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.     

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.     

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.     

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.     

Design and application of a novel coal-fired drum boiler using saline water in heavy oil recovery

In this paper, the design and operation of a novel coal-fired circulating fluidized bed (CFB) drum boiler that can generate superheated steam using saline water were introduced. The natural circulation water dynamics with a drum was adopted instead of the traditional once-through steam generator (OTSG) design, so that superheated steam can be generated for the better performance of the steam assisted gravity drainage (SAGD) technology in heavy oil recovery. The optimized staged evaporation method was proposed to further decrease the salinity of water in the clean water section of the boiler. The evaporating pipes of the salted water section were rearranged in the back pass of the boiler, where the heat load is low, to further improve the heat transfer safety. A CFB combustion technology was used for coal firing to achieve a uniform heat transfer condition with low heat flux. Pollutant control technologies were adopted to reduce pollutant emissions. Based on the field test, the recommended water standard for the coal-fired CFB drum boilers was determined. With the present technology, the treated recovery wastewater can be reused in steam-injection boilers to generate superheated steam. The engineering applications show that the boiler efficiency is higher than 90%, the blowdown rate is limited within 5.5%, and the superheat of steam can reach up to 30 K. Besides, the heavy oil recovery efficiency is significantly improved. Moreover, the pollutant emissions of SO₂, NO_x and dust are controlled within the ranges of 20–90 mg/(N·m³), 30–90 mg/(N·m³) and 2–10 mg/(N·m³) respectively.     

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.     

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.     

Power boiler fuel augmentation with a biomass fired atmospheric circulating fluid-bed gasifier

The successful experience in developing the advanced Foster Wheeler atmospheric circulating fluidized-bed (ACFB) combustion system subsequently led to the development of the ACFB gasification (ACFBG) Technology in the early 1980s. The driving force for the development work was the dramatic increase in oil price during this period. The primary advantage of the ACFBG technology is that it enables the substitution of expensive fuels (e.g. oil or gas) with cheaper solid fuels. Prior to this Lahti project, Foster Wheeler had supplied four waste wood fired commercial scale ACFBGs in the mid-1980s to the pulp and paper industry with capacities from 17 to 35 MW_th based on fuel input. These units are still in successful operation today. Lahden Lämpövoima Oy (LLOy) is a Finnish power company producing power and district heat for the City of Lahti. The company is jointly owned by the city of Lahti and by Fortum Oy, the largest utility power company in Finland. LLOy operates the Kymijärvi Power Plant located nearby the city of Lahti in Southern Finland. To keep the energy prices as low as possible, Plant personnel are continuously looking for the most economical fuel sources, and simultaneously, trying to improve the environmental aspects of the energy production. Currently, about 300 GW h/a of different types of biofuels and refuse fuels are available in the Lahti area. On an annual basis, the available amount of biofuels and refuse fuels is enough to substitute for about 15% of the fuels burned in the main boiler, equaling up to 30% of the coal typically used. The aim of the LLOy Kymijärvi Power Plant gasification project is a commercial scale demonstration of direct gasification of wet biofuel and the use of hot, raw and very low calorific gas directly in the existing coal fired boiler. The gasification of biofuels and co-combustion of gases in the existing coal-fired boiler offers many advantages such as: recycling of CO₂, decreased SO₂ and NO_x emissions, efficient way to utilize biofuels and recycled refuse fuels, low investment and operation costs, and utilization of the existing power plant capacity. Furthermore, only small modifications were required in the boiler and process upsets in the gasifier do not shut down the power plant. The first two years of operation (1998 and 1999) have been excellent with good availability for the gasifier and its related components. During 1998 several measurements and tests with different fuel combinations were performed. This paper presents the project in detail as well as the most important results of the first measurements. The EU Thermie program has supported this project.     

低低温电除尘及高效电源协同烟气处理技术的应用

为研究低低温电除尘及高效电源协同烟气处理技术的应用效果,以一循环流化床锅炉为研究对象,通过试验方法,对协同烟气处理技术投运前后烟气中的粉尘颗粒特性及排放质量浓度进行了测量及对比,并对该技术投运后的经济性进行了分析。结果表明:协同烟气处理技术投运后,机组排放的粉尘质量浓度由49.5 mg/m³降低至10.7 mg/m³,可显著提高除尘器的除尘效率;可降低机组供电标煤耗2.835 g/(kW·h),年节煤量1473.5 t;可进一步减少CO₂,SO₂,NO_x及粉尘等污染物的排放;可节约用电160 kW·h/h,每年节约电量6.16×10⁵ kW·h。     

燃煤锅炉烟气排放污染及两段清洁燃烧燃煤锅炉

文章分析了燃煤锅炉烟气排放污染现状及原因,着重介绍了两段清洁燃烧燃煤锅炉的结构及燃烧特点,为减少排放CO2等温室效应气体,降低烟尘、NOxSO2排放及烟气的黑,度提供了一条新的途径。     

Co-firing of biomass in coal-fired utility boilers

Co-firing tests with sawdust and coal have been carried out at FORTUM's Naantali-3 CHP power plant (315 MW_fuel). The Naantali-3 plant is a tangentially-fired pulverised-coal unit with a Sulzer once-through boiler that produces 79 MW electricity, 124 MW district heat and 70 MW steam. Naantali-3 is equipped with roller coal mills (Loesche), modern low-NO_x-burners (IVO RI-JET), over-fire air (OFA), electrostatic precipitator (ESP) and flue-gas desulphurization plant (FGD). Coal and sawdust were blended in the coal yard, and the mixture fed into the boiler through coal mills. Tests were carried out for three months during the April 1999 to April 2000 period with pine sawdust (50-65% moisture as received). During the tests, sawdust proportions of 2.5–8% (from the fuel input) were examined. The co-firing tests were successful in many ways, but the behaviour of the coal mills caused some problems, and therefore the simultaneous feed will not be the solution in a long-term use. Fortum has developed a new concept for co-firing coal and biofuels in large pulverised-coal fired boilers. The experiences gained from the Naantali co-firing tests and a good knowledge of low-NO_x burning and combustion behaviour of different fuels at Fortum, was used when the new co-firing concept was developed. This concept consists of a separate biofuel grinding system and bio- or bio-coal-burners. By using this system, it is possible to utilize many kinds of biofuels in PC-boilers as well as increase the share of biofuels, compared to the simultaneous feed of biofuel and coal.     

