Emissions from a conical FBC fired with a biomass fuel

The results of experimental tests conducted on a conical fluidised bed combustor (FBC), firing mixed sawdust of some Thai woods, are discussed. The concentration profiles for major gaseous emissions (CO, NO_x and CO₂), as well as the temperature and O₂ profiles along the combustor height, were obtained for various operating conditions (fuel feed rate and excess air) for three “fixed” bed heights (20, 30 and 40 cm). The influence of fuel quality (through varying the fuel's moisture content) on the formation of gaseous emissions was also studied. Both CO and NO_x axial profiles were found to have an extreme (maximum) in the active combustion zone. The effects of FBC load and excess air, as well as fuel moisture, on the CO_max were found to be very strong. The NO_x,max was less affected by the combustion conditions, approaching 1.5–3 times the NO_x values at the combustor outlet. The influence of the sand's bed height on the rate of gaseous emissions was found to be minor. The dependencies of the CO and NO_x emissions (i.e. concentrations in the waste flue gas) on the FBC operating conditions are shown for different values of fuel's moisture content.     

Optimisation of efficiency and emissions in pellet burners

There is a trade-off between the emissions of nitrogen oxides (NO_x) and of unburnt hydrocarbons and carbon monoxide (OGC and CO). Decreasing the excess air results in lower NO_x emission but also increased emission of unburnt. The efficiency increases, as the excess air is decreased until the losses due to incomplete combustion become too high. The often-high NO_x emission in today’s pellet burners can be significantly reduced using well-known techniques such as air staging. The development of different chemical sensors is very intensive and recently sensors for CO and OGC have been introduced on the market. These sensors may, together with a Lambda sensor, provide efficient control for optimal performance with respect to emissions and efficiency. In this paper, results from an experimental parameter study in a modified commercial burner, followed by Chemkin simulations with relevant input data and experiments in a laboratory reactor and in a prototype burner, are summarised. Critical parameters for minimisation of NO_x emission from pellet burners are investigated in some detail. Also, results from tests of a new sensor for unburnt are reported. In conclusion, relatively simple design modifications can significantly decrease NO_x emission from today’s pellet burners.     

Optimization of excess air for the improvement of environmental performance of a 150 MW boiler fired with Thai lignite

The results of an experimental study of the excess-air-dependent heat losses, as well as gaseous emissions (NO_x, SO₂ and CO), on a 150 MW boiler firing Thai lignite are discussed. The NO_x emissions were found to increase with the higher excess air ratios; the NO_x values in the flue gas (at 6% O₂) ranged from 257 to 325 ppm, whilst the excess air ratio varied from 1.06 to 1.32 at the economizer outlet. Owing to the highly-efficient operation of the flue gas desulfurization units, the SO₂ emissions from the unit were maintained at a relatively low level, 50–76 ppm for the above excess-air ratios, whereas they accounted for about 3100–3300 ppm at the inlet of the FGD units. The CO emissions were determined for the extremely low excess air ratios. Two approaches for the optimization of the excess air ratio were analyzed in this study. For the first, i.e. the conventional approach, the optimization was carried out based on minimizing the total excess-air-dependent heat losses. The second, the environmentally friendly approach, proposed in this work, was aimed at minimizing the “external” costs (or the costs of damage done by the boiler emissions to the environment and humans). As shown in this paper, the lignite firing at the optimal excess air results in a lower environmental impact by the boiler unit.     

Modeling of single coal particle combustion in O2/N2 and O2/CO2 atmospheres under fluidized bed condition

