under various temperatures. The effects of temperature on the process and mechanisms of NO consumption and N₂ formation are illustrated during NO reduction with HCN and NH₃, respectively. Additionally, we compare the performance of NO reduction by HCN and NH₃ and propose control strategies for the pyrolysis and reburn processes. The study provides new insights into the mechanisms of the NO reduction with nitrogen-containing species in coal pyrolysis gas, which may help optimize the operating parameters of the splitting and staging processes to decrease NOx emissions during coal combustion.     

Reduction effect of co-firing of torrefied straw and bituminous coal on PM1 emission under both air and oxyfuel atmosphere

Straw sample was torrefied at 260 °C and 300 °C in N₂, respectively, to prepare torrefied straw named as T-260 and T-300, and the reduction effect of co-firing straw or torrefied straw and steam coal on PM₁ is investigated. The combustion experiments were conducted in a high temperature drop tube furnace (DTF) at 1400 °C to collect the inorganic PM₁₀ for further analysis. Combustion atmosphere was air for all cases and 50% O₂/50% CO₂ (OXY50) for coal, T-260 and their blends of 1:1 and 4:1. The results show that all three biomass fuels show obvious emission reduction of PM with aerodynamic diameters of ≤?0.3?µm (PM_0.3) under both mix ratios. Reduction ratios of co-firing are overall higher at mix ratio of 1:1 than 4:1, and co-firing of T-260 or T-300 with coal shows higher reduction ratio than co-firing of straw. The higher ash content in torrefied straw leads to higher contents of alkali and alkaline earth metals (AAEM), which will further react with both Si and S during co-firing and coagulate into particles of larger sizes, leading to higher reduction ratios of PM_0.3 and unconspicuous reduction effects in PM_0.3–1 emitted from co-firing. During co-firing in oxyfuel atmosphere, a higher combustion temperature compared to air leads to an intensitive gasification, may resulting in effective and even higher reduction ratio in PM_0.3.     

Thermoluminescence characteristics of Na3 SO4 Cl:X (X=Ce,Dy, Mn) phosphor

In the present paper Na₃SO₄Cl:Ce, Na₃SO₄Cl:Dy, Na₃SO₄Cl:Mn, of Na₃SO₄Cl:Ce, Dy and Na₃SO₄Cl:Ce, Mn phosphor were synthesized by the wet chemical method. Thermoluminescence (TL) characteristics of Na₃SO₄Cl:Ce, Na₃SO₄Cl:Dy, Na₃SO₄Cl:Mn, Na₃SO₄Cl:Ce, Dy and Na₃SO₄Cl:Ce, Mn phosphors were studied for 5 Gy γ-ray dose. In TL glow curve, two peaks have been observed at 129°C and 224°C for different concentrations of Ce and Dy, whereas Mn peaks at 212°C. The same host is also tried for Ce, Dy (peaks at 126, 219) and Ce, Mn (248°C). A significant single peak is observed in the case of Na₃SO₄Cl:Mn and Na₃SO₄Cl:Ce, Mn. This may be due to the effect of activators. It is found that intensity tends to be increase with increased concentrations of the activators. The TL glow curves of the phosphors have been recorded and irradiated at a rate of 0.39 kGy h^?1 for 5 Gy γ-rays dose. It is also found that all the phosphors are less sensitive as compared with Thermoluminescence dosimetry-CaSO₄: Dy for the same γ-rays dose. The paper discuses the preliminary TL characteristics and effect of γ-rays on Na₃SO₄Cl:Ce, Na₃SO₄Cl:Dy, Na₃SO₄Cl:Mn, Na₃SO₄Cl:Ce, Dy and Na₃SO₄Cl:Ce, Mn phosphors.     

Effect of the torrefaction on the emission of PM10 from combustion of rice husk and its blends with a lignite

Torrefaction is a competitive biomass pretreatment technology. However, its impacts on particulate matter (PM) formation during biomass combustion and co-combustion with coal have little been investigated. This work provides new data on the formation of PM₁₀ (particulate matter with aerodynamic diameters less than or equal to 10 µm) from combustion of raw (RH), torrefied rice husk (TRH) and their blends with a lignite (SZ). All combustion experiments were carried out on a drop-tube furnace at 1300 °C and in air. The combustion-generated PM₁₀ was collected by a Dekati low pressure impactor and classified into 14 size fractions for further quantification and characterization. The results indicate that, compared with the RH, the TRH-derived PM₁₀₊ (particle size above 10?µm) contains more alkalis, leading to a decrease in the production of PM₁ (particle size below 1?µm). During co-combustion, fuel interactions promote the transformation of alkali chlorides to aluminosilicates. A considerable amount of water-soluble Ca and P in PM₁ transforms to PM_1–10 (particle size between 1–10?µm). As a result, the production of PM₁ (on an ash basis) decreases while that of PM_1–10 increases. Co-combustion of coal with torrefied rice husk is found to generate less PM₁ but more PM_1–10 than that with raw rice husk.     

