Improving Pulverized Coal and Biomass Co‐Combustion in a Cement Rotary Kiln by Computational Fluid Dynamics

The cement industry is one of the largest pollutant emitters. One way to cope with high pollutant emissions is to co‐combust biomass with pulverized coal. A mathematical model was developed, which is detailed enough to consider the complex physical and chemical behavior of the co‐combustion process but simple enough to perform simulations with a real geometry of cement rotary kiln within reasonable time. Numerical simulation with a 20‐% share of pulverized biomass of total fuel heating value was performed. An industrial rotary kiln geometry was simulated; temperature and velocity fields along with mass fractions of released volatiles and combustion products were analyzed. The model allows better insights in the co‐firing process with the main goal to reduce CO₂ emissions by optimizing the combustion process inside the rotary kiln.     

Development of fragmentation models for solid fuel combustion and gasification as subroutines for inclusion in CFD codes

This work arises from substantial problems found in the modelling of the gasification and combustion of solid fuel, both coal and biomass in the following different systems:

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Coal fired non-slagging cyclone combustors.

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Pre-calciners on cement plant.

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Fuel rich inverted cyclone combustors used to simulate the time temperature history of large utility boilers.

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Cyclonic gasifiers for sawdust and direct firing of small gas turbines.

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Deposition studies for slagging and fouling in large utility boilers.

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Prediction of final carbon in ash from pulverised coal systems.

Commercial CFD codes such as Fluent are well developed and have well proven routines for lagrangian tracking of burning particles through complex flow fields. However what has become apparent in numerous studies is that existing models for solid fuel combustion can be adjusted to predict the initial flow field aerodynamics, sometimes the temperature, but fall down when particles have to be followed completely through a system. This is manifested with cyclone combustors and gasifiers via enhanced retention of burning particles in centrifugal force fields, which can only be resolved by changes in the particle size distribution and thus fragmentation as the particle gasify or burn. This problem also becomes apparent in studies of processes in pre-calciners and in deposition in large utility boilers and furnaces.The paper will review the literature of the fragmentation of pulverised coal and biomass during gasification, devolatilisation and combustion and relate it to observed phenomena in the type of system under consideration. The difficulties of incorporating models of fragmentation in CFD codes such as Fluent are discussed. Then the implementation of such a model in Fluent is described, together with results from a number of different systems. It is concluded that the model so implemented shows improved prediction in many difficult areas, but still needs development to better reflect actual fragmentation conditions in different experimental systems.It is concluded that the model needs to be further extended beyond the single step fragmentation at present used, especially to include many of the fine particles known to be generated in such systems.     

Fluidized bed combustion of refuse-derived fuel in presence of protective coal ash

Combustion of refuse-derived fuel (RDF) alone or together with other biomass leads to superheater fouling and corrosion in efficient power plants (with high steam values) due to vaporization and condensation of alkali chlorides. In this study, means were found to raise the portion of RDF to 40% enb without risk to boilers. This was done by co-firing RDF with coal and optimizing coal quality. Free aluminum silicate in coal captured alkalies from vaporized alkali chlorides preventing Cl condensation to superheaters. Strong fouling and corrosion were simultaneously averted. Results from 100 kW and 4 MW CFB reactors are reported.     

Fluidized bed combustion of refuse-derived fuel in presence of protective coal ash

Combustion of refuse-derived fuel (RDF) alone or together with other biomass leads to superheater fouling and corrosion in efficient power plants (with high steam values) due to vaporization and condensation of alkali chlorides. In this study, means were found to raise the portion of RDF to 40% enb without risk to boilers. This was done by co-firing RDF with coal and optimizing coal quality. Free aluminum silicate in coal captured alkalies from vaporized alkali chlorides preventing Cl condensation to superheaters. Strong fouling and corrosion were simultaneously averted. Results from 100 kW and 4 MW CFB reactors are reported.     