A one-dimensional transient single coal particle combustion model was proposed to investigate the characteristics of single coal particle combustion in both O₂/N₂ and O₂/CO₂ atmospheres under the fluidized bed combustion condition. The model accounted for the fuel devolatilization, moisture evaporation, heterogeneous reaction as well as homogeneous reactions integrated with the heat and mass transfer from the fluidized bed environment to the coal particle. This model was validated by comparing the model prediction with the experimental results in the literature, and a satisfactory agreement between modeling and experiments proved the reliability of the model. The modeling results demonstrated that the carbon conversion rate of a single coal particle (diameter 6 to 8 mm) under fluidized bed conditions (bed temperature 1088 K) in an O₂/CO₂ (30:70) atmosphere was promoted by the gasification reaction, which was considerably greater than that in the O₂/N₂ (30:70) atmosphere. In addition, the surface and center temperatures of the particle evolved similarly, no matter it is under the O₂/N₂ condition or the O₂/CO₂ condition. A further analysis indicated that similar trends of the temperature evolution under different atmospheres were caused by the fact that the strong heat transfer under the fluidized bed condition overwhelmingly dominated the temperature evolution rather than the heat release of the chemical reaction.     

Integrated resource planning in the power sector and economy-wide changes in environmental emissions

This paper analyzes the roles of key factors (i.e., changes in structure, fuel mix and final demand) on total economy-wide changes in CO₂, SO₂ and NO_x emissions when power sector development follows the integrated resource planning (IRP) approach instead of traditional supply-based electricity planning (TEP). It also considers the rebound effect (RE) of energy efficiency improvements in the demand side and analyzes the sensitivity of the results to variations in the values of the RE. A framework is developed to decompose the total economy-wide change in the emission of a pollutant into four major components, i.e., structural change-, fuel mix- , final demand- and joint-effects. The final demand effect is further decomposed into three categories, i.e., construction of power plants, electricity final demand and final demand related to electricity using equipments. The factor decomposition framework is then applied in the case of the power sector in Indonesia. A key finding in the case of Indonesia is that in the absence of the RE, there would be total economy-wide reductions in CO₂, SO₂ and NO_x emissions of 431, 1.6 and 1.3 million tons respectively during the planning horizon of 2006–2025 under IRP as compared to that under TEP. The decomposition analysis shows that the final demand effect would account for 38% of the total CO₂ emission reduction followed by the structural change effect (35.1%) and fuel mix effect (27.6%) while the joint effect is negligible. The study also shows that economy-wide CO₂ emission reduction due to IRP considering the RE of 45% would be 241 million tons as compared to 333 million tons when the RE is 25%.     

Some aspects of the formation of nitric oxide during the combustion of biomass fuels in a laboratory furnace

The influence of bed-region stoichiometric ratio and fuel nitrogen content on the formation of gaseous species formed during grate combustion of biomass fuels is reported based on gas measurements made within the fuel bed. Three fuels were studied: two mixtures of pelletized bark and wood chips and one of pelletized straw. Experiments were performed in a vertical, cylindrical, laboratory-scale grate-furnace with 0.245 m i.d. and 1.8 m height. Primary air was supplied through a grate consisting of a steel plate with 340 holes of 3.7 mm diameter. Secondary air was supplied 0.66 m above the grate. Gas analysis was performed for O₂, CO₂, CO, H₂ and NO. Results were compared with values calculated using a computer program for thermochemical equilibrium conditions. The measured contents of O₂, CO₂, CO and H₂ show good agreement with calculated equilibrium conditions at all bed region stoichiometries. A higher formation of NO was found for the straw fuel (0.58% fuel nitrogen) than for the bark/wood chip fuels (≈0.25% fuel nitrogen). This is not in accordance with the thermochemical equilibrium calculations indicating that the formation of nitric oxide does not attain thermochemical equilibrium and that the nitrogen content of the fuel has an influence on the amount of NO that is formed. The fuel nitrogen conversion to NO ranged from 3 to 20% at reducing conditions and from 20 to 40% at bed region stoichiometries between 1.00 and 1.25.     