In situ parametric study of alkali release in pulverized coal combustion: Effects of operating conditions and gas composition

This work concerns a parametric study of alkali release in a lab-scale, pulverized coal combustor (drop tube reactor) at atmospheric pressure. Measurements were made at steady reactor conditions using excimer laser fragmentation fluorescence (ELIF) and with direct optical access to the flue gas pipe. In this way, absolute gas-phase alkali species could be determined in situ, continuously, with sub-ppb sensitivity, directly in the flue gas. A hard coal was fired in the range 1000–1300 °C, for residence times in the range 3–5 s and for air numbers λ (air/fuel ratios) from 1.15 to 1.50. In addition, the amount of chlorine, water vapor and sulfur, respectively, was increased in known amounts by controlled dosing of HCl, H₂O and SO₂ into the combustion gas to determine effects of these components on release or capture of the alkali species. The experimental results are also compared with values calculated using ash/fuel analyses and sequential extraction to obtain a fuller picture of alkali release in pulverized fuel combustion.     

Experimental and modeling study of H2S formation and evolution in air staged combustion of pulverized coal

High-concentration H₂S formed in the reduction zone of pulverized coal air-staged combustion can result into the high temperature corrosion of water wall tube of boiler, so it is of great importance to accurately predict H₂S concentration for the safe operation of boilers and burners. H₂S formation and evolution depends on two steps: the sulfur release from coal conversion and gas-phase reactions of sulfur species. In this study, the sulfur release characteristics from the pyrolysis of 17 coals, including 5 lignite, 9 bituminous coals and 3 anthracites, are investigated in a drop tube furnace (DTF). Sulfur release model is developed to describe the relationship between sulfur release and coal types. A global gas-phase reaction mechanism of sulfur species composed of ten reactions is used to calculate and predict the formation and evolution of H₂S, COS and SO₂ in the reduction zone of pulverized coal air-staged combustion. A wide range of air-staged combustion experiments of 17 coals are conducted in the DTF at different temperatures and stoichiometric ratios to validate the developed model. The results show that the prediction errors of sulfur species, including SO₂, H₂S and COS, are within ± 30%, which indicates that the developed prediction model of sulfur species is of great assistance for CFD modeling of actual engineering application.     

PM10 formation during the combustion of N2-char and CO2-char of Chinese coals

The formation of PM₁₀ (particles less than or equal to 10 μm in aerodynamic diameter) during char combustion in both air-firing and oxy-firing was investigated. Three Chinese coals of different ranks (i.e., DT bituminous coal, CF lignite, and YQ anthracite) were devolatilized at 1300 °C in N₂ and CO₂ atmosphere, respectively, in a drop tube furnace (DTF). The resulting N₂-chars and CO₂-chars were burned at 1300 °C in both air-firing (O₂/N₂ = 21/79) and oxy-firing (O₂/CO₂ = 21/79). The effects of char properties and combustion conditions on PM₁₀ formation during char combustion were studied. It was found that the formation modes and particle size distribution of PM₁₀ from char combustion whether in air-firing or in oxy-firing were similar to those from pulverized coal combustion. The significant amounts of PM_0.5 (particles less than or equal to 0.5 μm in aerodynamic diameter) generated from combustion of various chars suggested that the mineral matter left in the chars after coal devolatilization still had great contributions to the formation of ultrafine particles even during the char combustion stage. The concentration of PM₁₀ from char combustion in oxy-firing was generally less than that in air-firing. The properties of the CO₂-chars were different from those of the N₂-chars, which was likely due to gasification reactions coal particles experienced during devolatilization in CO₂ atmosphere. Regardless of the combustion modes, PM₁₀ formation in combustion of N₂-char and CO₂-char from the same coal was found to be significantly dependent on char properties. The difference in the PM₁₀ formation behavior between the N₂-char and CO₂-char was coal-type dependent.     