Agglomeration of biomass fired fluidized bed gasifier and combustor

Agglomeration is a major problem in biomass fired fluidized bed combustors and gasifiers. Mechanism, reduction options and detection techniques of agglomeration are reviewed. Agglomeration may be classified broadly into three types: defluidization induced agglomeration, melt‐induced agglomeration and coating‐induced agglomeration. Sodium and potassium content of the biomass are the major contributors to the agglomeration in biomass fired fluidized beds. Higher temperature, lower fluidizing velocity and coarser bed particles also increase the risk of agglomeration. Alternative bed materials, additives or the co‐combustion of biomass with other fuels can reduce agglomeration potential of a fluidized bed. Two agglomeration detection techniques are discussed: controlled fluidized bed agglomeration and early agglomeration recognition system.     

The influence of biomass co-combustion on boiler fouling and efficiency

The paper presents an attempt to evaluate the influence of biomass co-combustion on the fouling of boiler convection surfaces. In order to show the influence of co-firing biomass with bituminous coal on boiler efficiency, the calculations of pulverized fuel (PF) OP 140 steam generator have been carried out. Typical Upper Silesian coal with medium fouling inclination has been chosen as a basic fuel. Three kinds of biomass have been taken into consideration: straw, wood and dried sewage sludge. The results confirm that the properties of additional fuels cause deterioration of the boiler efficiency as well as the changes in boilers operational parameters (amount of water injected in attemperators, ash stream, hot air temperature). The biomass during cofiring in fact replaces the coal, but always the additional fuel consumption is higher than that of the substituted coal. Therefore, the actual decrease of coal consumption is smaller than the thermal fraction of the biomass.     

Examination of the combustion conditions of herbaceous biomass

Power generation from biomass is a fairly new area, and boilers that utilize various types of biomass have in many cases experienced serious problems with slagging, fouling and corrosion of boiler tubes. Mineral matter in these fuels can deposit on the heat-exchanger surfaces in the boiler and generate an insulating layer, which will significantly reduce the degree of heat-transfer from flue gas to water and steam. Our investigations were focused on the slag characteristics of different kinds of herbaceous biomass fuels. Since there is usually a reducing atmosphere present in the direct combustion zone of modern low-NO_x firing systems, it is important to study mineral matter transformation of burned fuel residues in a reducing atmosphere. An excellent device for this type of study is the electric-resistance heated Bunte–Baum softening temperature testing instrument, which was used in this work. Ash chemical composition was analyzed via flame atomic absorption spectrometry and the microstructure of ash was determined using a scanning electron microscope. Crystalline compounds of the ashes were identified by using X-ray powder diffraction. This paper provides an overview of results on the combustion and slag characteristics of herbaceous biomass fuels. The results include chemical compositions, morphology and softening properties of these fuels, with special attention to switch grass and sunflower seed shell.     

Leaching of Sodium and Chlorine from Coals

Abstract

The presence of sodium and chlorine in coals often causes fouling and corrosion of boiler tubes and downstream equipment in power plants. These elements are also suspected of augmenting materials problems in fluidized-bed combustors and coal-burning gas turbines. Battelle Columbus Laboratories has developed a process to remove the sodium and chlorine from coal. Experiments carried out with Illinois No. 6 coal suggest that substantial reductions in sodium and chlorine levels can be achieved by extracting the coal with water in the presence of calcium oxide under appropriate conditions. The degree of removal is a function of temperature, extraction time, and coal particle size, and is independent of the water-to-coal ratio. The effects of these process variables on removal are discussed. The data are also interpreted to postulate the mode of sodium and chlorine occurrence in coal.     

Combustion and air emissions from co‐firing a wood biomass,a Canadian peat and a Canadian lignite coal in a bubbling fluidised bed combustor

The effects of particle size, fuel blending ratio, moisture content and excess air ratio on combustion efficiency and air emissions (CO₂, CO, SO₂ and NO_x) from the co‐combustion of white pine or peat with a Canadian lignite coal, were examined in a pilot‐scale bubbling fluidised bed combustor. Pelletising was important for the efficient combustion of wood due to its high volatile content. Co‐firing lignite and pine pellets gave a proportional reduction in SO₂ and NO_x emissions with blending ratio, while co‐firing of peat and lignite resulted in increased SO₂ emissions, but decreased NO_x emissions. Moisture promotes combustion but with increased CO emissions, and results in increased NO_x emissions, and decreased SO₂ emissions. High excess air decreased CO, but moderately increased SO₂ and NO_x emissions. © 2011 Canadian Society for Chemical Engineering     

Hydrogen Production from Co‐Gasification of Coal and Biomass in the Presence of CaO as a Sorbent

Among the options for clean energy production, the gasification process is receiving increasing attention as it offers the best combination of investment and value of produced electricity compared to other methods. An Aspen Plus model of co‐gasification of biomass and coal with in situ CO₂ capture was developed to evaluate its potential for hydrogen production and cracking of organic impurities, i.e., tars. The effects of some critical operational variables on gas composition and yields of hydrogen gas and tar were investigated. The obtained results indicate that the fuel particle size plays a minor role in the process; smaller particles favor the conversion of tar and production of more hydrogen gas.     