Influence of nozzle height to width ratio on ignition and NOx emission characteristics of semicoke/bituminous coal blends in a 300 kW pulverized coal-fired furnace

To improve the ignition behavior and to reduce the high NO_x emissions of blended pulverized fuels (PF) of semicoke (SC), large-scale experiments were conducted in a 300 kW fired furnace at various nozzle settings, i.e., ratios (denoted by h_f/b) of the height of the rectangular burner nozzle to its width of 1.65, 2.32, and 3.22. The combustion tests indicate that the flame stability, ignition performance, and fuel burnout ratio were significantly improved at a nozzle setting of h_f/b = 2.32. The smaller h_f/b delayed ignition and caused the flame to concentrate excessively on the axis of the furnace, while the larger h_f/b easily caused the deflection of the pulverized coal flame, and a high-temperature flame zone emerged close to the furnace wall. NO_x emissions at the outlet of the primary zone decreased from 447 to 354 mg/m³ (O₂ = 6%), and the ignition distance decreased from 420 to 246 mm when the h_f/b varied from 1.65 to 3.22. Furthermore, the ratio (denoted by S_R/S_C) of the strong reduction zone area to the combustion reaction zone area was defined experimentally by the CO concentration to evaluate the reduction zone. The S_R/S_C rose monotonously, but its restraining effects on NO_x formation decreased as h_f/b increased. The results suggested that in a test furnace, regulating the nozzle h_f/b conditions sharply reduces NO_x emissions and improves the combustion efficiency of SC blends possessing an appropriate jet rigidity.     

Energy-related emissions and mitigation opportunities from the household sector in Delhi

Urban centers are the major consumers of energy, which is a major source of air pollution. Therefore, an insight into energy consumption and quantification of emissions from urban areas are extremely important for identifying impacts and finding solution to air pollution in urban centers. This paper applies the Long-range Energy Alternatives Planning (LEAP) system for modeling the total energy consumption and associated emissions from the household sector of Delhi. Energy consumption under different sets of policy and technology options are analyzed for a time span of 2001–2021 and emissions of carbon dioxide (CO₂), carbon monoxide (CO), methane (CH₄), non-methane volatile organic compounds (NMVOCs), nitrogen oxides (NO_x), nitrous oxide (N₂O), total suspended particulates (TSP) and sulfur dioxide (SO₂) are estimated. Different scenarios are generated to examine the level of pollution reduction achievable by application of various options. The business as usual (BAU) scenario is developed considering the time series trends of energy use in Delhi households. The fuel substitution (FS) scenario analyzes policies having potential to impact fuel switching and their implications towards reducing emissions. The energy conservation (EC) scenario focuses on efficiency improvement technologies and policies for energy-intensity reduction. An integrated (INT) scenario is also generated to assess the cumulative impact of the two alternate scenarios on energy consumption and direct emissions from household sectors of Delhi. Maximum reduction in energy consumption in households of Delhi is observed in the EC scenario, whereas, the FS scenario seems to be a viable option if the emission loadings are to be reduced.     

660 MW机组锅炉稳燃改造方案研究

  目的  为了解决电站锅炉深度调峰过程中出现稳燃差、水动力不稳定、SCR入口烟温过低等问题,锅炉稳燃改造势在必行。  方法  文章以某660 MW火电机组锅炉灵活性改造为例,提出了两种有效的稳燃改造方案,分析了燃烧及制粉系统改造后对炉膛和NO_x排放影响。  结果  两种改造方案均可提升锅炉超低负荷稳燃特性,燃烧及制粉系统改造后对炉膛和NO_x排放影响小,在技术可靠性、施工难度及工作量等方面水平相当,相比之下方案一略优。  结论  两种改造方案经理论验证切实可行,可为同类型机组进行灵活性改造提供技术参考。     

Selective removal of CO from hydrogen-rich stream over CuO/CexSn1−xO2–Al2O3 catalysts

The solid solutions of Ce_xSn_1−xO₂ incorporated with alumina to form Ce_xSn_1−xO₂–Al₂O₃ mixed oxides, by the suspension/co-precipitation method, were used to prepare CuO/Ce_xSn_1−xO₂–Al₂O₃ catalysts for the selective oxidation of CO in excess hydrogen. Incorporating Al₂O₃ increased the dispersion of Ce_xSn_1−xO₂, but did not change their main structures and did not weaken their redox properties. Doping Sn⁴⁺ into CeO₂ increased the mobility of lattice oxygen and enhanced the activity of the 7%CuO/Ce_xSn_1−xO₂–Al₂O₃ catalyst in the selective oxidation of CO. The selective oxidation of CO was weakened as the doped fraction of Sn⁴⁺ exceeded 0.5. Incorporating appropriate amounts of Sn⁴⁺ and Al₂O₃ could obtain good candidates 7%CuO/Ce_xSn_1−xO₂–Al₂O₃(20%), 1–x=0.1–0.5, for a preferential oxidation (PROX) unit in a polymer electrolyte membrane fuel cell system for removing CO. Its activity was comparable with, and its selectivity was much larger than, that of the noble catalyst 5%Pt/Al₂O₃.     

