Gas/particle flow and combustion characteristics and NOx emissions of a new swirl coal burner

Due to the limits of reserves and price for the high rank coal, the low rank coal has been employed as fuel for power generation in China and will be eventually employed in the world. To burn low rank coal, centrally fuel-rich swirl coal combustion burner has been studied in Harbin Institute of Technology. This paper reviews and analyzes the major research results. The work has included both experiments and numerical simulation. The experiments were conducted using small-scale single-phase experimental equipment, a gas/particle two-phase test facility and 200- and 300-MW_e wall-fired utility boilers. For the burner, the primary air and glass beads partially penetrate the central recirculation zone and are then deflected radially. At the center of the central recirculation zone, there is high particle volume flux and large particle size. For the burners the local mean CO concentrations, gas temperatures and temperature gradient are higher, and the mean concentrations of O₂ and NO_x in the jet flow direction in the burner region are lower. Moreover, the mean O₂ concentration is higher and the gas temperature and mean CO concentration are lower in the side wall region. Centrally fuel-rich burners have been successfully used in 200- and 300-MWe wall-fired pulverized coal utility boilers.     

Application of the Miller cycle to reduce NOx emissions from petrol engines

A conceptual analysis of the mechanism of the Miller cycle for reducing NO_x emissions is presented. Two versions of selected Miller cycle (1 and 2) were designed and realized on a Rover “K” series 16-valve twin-camshaft petrol engine. The test results showed that the application of the Miller cycle could reduce the NO_x emissions from the petrol engine. For Miller cycle 1, the least reduction rate of NO_x emission was 8% with an engine-power-loss of 1% at the engine’s full-load, compared with that of standard Otto cycle. For Miller cycle 2, the least reduction rate of NO_x emission was 46% with an engine-power-loss of 13% at the engine’s full-load, compared with that of standard Otto cycle.     

Investigation of NOx conversion characteristics in a porous medium

The conversion of nitric oxide (using CNG/air as fuel/oxidizer) inside a porous medium is investigated in this study. Unlike freely propagating flames, porous burners provide a solid medium that facilitates heat exchange with the gaseous phase. The heat exchange allows the stabilization of a variety of fuel mixtures from lean to rich and with a variety of calorific values. In addition, it allows the control of the reaction zone temperature and thus the control of pollutant formation while maintaining flame stability. An experimental porous burner was designed and manufactured for this purpose. The effects of equivalence ratio and flow velocity on the flame stabilization, NO_x and TFN (total fixed nitrogen) conversion ratios, and temperature profiles along the burner are investigated. In addition, numerical calculations using the PLUG flow simulator model and the GRI 3.0 kinetic mechanism reveals the key reactions which control the conversion efficiency. It was found that under slightly fuel-rich conditions (φ?1.3) NO_x mostly converts to N₂ with a maximum conversion ratio of 65%, while for higher equivalence ratios (φ>1.3) a large proportion of NO_x converts to NH₃. Results from experiments and numerical modeling showed that the temperature profile along the burner has significant effects on the NO_x and TFN conversion ratios. It was also found that temperatures between 1000 and 1500 K are most desirable for NO_x and TFN conversion in the porous burner. Analysis of the chemical paths for the low- and high-equivalence-ratio cases showed that the formation of nitrogen-containing species under very rich conditions (φ>1.3) is due to the increased importance of the HCNO path as compared to the HNO path. The latter is the dominant path at low equivalence ratios (φ?1.3) and leads to the formation of N₂. The NO concentration in the initial mixture was found to improve the conversion by up to 20% at low equivalence ratios (φ?1.3) and to have negligible effect at higher equivalence ratios.     

