NOx and H2S formation in the reductive zone of air-staged combustion of pulverized blended coals

Low NO_x combustion of blended coals is widely used in coal-fired boilers in China to control NO_x emission; thus, it is necessary to understand the formation mechanism of NO_x and H₂S during the combustion of blended coals. This paper focused on the investigation of reductive gases in the formation of NO_x and H₂S in the reductive zone of blended coals during combustion. Experiments with Zhundong (ZD) and Commercial (GE) coal and their blends with different mixing ratios were conducted in a drop tube furnace at 1200°C–1400°C with an excessive air ratio of 0.6–1.2. The coal conversion and formation characteristics of CO, H₂S, and NO_x in the fuel-rich zone were carefully studied under different experimental conditions for different blend ratios. Blending ZD into GE was found to increase not only the coal conversion but also the concentrations of CO and H₂S as NO reduction accelerated. Both the CO and H₂S concentrations inblended coal combustion increase with an increase in the combustion temperature and a decrease in the excessive air ratio. Based on accumulated experimental data, one interesting finding was that NO and H₂S from blended coal combustion were almost directly dependent on the CO concentration, and the CO concentration of the blended coal combustion depended on the single char gasification conversion.Thus, CO, NO_x, and H₂S formation characteristics from blended coal combustion can be well predicted by single char gasification kinetics.     

Biomass as a reburning fuel: a specialized cofiring application

Reaction Engineering International has performed a series of combustion tests to evaluate the potential for utilizing wood biomass as a reburn fuel for nitrogen oxides (NO_x) control. Reburning is an effective NO_x reduction technology that utilizes fuel injection above the main burner zone. Studies with other hydrocarbon fuels such as coal and natural gas as reburn fuels have shown that NO_x emissions can be reduced by more than 50–60% with about 15% of the heat input coming from the reburn fuel. Two different biomasses, a hardwood and softwood, were evaluated as reburning fuels and compared to coal and natural gas. The use of wood to reduce NO_x is attractive for several reasons. First, wood contains little nitrogen, as compared with coal which is also used as a reburning fuel. This results in lower NO_x production from fuel nitrogen species for wood. In addition, wood contains virtually no sulfur, so sulfur dioxide (SO₂) emissions are reduced in direct proportion to the coal replacement. Wood is a regenerable biofuel; when a fossil fuel is replaced by a biofuel, there is a net reduction in carbon dioxide (CO₂) emissions. Finally, since the reburning fuel is normally 10–20% of the total heat input, large quantities of wood are not necessary. Experimental results showed NO_x reductions of as high as 70% were obtained with approximately 10–15% wood heat input. The stoichiometric ratio in the reburn zone was the single most important variable affecting NO_x reduction. The highest reductions were found at a reburn stoichiometric ratio of 0.85. NO_x reduction fell to about 40–50% at slightly higher stoichiometric ratios (0.9x reduction was strongly dependent on initial NO_x concentration and only slightly dependent upon temperature, where increased temperature increased NO_x reduction. Finally, the experimental results suggest that wood is as effective as natural gas or coal as a reburning fuel. In addition, REI has completed computer simulations of a full-scale boiler to evaluate the conditions that maximize the NO_x reduction efficiency using biomass as the reburn fuel. Computer modeling of the TVA Allen Station Unit 3, a 265 MW_e cyclone-fired boiler, showed that NO_x reductions as high as 50–60% could be achieved within the constraints set by the boiler and operations. The most important parameters affecting final NO_x emissions are the cyclone barrel stoichiometry, residence time in the reburn zone, and mixing in both the reburn and overfire air zones. The combination of computer simulations and experimental programs has provided the engineers with the tools needed to optimize biomass as a reburn fuel to maximize NO_x reduction.     

Electron-beam flue-gas treatment for multicomponent air-pollution control

During coal combustion, different pollutants such as fly ash, sulfur oxides (SO₂ and SO₃), nitrogen oxides (NO_x=NO+NO₂) and volatile organic compounds (VOCs) are emitted. These pollutants are harmful to the environment and human health. Therefore different air-pollution-control technologies are used. Usually these technologies are designed for removing only a single pollutant. An integrated system for SO₂, NO_x and VOC simultaneous emission control is presented in the paper. The technology uses a high-energy electron-beam from an accelerator and ammonia to treat simultaneously SO₂ and NO_x, the obtained by-product can be used as a fertilizer. The industrial-demonstration plant at EPS Pomorzany in Szczecin is under optimization tests now. Moreover, the tests carried out with the pilot plant at EPS Kawêczyn have demonstrated the possibility of volatile-organic-compounds destruction and their final toxicity reduction.     