Emissions of nitrous oxide from combustion sources

Nitrous oxide (N₂0) has recently become the subject of intense research and debate, because of its increasing concentrations in the atmosphere and its known ability to deplete the ozone layer and also to contribute to the greenhouse effect. There are both natural and anthropogenic sources for N₂O; however, the man-made sources are increasing at a much higher rate than natural ones. Until very recently it was believed that the combustion of fossil fuels, especially coal, was the major contributing factor to these anthropogenic sources. For example, 30% of all N₂0 released into the atmosphere was once attributed to combustion sources, with 83% of the combustion sources coming from coal combustion. Correction of a recently discovered sampling artifact, whereby SO₂, H₂O and NO in combustion gases react in a sampling vessel to produce N₂O, has revealed that, in fact, less than 5 ppm of N₂0 are found in most product gases from combustion systems. Fluidized bed coal combustors are the exception, though, yielding N₂O levels of ca. 50ppm in their off-gases.

The gas-phase reactions of N₂0 in flames are reviewed first. It is clear that in most cases N₂0 is a very reactive intermediate, which is quickly destroyed before being emitted from a flame. The important homogeneous reactions removing N₂0 are thermal decomposition to N₂ and O₂ and also radical attack in e.g. N₂O + H → N₂ + OH. Nitrous oxide is formed from nitrogen-containing species by NO reacting with a radical derived from either HCN or NH₃; the reactions are NCO + NO → N₂0 + CO and NH + NO → N₂0 + H. The levels of N₂O observed are a balance betwen its rates of formation and destruction. It turns out that HCN is a more efficient precursor than NH₃ at producing N₂0. The removal of N₂O is fastest at high temperatures and in fuel-rich systems, where free hydrogen atoms are present in relatively large amounts.

When coal burns in a fluidized bed, most of the N₂O detected is produced during devolatilization, rather than in the subsequent stage of char combustion. It is clear that HCN and NH₃ are produced from nitrogenous material released during devolatilization; these two compounds give N₂0 when the volatiles burn. The burning of char, on the other hand, involves the chemi-sorption of O₂ on to sites containing carbon or nitrogen atoms, followed by surface reaction, with one of the products being N₂0, in addition to CO, CO₂ and NO. Fluidized coal combustors have temperatures around 900°C, which is low enough for the thermal decomposition of N₂O to be relatively slow. In addition, the presence of the solid phase provides a large area for radical recombination, which in turn reduces the rate of removal of N₂O by free radicals. Parametric studies of fluidized bed combustors have shown that factors such as: temperature, amount of excess air, carbon content and O/N ratio of the coal, all have a significant effect on N₂O emissions. It is important to note that heterogeneous reactions with solids, such as CaO and char, can cause large decreases in the amount of N₂O produced during the combustion of coal in a fluidized bed. In fact, there are several methods available for lowering the yields of N₂O from fluidized bed combustors generally. Areas of uncertainty in the factors affecting N₂O emissions from fluidized bed combustors are identified.     

Combustion technology developments in power generation in response to environmental challenges

Combustion system development in power generation is discussed ranging from the pre-environmental era in which the objectives were complete combustion with a minimum of excess air and the capability of scale up to increased boiler unit performances, through the environmental era (1970–), in which reduction of combustion generated pollution was gaining increasing importance, to the present and near future in which a combination of clean combustion and high thermodynamic efficiency is considered to be necessary to satisfy demands for CO₂ emissions mitigation.