Combustion and NOx emission characteristics of a retrofitted down-fired 660 MWe utility boiler at different loads

Industrial experiments were performed for a retrofitted 660 MW_e full-scale down-fired boiler. Measurements of ignition of the primary air/fuel mixture flow, the gas temperature distribution of the furnace and the gas components in the furnace were conducted at loads of 660, 550 and 330 MW_e. With decreasing load, the gas temperature decreases and the ignition position of the primary coal/air flow becomes farther along the axis of the fuel-rich pipe in the burner region under the arches. The furnace temperature also decreases with decreasing load, as does the difference between the temperatures in the burning region and the lower position of the burnout region. With decreasing load, the exhaust gas temperature decreases from 129.8 °C to 114.3 °C, while NO_x emissions decrease from 2448 to 1610 mg/m³. All three loads result in low carbon content in fly ash and great boiler thermal efficiency higher than 92%. Compared with the case of 660 MW_e before retrofit, the exhaust gas temperature decreased from 136 to 129.8 °C, the carbon content in the fly ash decreased from 9.55% to 2.43% and the boiler efficiency increased from 84.54% to 93.66%.     

Measurement of gas species,temperatures, char burnout,and wall heat fluxes in a 200-MWe lignite-fired boiler at different loads

We measured various operational parameters of a 200-MW_e, wall-fired, lignite utility boiler under different loads. The parameters measured were gas temperature, gas species concentration, char burnout, component release rates (C, H and N), furnace temperature, heat flux, and boiler efficiency. Cold air experiments of a single burner were conducted in the laboratory. A double swirl flow pulverized-coal burner has two ring recirculation zones that start in the secondary air region of the burner. With increasing secondary air flow, the air flow axial velocity increases, the maximum values for the radial velocity, tangential velocity, and turbulence intensity all increase, and there are slight increases in the air flow swirl intensity and the recirculation zone size. With increasing load gas, the temperature and CO concentration in the central region of burner decrease, while O₂ concentration, NO_x concentration, char burnout, and component release rates of C, H, and N increase. Pulverized-coal ignites farther into the burner, in the secondary air region. Gas temperature, O₂ concentration, NO_x concentration, char burnout and component release rates of C, H, and N all increase. Furthermore, CO concentration varies slightly and pulverized-coal ignites closer. In the side wall region, gas temperature, O₂ concentration, and NO_x concentration all increase, but CO concentration varies only slightly. In the bottom row burner region the furnace temperature and heat flux increase appreciably, but the increase become more obvious in the middle and top row burner regions and in the burnout region. Compared with a 120-MW_e load, the mean NO_x emission at the air preheater exits for 190-MW_e load increases from 589.5 mg/m³ (O₂ = 6%) to 794.6 mg/m³ (O₂ = 6%), and the boiler efficiency increases from 90.73% to 92.45%.     

Combining GTL fuel,reformed EGR and HC-SCR aftertreatment system to reduce diesel NOx emissions. A statistical approach

An ultra-low sulphur diesel (ULSD) fuel and a synthetic gas-to-liquid (GTL) fuel, besides different types of standard and reformed EGR, were evaluated in a single-cylinder, direct injection, diesel engine equipped with hydrocarbon-selective catalytic reduction (HC-SCR) aftertreatment system. The results obtained were statistically analysed (at 95% statistical significance) to identify the most significant factors that affect NO_x emissions and to search for the optimum operation conditions in order to minimize these emissions. For that purpose, a fully crossed factorial experimental design was used, including two different engine speeds (1200 and 1500 rpm), two engine loads (25% and 50%), and four EGR/REGR ratios (0%, 10%, 20% and 30%) resulting in almost one hundred tests. An optimal combination of fuel type, REGR type and REGR ratio was proved to reduce around 89–95% of the reference NO_x emissions. In general, at 25% engine load GTL fuelling combined with the reformed EGR with the highest hydrogen content was found the most desirable, as the hydrogen sharply increased the NO_x conversion in the SCR catalyst. Differently, at 50% load standard EGR was sufficient to reach high NO_x reductions. These findings may be used for the implementation of a system on-board capable to switch from EGR to REGR, which will help engine manufacturers to meet the future emission regulations.     