Über die Anreicherung des Stabilen Isotops 34S im System Hydrogensulfit/Schwefeldioxid, 2. Mitt.: Das System NH4HSO3/SO2

Es wurde im System NH₄HSO₃/SO₂ die Abhängigkeit des Gesamttrennfaktors für die ³¹S-Anreicherung von der NH₄HSO₃-Konzentration und bei Temperaturen von 20 °C und 80 °C untersucht. Für den elementaren Trennfaktor wurde bei 20 °C eine Abhängigkeit von der NH₄HSO₃-Konzentration festgestellt, während er bei 80 °C für 3…10 Mol NH₄HSO₃/l ～ = 1,007 beträgt. Die Trennstufenhöhen lagen bei 20 °C zwischen 1,23 cm und 5,55 cm und bei 80 °C zwischen 0,93 cm und 2,12 cm.     

Phase transitions and energy storage properties of some compositions in the (1−x)Li2SO4–xNa2SO4 system

In the binary system (1?x)Li₂SO₄–xNa₂SO₄, the solid–solid phase transitions and energy storage properties of Li₂SO₄, Na₂SO₄, the binary compound LiNaSO₄ and two eutectoids (E₁: 0.726Li₂SO₄–0.274Na₂SO₄; E₂: 0.03Li₂SO₄–0.97Na₂SO₄) were investigated by X-ray diffraction and differential scanning calorimetry. Li₂SO₄ has a solid–solid phase transition at 578 °C with the transition enthalpy 252 J g^?1. The binary compound LiNaSO₄ gives a slightly lower enthalpy value, 214 J g^?1 and its transition temperature is clearly reduced to 514 °C. The transition enthalpy of the eutectoid E₁ is maintained to 177 J g^?1 and its transition temperature is further reduced to 474 °C. Li₂SO₄, LiNaSO₄ and the eutectoid E₁ are applicable phase transition materials because of their large transition enthalpies. The enthalpies of Na₂SO₄ and the eutectoid E₂ are not very high (～45 J g^?1), but their transition temperatures are quite low (～250 °C); thus their transition properties may be applied at such low temperatures.     

Influence of combustion temperature and coal types on alumina crystal phase formation of high-alumina coal ash

The alumina content (more than 40%) of high-alumina coal ash is comparative to the middle content bauxite ores in China. So far, in order to meet the high demand of alumina and the rise of circular economy industrial chain, extracting alumina from coal ash has become a way to comprehensively utilize high-alumina coal ash. However, this process has high requirements on the crystal phase and stability of alumina. Different from most studies, this paper focuses on how to produce coal ash more beneficial to the later refining of aluminum. Therefore, the effects of combustion temperature and coal types by classifying high-alumina coal into dull coal and bright coal on alumina crystal phase formation were studied. Through proximate analysis, ultimate analysis, calorific value analysis, X-ray fluorescence spectroscopy, X-ray diffraction (XRD) and scanning electron microscope (SEM) and other methods, it is found that γ-Al₂O₃ in high-alumina coal ash translated into more stable θ-Al₂O₃ and finally α-Al₂O₃ when combustion temperature is higher than 1000°C. Thus compared with pulverized coal boilers, circulating fluidized bed (CFB) boilers with lower combustion temperature can produce higher quality coal ash. Moreover, at the same combustion temperature, alumina crystal phase in dull coal ash is relatively less stable than that in bright coal ash, which is more suitable to the later refining and electrolysis of aluminum.     

Low temperature electrical conductivity,dielectric constant,and phase transitions in (NH4)2SO4 and (ND4)2SO4

Careful axiswise measurements of d.c. conductivity and dielectric constants of (NH₄)₂SO₄ from 50 to - 196°C establish two distinct phase transitions, instead of one, at temperatures -49.5 and -58°C which remain unchanged in (ND₄)₂SO₄. Explanation based on successive distortions of non-equivalent (NH₄)⁺ is offered. Low temperature transport process in the crystal also is discussed.     