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.     

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.     

重型柴油机SCR后处理系统尿素喷射电子控制单元开发

针对重型柴油机尿素选择催化还原(SCR)后处理系统,开发了尿素喷射电子控制单元(DCU),包括硬件平台与控制策略的设计开发。该电子控制单元采用高速16位单片机,具有结构紧凑、工作可靠的优点。通过采用基于发动机MAP图和实际工况的稳态和瞬态修正控制策略,使所匹配的柴油机尿素SCR后处理系统达到重型柴油车国-Ⅳ或更严格的排放标准。     

Exhaust gas recirculation for Nox control in a multicylinder hydrogen-supplemented S.I. engine

This paper describes the results of an experimental investigation carried out on a hydrogen-supplemented multicylinder spark ignition (S.I.) engine to control the level of NO_x (oxides of nitrogen) emission by adopting exhaust gas recirculation (EGR). It was observed that the NO_x level was substantially reduced over a wide range of engine operation conditions. Performance characteristics of the system were also evaluated corresponding to these operating conditions.     

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.     

IrxCo1−x (x = 0.3–1.0) alloy electrocatalysts, catalytic activities, and methanol tolerance in oxygen reduction reaction

Novel methanol-tolerant catalysts for oxygen reduction reaction (ORR), Ir_xCo_1−x/C (x = 0.3–1.0), were synthesized by a conventional impregnation method. These carbon-supported catalysts showed particle sizes of 2.7–5.0 nm. The catalyst activity and the catalyzed ORR kinetics were characterized by cyclic voltammetry and rotating disk electrode methods. Among these Ir_xCo_1−x/C catalysts, the alloy with a formula of Ir_xCo_1−x/C with x value in the range of 0.7–0.8 exhibited the highest mass and specific activities. Compared to a Pt/C catalyst, these alloy catalysts have much stronger methanol tolerance in terms of ORR onset potential (or open-circuit potential). Based on the rotating disk electrode measurements, it was confirmed that these Ir_xCo_1−x/C alloy catalysts could catalyze a complete four-electron transfer reaction of oxygen to water. These results strongly suggest that the novel Ir–Co metal alloy catalysts synthesized in this work could be promising for DMFC cathodes.     

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.     

Cofiring coal and straw in a 150 MWe power boiler experiences

In 1995 ELSAM/MIDTKRAFT equipped the 150 MW_e pulverised coal-fired Studstrup power station, unit 1, for a technology demonstration cofiring of coal and straw. The conversion consisted of establishing a straw pre-processing plant and modifying the burner system. After plant commissioning in January 1996, a 2-year demonstration program was initiated. The objective of the program was to evaluate the influence of cofiring on boiler plant performance, combustion chemistry, heat surface deposits and corrosion, residue quality, emissions, and selective catalytic reduction (SCR) systems. This paper presents the plant conversion and results from the demonstration period.     

Ti4O7 supported IrOx for anode reversal tolerance in proton exchange membrane fuel cell

Fuel starvation can occur and cause damage to the cell when proton exchange membrane fuel cells operate under complex working conditions. In this case, carbon corrosion occurs. Oxygen evolution reaction (OER) catalysts can alleviate carbon corrosion by introducing water electrolysis at a lower potential at the anode in fuel shortage. The mixture of hydrogen oxidation reaction (HOR) and unsupported OER catalyst not only reduces the electrolysis efficiency, but also influences the initial performance of the fuel cell. Herein, Ti₄O₇ supported IrO_x is synthesized by utilizing the surfactant-assistant method and serves as reversal tolerant components in the anode. When the cell reverse time is less than 100 min, the cell voltage of the MEA added with IrO_x/Ti₄O₇ has almost no attenuation. Besides, the MEA has a longer reversal time (530 min) than IrO_x (75 min), showing an excellent reversal tolerance. The results of electron microscopy spectroscopy show that IrO_x particles have a good dispersity on the surface of Ti₄O₇ and IrO_x/Ti₄O₇ particles are uniformly dispersed on the anode catalytic layer. After the stability test, the Ti₄O₇ support has little decay, demonstrating a high electrochemical stability. IrO_x/Ti₄O₇ with a high dispersity has a great potential to the application on the reversal tolerance anode of the fuel cell.     