Sensitizing effects of NOx on CH4 oxidation at high pressure

The CH₄/O₂/NO_x system is investigated in a laboratory-scale high pressure laminar flow reactor with the purpose of elucidating the sensitizing effects of NO_x on CH₄ oxidation at high pressures and medium temperatures. Experiments are conducted at 100, 50, and 20 bar, 600-900 K, and stoichiometric ratios ranging from highly reducing to oxidizing conditions. The experimental results are interpreted in terms of a detailed kinetic model drawn from previous work of the authors, including an updated reaction subset for the direct interactions of NO_x and C_1-2 hydrocarbon species relevant to the investigated conditions. The results reveal a significant decrease in the initiation temperature upon addition of NO_x. A similar effect is observed with increasing pressure. The sensitizing effect of NO_x is related to the hydrocarbon chain-propagating NO/NO₂ cycle operated by NO₂+CH₃?NO+CH₃O and NO+CH₃OO?NO₂+CH₃O as well as the formation of chain-initiating OH radicals from interactions between NO/NO₂ and the H/O radical pool. At low temperatures, reactions between NO/NO₂ and CH₃O/CH₂O also gain importance. The results indicate a considerable intermediate formation of nitromethane (CH₃NO₂) as a characteristic high-pressure phenomenon. The formation of CH₃NO₂ represents an inactivation of NO_x, which may result in a temporary reduction of the overall hydrocarbon conversion rate.     

A numerical study on NOx formation in laminar counterflow CH4/air triple flames

Formation of NO_x in counterflow methane/air triple flames at atmospheric pressure was investigated by numerical simulation. Detailed chemistry and complex thermal and transport properties were employed. Results indicate that in a triple flame, the appearance of the diffusion flame branch and the interaction between the diffusion flame branch and the premixed flame branches can significantly affect the formation of NO_x, compared to the corresponding premixed flames. A triple flame produces more NO and NO₂ than the corresponding premixed flames due to the appearance of the diffusion flame branch where NO is mainly produced by the thermal mechanism. The contribution of the N₂O intermediate route to the total NO production in a triple flame is much smaller than those of the thermal and prompt routes. The variation in the equivalence ratio of the lean or rich premixed mixture affects the amount of NO formation in a triple flame. The interaction between the diffusion and the premixed flame branches causes the NO and NO₂ formation in a triple flame to be higher than in the corresponding premixed flames, not only in the diffusion flame branch region but also in the premixed flame branch regions. However, this interaction reduces the N₂O formation in a triple flame to a certain extent. The interaction is caused by the heat transfer and the radical diffusion from the diffusion flame branch to the premixed flame branches. With the decrease in the distance between the diffusion flame branch and the premixed flame branches, the interaction is intensified.     

NOx emission and thermal efficiency of a 300 MWe utility boiler retrofitted by air staging

Full-scale experiments were performed on a 300 MWe utility boiler retrofitted with air staging. In order to improve boiler thermal efficiency and to reduce NO_x emission, the influencing factors including the overall excessive air ratio, the secondary air distribution pattern, the damper openings of CCOFA and SOFA, and pulverized coal fineness were investigated. Through comprehensive combustion adjustment, NO_x emission decreased 182 ppm (NO_x reduction efficiency was 44%), and boiler heat efficiency merely decreased 0.21%. After combustion improvement, high efficiency and low NO_x emission was achieved in the utility coal-fired boiler retrofitted with air staging, and the unburned carbon in ash can maintain at a desired level where the utilization of fly-ash as byproducts was not influenced.     

Studies on B sites in Fe-doped LaNiO3 perovskite for SCR of NOx with H2

A series of LaNi_1−xFe_xO₃ (x = 0.0, 0.2, 0.4, 0.7, and 1.0) perovskites were synthesized and characterized by X-ray diffraction (XRD), N₂ physisorption, scanning electron microscopy (SEM), H₂-temperature-programmed reduction (H₂-TPR), and X-ray photoelectron spectroscopy (XPS). The perovskites were investigated for selective catalytic reduction of NO_x by hydrogen (H₂-SCR). It is shown that Fe addition into LaNiO₃ leads to a promoted efficiency of NO_x removal, as well as a high stability of perovskite structure. Moreover, easy reduction of Ni³⁺ to Ni²⁺ with the aid of appropriate Fe component mainly accounts for the enhanced activity. Meanwhile, deactivation of the sulfated catalysts is due to that sulfates mainly deposit on active Ni component while doping of Fe can protect Ni to some extent at the expense of partial sulfation.     