A deep insight into arsenic adsorption over γ-Al2O3 in the presence of SO2/NO

Arsenic is easily evaporated during coal combustion, which not only raises serious environmental concerns but also results in the deactivation of catalyst in selective catalytic reduction (SCR) systems. It is a promising method to use sorbents for the capture of arsenic vapors (As₂O₃(g)) before As-containing flue gas entering SCR catalyst. However, arsenic has a strong affinity with sulfur in coal and SO₂ in the coal combustion flue gas strongly suppresses As₂O₃(g) capture by typical Ca/Fe-based sorbents. This study estimated the selective capture of As₂O₃(g) by γ-Al₂O₃ and the effects of SO₂ and NO on the arsenic adsorption were investigated. The results showed that As₂O₃(g) adsorption over γ-Al₂O₃ was effectively conducted at temperatures ranging from 300 to 400 °C. In the reacted γ-Al₂O₃, arsenic was predominantly in the form of As³⁺ through reactions with Al-O bonds and positive charged alumina ions. SO₂ was slightly adsorbed on γ-Al₂O₃, which had a limited effect on arsenic adsorption. The adsorption of SO₂ on γ-Al₂O₃ mainly occurred on the sites of hydroxyl groups (Al-OH) and few adsorbed SO₂ was bound with positive charged alumina ions. NO was catalytically oxidized by γ-Al₂O₃ and released as NO₂. Nevertheless, NO competed with As₂O₃(g) to adhere to positive charged alumina ions and strongly suppressed arsenic adsorption over γ-Al₂O₃. Fortunately, in the presence of SO₂, NO was mostly transformed into intermediate (-SO₃NO) at the sites of Al-OH on γ-Al₂O₃. As a result, the adverse effect of NO on the adsorption of As₂O₃(g) was weakened.     

Online deposition measurement and slag bubble behavior in the reduction zone of pulverized coal staged combustion

An online thermogravimetric measurement method of ash deposition was developed. Ash deposition and slag bubble in the reductive zone of pulverized coal staged combustion were investigated. Firstly, a steady pulverized coal staged combustion was achieved in an electrically heated down-fired furnace. Additionally, gas species, coal conversion, and particle size distribution were quantitatively measured. Secondly, real-time ash deposition rates at different temperatures (1100–1400 °C) were measured, and deposition samples were carefully collected with an N₂ protection method. The morphologies of collected samples were investigated through a scanning electron microscope. It was found that the deposited ash transformed from a porous layer composed of loosely bound particles to a solid layer formed by molten slag. Different behaviors of the slag bubble were observed, and bubble sizes were significantly affected by the deposition temperature. A deposition and bubble formation mechanism was proposed and used for modeling. Results showed that the proposed model well predicted the observed ash deposition and bubble formation process.     

Experimental investigation of NOx emission and ash-related issues in ammonia/coal/biomass co-combustion in a 25-kW down-fired furnace

Co-firing ammonia in coal units is a promising approach for the phasedown of coal power. In this paper, we demonstrate the feasibility of burning ammonia with coal and biomass in a 25- kW down-fired furnace with a swirl-stabilized burner. Ammonia is injected from the central tube at thermal ratios ranging from 0 to30% and can be completely burnt out in most co-firing cases. We investigate the NO_x emission, unburnt carbon in fly ash, particulate matter formation and ash deposition behaviors when co-firing NH₃ with either SH lignite coal or the coal/biomass blend. With a fixed air staging ratio, the NO_x emission increases linearly with the NH₃ fuel ratio. By increasing the percentage of secondary air, the emitted NO_x can be reduced to 300 ppm with an NH₃ thermal ratio of 30%. The unburnt carbon is affected by NH₃ addition in a complex manner. With a 30% (thermal) NH₃ addition, the unburnt carbon increases from 0.4% to 5.6% for the SH coal mainly due to a temperature drop, but decreases from 2.2% to 0.7% for the SH coal/biomass blend. As for the ash-related issues, the addition of NH₃ to either coal or coal/biomass blend is found to alleviate both the fouling intensity and the ultrafine particulate matter formation ability. This is a major advantage over biomass combustion.     

Measurements and modelling of oxy-fuel coal combustion

Oxy-fuel combustion is one of the most promising technologies to isolate efficiently and economically CO₂ emissions in coal combustion for the ready carbon sequestration. The high proportions of both H₂O and CO₂ in the furnace have complex impacts on flame characteristics (ignition, burnout, and heat transfer), pollutant emissions (NO_x, SO_x, and particulate matter), and operational concerns (ash deposition, fouling/slagging). In contrast to the existing literature, this review focuses on fundamental studies on both diagnostics and modelling aspects of bench- or lab-scale oxy-fuel combustion and, particularly, gives attention to the correlations among combustion characteristics, pollutant formation, and operational ash concerns. First, the influences of temperature and species concentrations (e.g., O₂, H₂O) on coal ignition, volatile combustion and char burning processes, for air- and oxy-firing, are comparatively evaluated and modelled, on the basis of data from optically-accessible set-ups including flat-flame burner, drop-tube furnace, and down-fired furnace. Then, the correlations of combustion-generated particulate/NO_x emissions with changes of combustion characteristics in both air and oxy-fuel firing modes are summarized. Additionally, ash deposition propensity, as well as its relation to the formation of fine particulates (i.e. PM_0.2, PM₁ and PM₁₀), for both modes are overviewed. Finally, future research topics are discussed. Fundamental oxy-fuel combustion research may provide an ideal alternative for validating CFD simulations toward industrial applications.     