Nationwide energy supply chain analysis for hybrid feedstock processes with significant CO2 emissions reduction

Integrating diverse energy sources to produce cost‐competitive fuels requires efficient resource management. An optimization framework is proposed for a nationwide energy supply chain network using hybrid coal, biomass, and natural gas to liquids (CBGTL) facilities, which are individually optimized with simultaneous heat, power, and water integration using 162 distinct combinations of feedstock types, capacities, and carbon conversion levels. The model integrates the upstream and downstream operations of the facilities, incorporating the delivery of feedstocks, fuel products, electricity supply, water, and CO₂ sequestration, with their geographical distributions. Quantitative economic trade‐offs are established between supply chain configurations that (a) replace petroleum‐based fuels by 100%, 75%, and 50% and (b) utilize the current energy infrastructures. Results suggest that cost‐competitive fuels for the US transportation sector can be produced using domestically available coal, natural gas, and sustainably harvested biomass via an optimal network of CBGTL plants with significant GHG emissions reduction from petroleum‐based processes. © 2012 American Institute of Chemical Engineers AIChE J, 2012     

煤与生物质的共热解液化研究进展

煤与生物质共热解液化将是燃料与化学品重要的转化技术之一。本文从共热解液化机理、共热解液化反应动力学、煤与生物质的协同作用、催化剂、共热解液化工艺、共热解液化产物等方面对煤与生物质共热解液化研究进展进行了综述,指出煤与生物质的快速共热解液化将是重要的发展方向,催化剂的应用和液化产物的精制将对提升液化油的品位和降低成本,对实现共液化油替代现行石化液体油具有更重要的意义。     

Comparison of the high-temperature corrosion of aluminium nitride,alumina, magnesia and zirconia ceramics by coal ashes

Higher-grade reactor lining materials are needed to withstand the higher working temperatures and gas pressures used in gasification reactors to improve their efficiency. Both conventional oxide materials and nonoxide materials such as SiC and AlN are suitable refractory materials for the reducing atmospheres prevailing in gasifiers. Interactions between the reactor lining and the slag constitute the main corrosion mechanism. In previous investigations on the high-temperature corrosion of aluminium nitride by coal ash [1] AlN materials were tested in basic and acidic coal ashes at temperatures of 900–1300 °C. In the present work the high-temperature corrosion of aluminium nitride by different coal ashes is compared with that of alumina, magnesia and zirconia.     

A study on the char burnout characteristics of coal and biomass blends

The char burnout characteristics of coal/biomass blends under conditions pertinent to pulverised fuel combustors were investigated by a combined modelling and experimental approach. Results indicate that blending of coal with biomass increases the likelihood of char extinction (i.e. extinction potential of the char particle in the blend), in turn, decreasing the char burnout level. Our modelling results attribute this to a reduction in the char particle size to levels below a critical dimension which appears to be a strong function of the fuel blending ratio (the weight percentage of biomass in the blend), fuel reactivity, char cloud shape and particle density number. It is demonstrated here that the drop in the char burnout level during co-firing can be effectively resolved when a more reactive secondary coal is added to the blend to minimise its extinction potential.     

Validation and Application of a Kinetic Model for Downdraft Biomass Gasification Simulation

Biomass gasification is widely recognized as an effective method to obtain renewable energy. To accurately predict the syngas and tar compositions is a challenge. A chemical reaction kinetics model based on comprehensive gasification kinetics is proposed to simulate downdraft biomass gasification. The kinetic model is validated by direct comparison to experimental results of two downdraft gasifiers available in the literature and is found to be more accurate than the widely used Gibbs energy‐minimizing model (GEM model). The kinetic model is then applied to investigate the effects of equivalence ratio (ER), gasification temperature, biomass moisture content, and biomass composition on syngas and tar production. Accurate water‐gas shift and CO shift reaction kinetics are found critical to achieve good agreement with experimental results.     