Operator-guidance systems for industrial processes: A case study evaluating impact on energy use and NOx control

Systems for process-operator guidance are now found in all industrial branches. Their impact on process performance, particularly energy use, is discussed. A case study includes an evaluation of a system in a steel plant; energy efficiency of the process was about 4% higher when the system was in use. A proposed extension of the system includes calculation of NO_x emissions and emission charges. The expected process performance for simulated operating conditions is studied. Four performance criteria are used: operating cost, NO_x emissions, energy and exergy efficiencies. Measurement accuracy, process modelling, and the emission-charge scheme are found to be critical for the guidance given by the system and hence the performance of the process.     

Effects of pyrolyzed semi-char blend ratio on coal combustion and pollution emission in a 0.35 MW pulverized coal-fired furnace

The effects of blend ratio on combustion and pollution emission characteristics for co-combustion of Shenmu pyrolyzed semi-char (SC), i.e., residuals of the coal pyrolysis chemical processing, and Shenhua bituminous coal (SB) were investigated in a 0.35 MW pilot-scale pulverized coal-fired furnace. The gas temperature and concentrations of gaseous species (O₂, CO, CO₂, NO_x and HCN) were measured in the primary combustion zone at different blend ratios. It is found that the standoff distance of ignition changes monotonically from 132 to 384 mm with the increase in pyrolyzed semi-char blend ratio. The effects on the combustion characteristics may be neglected when the blend ratio is less than 30%. Above the 30% blend ratio, the increase in blend ratio postpones ignition in the primary stage and lowers the burnout rate. With the blend ratio increasing, NO_x emission at the furnace exit is smallest for the 30% blend ratio and highest for the 100% SC. The NO_x concentration was 425 mg/m³ at 6% O₂ and char burnout was 76.23% for the 45% blend ratio. The above results indicate that the change of standoff distance and NO_x emission were not obvious when the blend ratio of semi-char is less than 45%, and carbon burnout changed a little at all blend ratios. The goal of this study is to achieve blending combustion with a large proportion of semi-char without great changes in combustion characteristics. So, an SC blend ratio of no more than 45% can be suitable for the burning of semi-char.     

Synthesis and characterization of PtNx/C as methanol-tolerant oxygen reduction electrocatalysts for a direct methanol fuel cell

Platinum nitride supported on carbon (PtN_x/C) is synthesized by the novel strategy of chelating the Pt precursor followed by pyrolysis and is characterized as a possible cathode electrocatalyst for direct methanol fuel cells (DMFCs). The prepared PtN_x/C is shown to possess high methanol tolerance and catalytic activity for the oxygen reduction reaction (ORR). The results indicate that the temperature of the heat treatment and the molar ratio of Pt to N in the precursor solution play important roles in the catalytic performance. A sample of PtN_x/C prepared at 700 °C with a Pt:N ratio of 1:2 shows a significant decrease in the potential loss associated with the mixed potential and the poisoning effect by adsorbed methanol, and this results in a high power density of 180 mW cm⁻². The performance is 30% higher than that of Pt/C under 4 M of methanol concentration.     

Distributed grid-connected photovoltaic power system emission offset assessment: statistical test of simulated- and measured-based data

This study assesses the pollutant emission offset potential of distributed grid-connected photovoltaic (PV) power systems. Computer-simulated performance results were utilized for 214 PV systems located across the US. The PV systems’ monthly electrical energy outputs were based on a performance calculator called PVWATTS. Offset emissions of sulfur dioxide (SO₂), carbon dioxide (CO₂), and nitrogen oxides (NO_x) were determined from PV system outputs and average utility emissions data from each state. For validation, the simulated monthly results were statistically compared with measurement-based data (both production and corresponding emissions data) from 29 PV systems installed at different sites across the US.

While the data shows high (geographic) variability, the substantial number of measurements allows reliable statistical analysis. The methods are found to give consistent results in spite of the necessity to employ some even quite crude input approximations—such as the use of statewide rather than specific emissions data for the systems. No significant differences between simulated and measured monthly means for any of the pollutants were noted on the basis of individual monthly analyses, though the results for NO_x suggest the possible existence of some difference in that case. A more detailed statistical modeling using all monthly data in one combined analysis (allowing improved variability estimation) confirms these conclusions. Even the shorter confidence intervals for expected offsets obtained through the combined analysis show no significant differences between simulated and measured methods for SO₂ and CO₂. The differences for NO_x are statistically significant but consistent—suggesting useful prediction by the simulations via a constant correction factor. As expected, significant differences between months are evident for both simulated and measured offsets.     