Salt deposition from hydrothermal solutions in a flow reactor

A flow hydrothermal setup with a tubular reactor equipped with a plunger pump and back pressure valves is used to study the effects of scaling in the K₂SO₄-KCl-H₂O, K₂SO₄-K₂CO₃-H₂O, and Na₂SO₄-NaCl-H₂O systems at pressures of up to 270–340 kg/cm², temperatures of 400–600°C, and flow rates of 5.0 and 2.5 ml/min in order to establish conditions for the formation of salt plugs of type 2 (K₂SO₄, Na₂SO₄) in the flow mode at supercritical (SC) state parameters and to explore ways of eliminating such salt deposits by means of hydrothermal solvents, more specifically, high-temperature aqueous solutions of salts of type 1 (KCl, K₂CO₃, and NaCl). The concentrations of hydrothermal solvents sufficient to prevent the plugging of flow systems with solutions containing 0.26–0.27 mol % K₂SO₄ or Na₂SO₄ are determined, and the effects of the flow rate and chemical composition of type 1 salts on this process are studied. The results show that the phenomenon of scaling with the formation of salt plugs, which hinders the practical use of supercritical water oxidation technology, can be eliminated by adding readily soluble electrolytes, salts of type 1, to initial aqueous solution of type 2 salts.     

X-ray diffraction study of sub-lattice order parameters in ferrielectric ammonium sulphate

Single-crystal X-ray diffraction has been used to study the sublattice order parameters corresponding to the ordering of the two kinds of symmetrically independent ammonium ions in (NH₄)₂SO₄. The diffraction data were collected on an automatic computer-controlled diffractometer using the 2θ-ω scan technique in the temperature range 22 to 130°C. The temperature dependence of the integrated intensities of Bragg reflections is well explained not by a pseudo-proper ferroelectric model, but by a ferrielectric one. It is concluded that (NH₄)₂SO₄ undergoes a ferrielectric phase transition at about ?50°C.     

Synthesis of indium oxide nanoparticles by solid state chemical reaction

A three stage process consisting of mechanical milling, heat treatment, and washing has been investigated as a means of manufacturing nanoparticulate powders of In₂O₃. In the first stage of processing, mechanical milling was used to prepare a nanocrystalline mixture of In₂(SO₄)₃, Na₂CO₃, and NaCl. Subsequent heat treatment of the milled reactant resulted in the formation of nanocrystalline In₂O₃ particles embedded within a matrix of Na₂SO₄ and NaCl. In the final stage of processing, the In₂O₃ powder was recovered by washing with deionised water. The duration of milling was found to determine the degree of hard agglomeration in the final washed powder. It was also found that the average particle size of the powder could be controlled between 8 and 18 nm by simply varying the temperature of the post-milling heat treatment over the range of 400 to 550°C. These results demonstrate that solid state chemical reaction can be used as a technically simple method for manufacturing nanoparticulate In₂O₃ powders with a controlled particle size and low levels of hard agglomeration.     

Thermoluminescence of γ-irradiated BaCa(SO4)2:Eu,Dy

In this paper, thermoluminescence (TL) studies of BaCa(SO₄)₂:Eu,Dy phosphor are reported. A microcrystalline sample of BaCa(SO₄)₂:Eu,Dy was prepared by a solid state diffusion method and the formation of the compound was confirmed by the X-ray diffraction study. Morphology of the phosphor was analyzed by scanning electron microscopy (SEM). The sample is found to have an average particle size of 5?µm. TL glow curves of the γ-irradiated samples with different concentrations of Eu and Dy were studied and compared with BaCa(SO₄)₂:Eu and BaCa(SO₄)₂:Dy. It has been found that a single peak was located at around 230°C with the highest TL intensity in BaCa(SO₄)₂:Eu,Dy which is eight times and two times more than singly Dy- and Eu-doped BaCa(SO₄)₂ phosphor, respectively. For TL analysis, BaCa(SO₄)₂:Eu,Dy (0.2?mol%, 1?mol%) is annealed at different temperatures ranging from 900°C to 1100°C. Analysis of the TL glow curve was carried out by a glow curve deconvoluted method. Trapping parameters (activation energy and frequency factor) of all TL glow curves were evaluated by Chen's peak shape method. A comparison of trapping parameters between BaCa(SO₄)₂:Eu,Dy; BaCa(SO₄)₂:Eu and BaCa(SO₄)₂:Dy phosphors at 900°C, 1000°C and 1100°C is also reported in this paper.