Biochemical Conversion of Microalgae Biomass into Biofuel

Biofuel production via microalgae is a promising and sustainable alternative to replace the typical fossil fuel that is the main contributor to the global warming. However, for a cost‐effective biofuel production, further advanced research is still needed for large‐scale operation. This article is a tutorial review on conversion processes of microalgae into biofuel, with emphasis on biochemical conversion. The following topics are discussed: (i) microalgae biomass and its composition, (ii) thermochemical conversion, (iii) chemical conversion, and (iv) biochemical conversion. In addition, various aspects of anaerobic digestion, digester designs, and effects of operating conditions on the production of methane, bioethanol, and biohydrogen are discussed. The general kinetics of biomass conversion into biofuel is presented. This study suggests, if that biomass contains less than 50 % moisture, then it is recommended to use the direct combustion method; otherwise, biochemical conversion is the most suitable process to biofuel production.     

Rheology of co‐continuous phase in physical thermoplastic elastomers

Nowadays, chemical and physical thermoplastic elastomers have found many applications in different industries. Compatibilizer and co‐continuous composition are among the important factors which control the optimum properties of a number of physical blends. Morphology, mechanical and rheological properties of various blends of polystyrene (PS) and polybutadiene were investigated. The results show that the co‐continuous composition for these blends is located in‐between 24 and 35 wt% of PS. These blends show a negative deviation from the viscosity logarithmic additivity rule. This observation suggests an improvement in the interactions between the phases in co‐continuous composition. Rheological and mechanical properties were used in determination of the co‐continuous phase. Copyright © 2004 Society of Chemical Industry     

Influence of phase structure on impact toughening of isotactic polypropylene by metallocene‐catalyzed linear low‐density polyethylene

In the present study isotactic polypropylene (PP) and metallocene‐catalyzed linear low‐density polyethylene (mLLDPE) were blended together to obtain thermoplastic materials (compositions) with improved toughness. Structure–property relationships were determined for these compositions with the help of scanning electron microscopy (SEM). Special emphasis was made on tracing the morphological features that led to the optimum mechanical performance. A co‐continuous type of structure was found to have much superior toughness as compared to a dispersed‐matrix structural type, for blends comprised of the same components (PP and mLLDPE). The study showed the fascinating possibility of creating toughened PP blends by inducing a co‐continuous structure. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1011–1018, 2000     

Ash properties and environmental impact of various biomass and coal fuels and their blends

Aiming at investigating the influence of minerals in co-firing applications in existing and developing systems, as well as their environmental impact upon recycling to soils, we used a combination of techniques such as X-ray fluorescence spectroscopy, ultraviolet and visible spectroscopy, inductive coupled plasma spectroscopy, X-ray diffractometry, differential thermal analysis and fusibility analysis to characterize various biomass and coal ashes and their blends, with biomass proportions up to 20%. Slagging and fouling propensities were predicted.The results showed that biomass ashes were richer in calcium, silicon and alkali minerals and micronutrients such as Zn, Cu and Mn, in comparison to coal ashes. Some could be useful for soil amendment or the cement industry. Slagging/fouling problems should be expected in boilers operating above 1000 °C, especially those firing cotton residue, vine shoots and bituminous coal without pre-treatment. However, the environmental impact of either biomass or coal ashes upon their disposal is expected to be very low, as leaching tests have shown. For coal/biomass blends, the composition and the fusibility of the ashes varied between those of the individual components. Thus co-firing processes using the alternative fuels studied up to 20% would not entail significant limitations in the system operation or the management strategies of ashes.     

Steam assisted biomass gasification—an overview

Biomass gasification technologies have matured over the years. In the past decade, numerous types of gasifiers have been developed and tested. This article reviews the status of steam assisted biomass gasifiers. It first discusses the challenges and the key operating parameters for steam‐biomass gasification and their effects on the gasifier performance. The article then summarises the progress and status of commercial or demonstration scale steam assisted biomass gasifiers available in the world.