Factors affecting photocatalytic performance through the evolution of the properties due to the phase transition from NaBiO3·2H2O to BiO2–x

The phase transition process of a photocatalytic system from NaBiO₃·2H₂O to BiO_2–x has been investigated to understand the important factors that affect photocatalytic performance in a composite system. It is found that a proper amount of BiO_2–x on the surface of NaBiO₃·2H₂O could effectively suppress the electron/hole recombination and increase the exposed reactive sites for photocatalytic reaction. A fully covered BiO_2–x on NaBiO₃·2H₂O results in a dramatical decrease of photocatalytic degradation of dye. An over long hydrothermal process can result in BiO_2–x with reduced oxygen vacancies, which degrades the photocatalytic activity. Furthermore, the photocatalytic reduction ability of CO₂ conversion has been investigated, indicating that the surface activity to different reactants also directly affects the catalytic performance. The investigation of the gradient phase transition process presents a clear guidance to construct a desired photocatalytic system, in addition to selecting gradient materials with suitable bandgap structure and a morphology with different fraction and distribution of each component. The defect evolution of each component during construction of a composite is also an important factor that should be optimized and considered in making a composite to achieve high photocatalytic efficiency.     

Stability of some ternary and quaternary CeNi5-based and PrNi5-based hydride systems

The Ce_1−xR_xNi_2.5Cu_2.5 (R = La,Pr; 0.8 x 0.3) and PrNi_5−xM_x (M = Cu, Fe; 0.5 x 2.5) alloys were investigated for their hydriding characteristics in the temperature range 0–70°C and hydrogen pressure range 0.01–50 atm. The nonlinear behaviour of unit cell volume vs x in Ce_1–xLa_xNi_2.5Cu_2.5 suggests that both size and electronic effects are involved. The partial replacement of Ce by La and Ni by Cu in CeNi₅ causes a substantial reduction in the hydrogen sorption pressures without significantly impairing its hydrogen capacity. It was observed that Fe is more effective than Cu in stabilizing PrNi₅-H₂. The high values of the molar entropy of hydrogen of the β-hydrides studied, S^β_H, are attributed to extensive hydrogen disorder in the interstitial sites of the host lattice. A linear correlation between the hydride decomposition pressures (or free energy) and the unit cell volume. V_c, of the host alloys was observed. This behavior is helpful in predicting the stabilities of new hydrides in a given substitutional alloy series.     

Hydrogen as a fuel for the transportation sector: possibilities and views for future applications in Libya

World-wide energy consumption in the transportation sector accounts for about one quarter of the total energy consumption. This implies that thousands of tons of pollutants are emitted each year. The total pollutants include CO, CO₂, HC, NO_x, SO₂ and soot particles. In Libya, the transportation sector counts for a big share of the total energy demand. So if this sector would be changed to clean fuel,the pollution will be reduced dramatically. Hydrogen is proposed (hypothetically) to be used for the transportation sector in Libya. This paper will review the advancement of this technology world wide, in a sense of hydrogen production, storage, transportation and refueling systems. The possibilities of using hydrogen in the transportation sector in Libya and the expected advantages, obstacles and constraints associated with its application and public acceptance.     

On the computer simulation of the P–C isotherms of ZrFe2 type hydrogen storage materials

This paper deals with computer simulation of the P–C isotherms of some ZrFe₂ type (Zr(Fe_1−xCr_x)₂, Zr_1−xTi_xFe_1.4Cr_0.6, Zr_1−2xMm_xTi_xFe_1.4Cr_0.6 : x00.4) of hydrogen storage materials. A feasible mathematical model has been developed to simulate the P–C isotherms. The randomized variables in the model applied for simulating the P–C isotherms of the above-mentioned ZrFe₂ type hydrogen storage materials correspond to change in enthalpy (ΔH) and entropy (ΔS) of hydride formation. Several ZrFe₂ type materials as in above have been synthesized and their P–C isotherms, enthalpy and entropy change has been evaluated experimentally in order to have input data for simulation. A special software was developed to simulate the P–C isotherms using the said model. A close match between the experimentally observed and simulated P–C isotherms for the above said ZrFe₂ type alloys has been obtained.