From the 1970s on, attention has increasingly turned towards emission control technologies for the reduction of oxides of nitrogen and sulfur, the so-called acid rain precursors. By a better understanding of the NO_x formation and destruction mechanisms in flames, it has become possible to reduce significantly their emissions via combustion process modifications, e.g. by maintaining sequentially fuel-rich and fuel-lean combustion zones in a burner flame or in the combustion chamber, or by injecting a hydrocarbon rich fuel into the NO_x bearing combustion products of a primary fuel such as coal.

Sulfur capture in the combustion process proved to be more difficult because calcium sulfate, the reaction product of SO₂ and additive lime, is unstable at the high temperature of pulverized coal combustion. It is possible to retain sulfur by the application of fluidized combustion in which coal burns at much reduced combustion temperatures. Fluidized bed combustion is, however, primarily intended for the utilization of low grade, low volatile coals in smaller capacity units, which leaves the task of sulfur capture for the majority of coal fired boilers to flue gas desulfurization.

During the last decade, several new factors emerged which influenced the development of combustion for power generation. CO₂ emission control is gaining increasing acceptance as a result of the international greenhouse gas debate. This is adding the task of raising the thermodynamic efficiency of the power generating cycle to the existing demands for reduced pollutant emission. Reassessments of the long-term availability of natural gas, and the development of low NO_x and highly efficient gas turbine–steam combined cycles made this mode of power generation greatly attractive also for base load operation.

However, the real prize and challenge of power generation R&D remains to be the development of highly efficient and clean coal-fired systems. The most promising of these include pulverized coal combustion in a supercritical steam boiler, pressurized fluid bed combustion without or with topping combustion, air heater gas turbine-steam combined cycle, and integrated gasification combined cycle. In the longer term, catalytic combustion in gas turbines and coal gasification-fuel cell systems hold out promise for even lower emissions and higher thermodynamic cycle efficiency. The present state of these advanced power-generating cycles together with their potential for application in the near future is discussed, and the key role of combustion science and technology as a guide in their continuing development highlighted.     

燃煤机组耦合固废焚烧关键技术研究进展

[目的]燃煤与固体废弃物混合掺烧不仅可以实现固废的能量回收利用,也是实现燃煤发电的碳减排的路径之一。[方法]文章综述讨论了燃煤电站掺混固废的研究工作,主要介绍了基于目前主流的电站锅炉为反应器开展燃煤与不同固废掺混的燃烧应用与技术发展;从燃料经济性、混合燃料的飞灰特征、污染物排放以及碳税角度评价燃煤掺混固废的燃烧技术发展;最后讨论了直接掺混和间接掺混的技术的特点。[结果]燃煤直接掺混固废燃烧时需要尽可能减少对锅炉运行的影响,特别是气体污染物的排放以及飞灰对换热面的影响和飞灰无害化处置。间接掺混可以避免混合燃料燃烧对炉膛的影响,但是需要较高的硬件成本投资且耦合技术较为复杂。富氧燃烧技术依旧需要对现有锅炉结构优化来提高该技术的适用性。[结论]直接掺混可实现性与成本优于间接掺混,且循环流化床燃料适应广的特点有利于燃煤直接掺混固废燃烧技术的应用,随着基于循环流化床的富氧燃烧技术的发展将更有利于实现火电厂的碳减排。     

Reducing the minimum load and NOx emissions for lignite-fired boiler by applying a stable-flame concept

For the purpose of improving the load range and NO_x emission level of lignite-fired power plants, a new combustion technology, called NR-LE burners (NO_x Reduction-Load Extension), has been developed in co-operation between Babcock-Hitachi and Fortum. A single-burner combustion test was performed in Japan with this new NR-LE type burner using Czech lignite. Adapting the flame-stabilization ring and a special additional air-nozzle resulted in achieving a stable flame, which enables:

• The burner minimum load to be less than 50% (Boiler load: 30–40%)

• Low NO_x emissions of less than 200 mg/m³ (6% O₂, dry base)

The first commercial full-scale application of the NR-LE burner was by the IPP power producer in the Czech Republic (Sokolovská Uhelná, a.s. at Vresová Unit2 boiler with steam parameters 325 t/h, 535 °C, 13.5 MPa). The commissioning test runs of the new burners were carried out during September to October 2001. The boiler is now in commercial operation, with (i) a 30% minimum load without supplementary fuel, and (ii) lower NO_x emission levels.     