PDA research on a novel pulverized coal combustion technology for a large utility boiler

In this paper, a new technology for a tangential firing pulverized coal boiler, high efficiency and low NO_x combustion technology with multiple air-staged and a large-angle counter flow fuel-rich jet (ACCT for short) is proposed. To verify the characteristics of this technology, experiments of two combustion technologies, ACCT and CFS-1 (Concentric Firing System-1), are carried out under a cold model of a 1025 t/h tangential firing boiler with a PDA (particle dynamics anemometer). The distributions of velocity, particle concentration, particle diameters and the particle volume flux of primary air and secondary air are obtained. The results show that the fuel-rich primary air of ACCT can go deeper into the furnace and mix with the main flow better, which means that the counter flow of fuel-rich jets in ACCT can realize stable combustion, low NO_x emission and slagging prevention.     

An experimental and numerical investigation of n-heptane/air counterflow partially premixed flames and emission of NOx and PAH species

An experimental and numerical investigation of counterflow prevaporized partially premixed n-heptane flames is reported. The major objective is to provide well-resolved experimental data regarding the detailed structure and emission characteristics of these flames, including profiles of C₁-C₆, and aromatic species (benzene and toluene) that play an important role in soot formation. n-Heptane is considered a surrogate for liquid hydrocarbon fuels used in many propulsion and power generation systems. A counterflow geometry is employed, since it provides a nearly one-dimensional flat flame that facilitates both detailed measurements and simulations using comprehensive chemistry and transport models. The measurements are compared with predictions using a detailed n-heptane oxidation mechanism that includes the chemistry of NO_x and PAH formation. The reaction mechanism was synergistically improved using pathway analysis and measured benzene profiles and then used to characterize the effects of partial premixing and strain rate on the flame structure and the production of NO_x and soot precursors. Measurements and predictions exhibit excellent agreement for temperature and major species profiles (N₂, O₂, n-C₇H₁₆, CO₂, CO, H₂), and reasonably good agreement for intermediate (CH₄, C₂H₄, C₂H₂, C₃H_x) and higher hydrocarbon species (C₄H₈, C₄H₆, C₄H₄, C₄H₂, C₅H₁₀, C₆H₁₂) and aromatic species (toluene and benzene). Both the measurements and predictions also indicate the existence of two partially premixed regimes; a double flame regime for ?<5.0, characterized by spatially separated rich premixed and nonpremixed reaction zones, and a merged flame regime for ?>5.0. The NO_x and soot precursor emissions exhibit strong dependence on partial premixing and strain rate in the first regime and relatively weak dependence in the second regime. At higher levels of partial premixing, NO_x emission is increased due to increased residence time and higher peak temperature. In contrast, the emissions of acetylene and PAH species are reduced by partial premixing because their peak locations move away from the stagnation plane, resulting in lower residence time, and the increased amount of oxygen in the system drives the reactions to the oxidation pathways. The effects of partial premixing and strain rate on the production of PAH species become progressively stronger as the number of aromatic rings increases.     

Effect of ignition timing retard strategy on NOx reduction in hydrogen-compressed natural gas blend engine with increased compression ratio

Hydrogen-compressed natural gas blend (HCNG) engines can extend the lean burn limit because of the wide flammability range of hydrogen. Lean combustion helps facilitate high efficiency and fundamentally reduces NO_x emission. Increasing the compression ratio (CR) of an HCNG engine was reported to improve its thermal efficiency. However, the high risk of knock occurrence and the increase in NO_x emission can hinder CR increase.     

Performances of a heat exchanger and pilot boiler for the development of a condensing gas boiler

In this research, design factors for a heat exchanger and boiler were investigated using a simplified model of a heat exchanger and pilot condensing boiler, respectively. Specifications of each heat exchanger component (e.g., upper heat exchanger (UHE) and lower heat exchanger (LHE); coil heat exchanger (CHE); baffles) were investigated using a model apparatus, and the comprehensive performance of the pilot gas boiler was examined experimentally. The heating efficiency of the boiler developed was about 90% when using the optimal designed heat exchangers. Compared to a conventional Bunsen-type boiler, the heating efficiency was improved about 10%. Additionally, NO_x and CO emissions were about 30 ppm and 160 ppm, respectively, based on a 0% O₂ basis at an equivalence ratio of 0.70, which is an appropriate operating condition. However, the pollutant emission of the boiler developed is satisfactory considering the emission performance of a condensing boiler, even though CO emission must be reduced.     