污泥掺烧方式对燃煤锅炉燃烧影响的模拟研究

为了确保燃煤锅炉掺烧污泥后炉内燃烧安全稳定并控制NO_x的生成,以国内某典型1 000 MW超超临界燃煤锅炉为研究对象,利用CFD软件计算研究了不同的污泥掺烧方式对锅炉温度场和NO_x生成的影响。结果表明：在燃煤锅炉不同层的燃烧器掺烧污泥,掺烧污泥的燃烧器对应高度均出现了温度的下降和NO_x排放浓度的降低;随着污泥分别由下往上在B,D,F层燃烧器进行掺烧,在炉膛出口处烟温升高,NO_x排放浓度降低;在保持F层燃烧器总热值不变的情况下进行掺烧时,能保证锅炉整体温度水平,掺烧污泥比例越高,炉膛出口烟温越低,NO_x生成量越少;在F层燃烧器掺烧污泥燃烧效果较好,有利于NO_x减排,是最适合污泥掺烧的燃烧器层。     

Influence of division cone angles between the fuel-rich and the fuel-lean ducts on gas–particle flow and combustion near swirl burners

Radial bias combustion pulverized coal swirl burners are used in boilers when burning low-grade coals, which are often low-volatility anthracite and lean coals that do not burn stably. In a gas–particle test facility, a three-dimensional particle-dynamics anemometer was used to measure gas–particle flows in the near-burner region. Division cones between the fuel-rich and the fuel-lean ducts of the burner were set at angles of 43.2° and 0°. Velocity and particle volume flux profiles were obtained and the influence of the cones on combustion was analyzed. Burners with a division cone angle of 43.2° were used in a lean-coal fired boiler (volatility 13.2% as received, ash 27.46% as received), rated at 670 t/h. Pulverized coal was conveyed by cold moisture-laden exhaust air from the pulverizer. In situ aerodynamic field measurements in cold state were made and combustion tests were carried out. Low NO_x emission and high thermal efficiency resulted and the turn-down ratio was 50%.     

用活动遮热板装置改善层燃炉的煤种负荷适应性

分析了国内现有燃煤层燃炉燃烧过程中存在的主要问题，在此基础上，提出了用活动遮热板装置来改善层燃炉的煤种，负荷适应性的思想；简单介绍了活动遮热板的物理模型。工作原理及其主要结构组成；用试验与传热计算的方法论证了活动遮热板用于层燃炉中的可行性及其对层燃炉内温度场，燃烧效率的影响，试验与传热计算表明。活动遮热板应用于层燃炉是完全可行的，并且可大大改善其煤种和负荷的适应性。     

燃煤工业锅炉的发展与解耦燃烧技术的开发

比较了我国现有典型燃煤工业锅炉,包括链条炉排锅炉、循环流化床锅炉和煤粉燃烧锅炉的技术现状,分析了这三种锅炉的燃烧特性及其存在的优势和劣势。介绍了一种新型的煤炭解耦燃烧工业锅炉的技术原理和研究开发现状,从燃烧效率、烟气排放、设备投资、运行维护和负荷及燃料的适应性等几方面综合考虑,解耦燃烧工业锅炉具有良好的发展和应用前景。     

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.     

NO control in large-scale power plant boilers through superfine pulverized coal technology

Superfine pulverized coal technology can effectively reduce NO_x emission in coal-fired power plant boilers. It can also economize the cost of the power plant and improve the use of the ash in the flue gas. Superfine pulverized coal technology, which will be widely used in China, includes common superfine pulverized coal technology and superfine pulverized coal reburning technology. The use of superfine pulverized coal instead of common coal in large-scale power plants will not only reduce more than 30% of NO_x emission but also improve the thermal efficiency of the boiler.     

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。     

Thermodynamic and economic analyses of a coal and biomass indirect coupling power generation system

The coal and biomass coupling power generation technology is considered as a promising technology for energy conservation and emission reduction. In this paper, a novel coal and biomass indirect coupling system is proposed based on the technology of biomass gasification and co-combustion of coal and gasification gas. For the sake of comparison, a coal and biomass direct coupling system is also introduced based on the technology of co-combustion of coal and biomass. The process of the direct and the indirect coupling system is simulated. The thermodynamic and economic performances of two systems are analyzed and compared. The simulation indicates that the thermodynamic performance of the indirect coupling system is slightly worse, but the economic performance is better than that of the direct coupling system. When the blending ratio of biomass is 20%, the energy and exergy efficiencies of the indirect coupling system are 42.70% and 41.14%, the internal rate of return (IRR) and discounted payback period (DPP) of the system are 25.68% and 8.56 years. The price fluctuation of fuels and products has a great influence on the economic performance of the indirect coupling system. The environmental impact analysis indicates that the indirect coupling system can inhibit the propagation of NO_x and reduce the environmental cost.     

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.