The long-term life cycle private and external costs of high coal usage in the US

Using four times as much coal in 2050 for electricity production need not degrade air quality or increase greenhouse gas emissions. Current SO_x and NO_x emissions from the power sector could be reduced from 12 to less than 1 and from 5 to 2 million tons annually, respectively, using advanced technology. While direct CO₂ emissions from new power plants could be reduced by over 87%, life cycle emissions could increase by over 25% due to the additional coal that is required to be mined and transported to compensate for the energy penalty of the carbon capture and storage technology. Strict environmental controls push capital costs of pulverized coal (PC) and integrated coal gasification combined cycle (IGCC) plants to $1500–1700/kW and $1600–2000/kW, respectively. Adding carbon capture and storage (CCS) increases costs to $2400–2700/kW and $2100–3000/kW (2005 dollars), respectively. Adding CCS reduces the 40–43% efficiency of the ultra-supercritical PC plant to 31–34%; adding CCS reduces the 32–38% efficiency of the GE IGCC plant to 27–33%. For IGCC, PC, and natural gas combined cycle (NGCC) plants, the carbon dioxide tax would have to be $53, $74, and $61, respectively, to make electricity from a plant with CCS cheaper. Capturing and storing 90% of the CO₂ emissions increases life cycle costs from 5.4 to 11.6 cents/kWh. This analysis shows that 90% CCS removal efficiency, although being a large improvement over current electricity generation emissions, results in life cycle emissions that are large enough that additional effort is required to achieve significant economy-wide reductions in the US for this large increase in electricity generation using either coal or natural gas.     

Chimney emissions from small-scale burning of pellets and fuelwood—examples referring to different combustion appliances

Most wood boilers used for residential heating today are old-fashioned and emit large quantities of organic compounds. The installation of a pellet burner and a change to wood pellets as fuel normally decreases the emissions remarkably. In this study, the emissions from different equipment for burning of wood and pellets are compared. The organic fraction of smoke from traditional wood burning is to a great extent composed of methoxyphenols, with antioxidant effects. Methoxyphenols were also identified in smoke from pellet stoves. A fuelwood boiler or a furnace with an inserted pellet burner is heated to a higher combustion temperature, decreasing the total amount of organic compounds in the smoke. Above 800 °C, methoxyphenols are thermally decomposed and carcinogenic polycyclic aromatic compounds (PACs) are formed. The combustion-formed aromatic hydrocarbon benzene is present in smoke from all kinds of burning, but the proportion relative to primary organic compounds increases with increasing combustion temperature. In smoke from an environmentally labelled wood boiler and from some pellet burning devices, the levels of PAC and benzene were found to be low. Evidently, the combustion was nearly complete. Although the change from wood to pellets significantly decreases the emissions, considerable differences exist between various combinations of pellet burners and boiler furnaces.     

Emissions from multiple-spouted and spout-fluid fluidized beds using rice husks as fuel

This paper presents the experimental results of the emissions of CO and CO₂ using rice husks as fuel on different configurations of spout-sluidized beds namely, multiple-spouted and spout-fluid fluidized bed. The emission of pollutants from the multiple-spouted bed and spout-fluid bed was investigated with rice husk fuel. The operating parameters considered were the different levels of excess air, different primary-to-secondary air ratios at each level of excess air and method of feeding. It was found that emission of CO from the multiple-spouted bed seemed to be lower with under-bed feeding of the rice husk fuel compared to over-bed feeding. However, the emission of CO₂ did not change significantly for both methods of feeding. Changes in excess air levels influenced the emissions of CO and CO₂ from the multiple-spouted bed within the excess air range investigated. It was found that emission of CO was less at 10% excess air with over-bed feeding; emission of CO in the case of under-bed feeding was lowest at 20% excess air level. It was found that the method of feeding had not significantly influenced the emission of CO and CO₂ in the spout-fluid bed. The combustion efficiency however, in general, was slightly higher in the case of under-bed feeding compared to over-bed feeding. Emission of CO was less in the spout-fluid bed compared with the emission of CO in the multiple-spouted bed. The result can be likely attributed to the higher combustion efficiency attained by the spout-fluid bed compared with that of multiple-spouted bed.     