Influence of primary air ratio on flow and combustion characteristics and NOx emissions of a new swirl coal burner

Cold airflow experiments on a small-scale burner model, as well as in situ experiments on a centrally fuel-rich swirl coal combustion burner were conducted. Measurements were taken from within a 300 MW_e wall-fired pulverized-coal utility boiler installed with eight of centrally fuel-rich swirl coal combustion burners in the bottom row of the furnace during experiments. Various primary air ratios, flow characteristics, gas temperature and gas species concentrations in the burner region were measured. The results of these analyses show that with decreasing primary air ratio, the swirl intensity of air, divergence angles and maximum length and diameter of the central recirculation zone all increased, and the turbulence intensity of the jet flow peaked but decayed quickly. In the burner nozzle region, gas temperature, temperature gradient and CO concentration increased with decreasing primary air ratio, while O₂ and NO_x concentration decreased. Different primary air ratios, the gas temperatures and gas species concentrations in the side-wall region varied slightly.     

A facile method to synthesize well-dispersed PtRuMoOx and PtRuWOx nanoparticles and their electrocatalytic activities for methanol oxidation

PtRuMoO_x and PtRuWO_x catalysts supported on multi-wall carbon nanotubes (MWCNTs) are prepared by ultrasonic-assisted chemical reduction method. XRD measurements indicate that Pt exists as face-centered cubic structure, Ru is alloyed with platinum, and the metal oxides exist as an amorphous structure. TEM pictures show that PtRuMoO_x and PtRuWO_x catalysts are well dispersed on the surface of MWCNTs with the particle size of about 3 nm and a narrow particle size distribution. The electrochemical properties of the catalysts for methanol electrooxidation are studied by cyclic voltammetry (CV), chronoamperometry (CA) and chronopotentiometry (CP). The onset potentials for methanol oxidation on PtRuMoO_x and PtRuWO_x are more negative than that of pure Pt catalyst, shifting negatively by about 0.20 V and have better electrocatalytic activities than PtRu/MWCNTs.     

燃气锅炉低氮排放的经济性分析

简要阐述了燃气锅炉低氮排放技术,并对烟气再循环（FGR）技术进行了详细介绍。从经济性方面分析,选择合适的烟气再循环率可减小FGR对锅炉效率的影响,并可实现减排的目标;提出燃烧器和锅炉的研究方向是如何提高锅炉传热效率,并将FGR对锅炉热效率的影响降到最低,以便更好地实现节能减排的目标。     

130t·h~(-1)循环流化床锅炉低氮燃烧改造实践

为达到锅炉污染物排放标准的要求,并实现在役循环流化床锅炉可采用选择性非催化还原(SNCR)方式低成本脱除NOx的目标,对NOx的生成、降低NOx原始生成的炉内条件、脱硝工艺的选择等方面进行了阐释.从工程实践出发,对1台130 t·h-1高温高压循环流化床锅炉的相应结构进行了分析,提出了改造方案.该方案实施后达到了良好的改造效果.     

Determination of Cu(In1−xGax)3Se5 defect phase in MBE grown Cu(In1−xGax)Se2 thin film by Rietveld analysis

Quantitative phase analysis of Cu(In_1−xGa_x)Se₂ (CIGS) thin film grown over Mo coated soda lime glass substrates was studied by Rietveld refinement process using room temperature X-ray data at θ-2θ mode. Films were found to contain both stoichiometric Cu(In_1−xGa_x)Se₂ and defect related Cu(In_1−xGa_x)₃Se₅ phases. Best fitting was obtained using crystal structure with space group I-42d for Cu(In_1−xGa_x)Se₂ and I-42m for Cu(In_1−xGa_x)₃Se₅ phase. The effects of Ga/III (=Ga/In+Ga=x) ratio and Se flux during growth over the formation of Cu(In_1−xGa_x)₃Se₅ defect phase in CIGS was studied and the correlation between quantity of Cu(In_1−xGa_x)₃Se₅ phase and solar cell performance is discussed.