440t_h CFB锅炉掺烧石油焦二氧化硫排放特性研究

在440 t/h大型循环流化床锅炉上进行了燃烧不同比例煤和石油焦混合燃料时二氧化硫排放特性的试验研究。研究了燃烧不同比例的混合燃料、炉膛温度、过量空气系数和钙硫摩尔比对二氧化硫排放特性的影响。研究结果表明,过量空气系数和钙硫摩尔比的增加可以降低二氧化硫排放浓度。存在一个最佳脱硫温度,二氧化硫排放浓度最低,对于各种混合燃料最佳脱硫温度应在830～850℃之间。     

Thermochemical hydrogen production from a two-step solar-driven water-splitting cycle based on cerium oxides

A new thermochemical cycle for H₂ production based on CeO₂/Ce₂O₃ oxides has been successfully demonstrated. It consists of two chemical steps: (1) reduction, 2CeO₂ → Ce₂O₃ + 0.5O₂; (2) hydrolysis, Ce₂O₃ + H₂O → 2CeO₂ + H₂. The thermal reduction of Ce(IV) to Ce(III) (endothermic step) is performed in a solar reactor featuring a controlled inert atmosphere. The feasibility of this first step has been demonstrated and the operating conditions have been defined (T = 2000 °C, P = 100–200 mbar). The hydrogen generation step (water-splitting with Ce(III) oxide) is studied in a fixed bed reactor and the reaction is complete with a fast kinetic in the studied temperature range 400–600 °C. The recovered Ce(IV) oxide is then recycled in first step. In this process, water is the only material input and heat is the only energy input. The only outputs are hydrogen and oxygen, and these two gases are obtained in different steps avoiding a high temperature energy consuming gas-phase separation. Furthermore, pure hydrogen is produced (it is not contaminated by carbon products like CO, CO₂), thus it can be used directly in fuel cells. The results have shown that the cerium oxide two-step thermochemical cycle is a promising process for hydrogen production.     

Industrial-scale investigations on effects of tertiary-air declination angle on combustion and steam temperature characteristics in a 350-MW supercritical down-fired boiler

Industrial-scale experiments were conducted to study the effects of tertiary air declination angle (TDA) on the coal combustion and steam temperature characteristics in the first 350-MW supercritical down-fired boiler in China with the multiple-injection and multiple-staging combustion (MIMSC) technology at medium and high loads. The experimental results indicated that as the TDA increased from 0° to 15°, the overall gas temperature in the lower furnace rose and the symmetry of temperature field was enhanced. The ignition distance of the fuel-rich coal/air flow decreased. In near-burner region, the concentration of O₂ decreased while the concentrations of CO and NO increased. The concentration of NO decreased in near-tertiary-air region. The carbon in fly ash decreased significantly from 8.40% to 6.45% at a load of 260 MW. At a TDA of 15°, the ignition distances were the shortest (2.07 m and 1.73 m) at a load of 210 MW and 260 MW, respectively. The main and reheat steam temperatures were the highest (557.2°C and 559.4°C at a load of 210 MW, 558.4°C and 560.3°C at a load of 260 MW). The carbon in fly ash was the lowest (4.83%) at a load of 210 MW. On changing the TDA from 15° to 25°, the flame kernel was found to move downward and the main and reheat steam temperatures dropped obviously. The change of TDA has little effect on NO_x emissions(660–681 mg/m³ at 6% O₂). In comprehensive consideration of the pulverized coal combustion characteristics and the unit economic performance, an optimal TDA of 15° is recommended.