Ash deposition behavior of cynara-coal blends in a PF pilot furnace

Biomass is nowadays considered as a very interesting option to substitute conventional fossil fuels. Although biomass could be burnt alone, it can also be co-fired together with coal in existing power plants, at a lower cost. One of the main problems related with biomass used in thermal applications is its propensity to form ash deposits. Slagging and fouling caused by ash may derive in heat transfer losses, corrosion in the tubes or even boiler shutdown. A deposition probe has been designed and proved to study this phenomenon. Several combustion tests have been performed in a 500 kW_th PF pilot test rig burning cynara blended with two coals at different shares in energy basis. Different analyses have been performed to those ash samples collected during the tests. From the results, it is observed that the quantity of collected ash in the deposition probe did not increase noticeably when increasing the biomass share up to 15% in energy basis. However, the opposite was detected in Spanish coal tests, due to its higher ash content. Major components of ash samples were aluminosilicates coming from coal clays. These components may act as protective ash coal compounds, but inorganic elements such as calcium or potassium also appeared and their presence increased with the biomass share. Although chlorine content in cynara was high, no important presence of this element was encountered in none of the ash samples collected. Experimental results agree with other experimental studies showing that aluminosilicates from coals may act as protective ash compounds, preventing chlorine deposition on heat transfer surfaces. The beneficial effect is also detected at pulverized fuel conditions.     

Mechanistic prediction of ash deposition in a pilot-scale test facility

A mechanistic approach has been used for assessing the ash deposition tendency and has been used for predicting the ash deposition behavior of Australian bituminous coals in a pilot-scale test furnace. The detailed analysis of mineral matter in coal determined by QemSCAN analytical technique was incorporated into an ash formation model to estimate the character of ash particles. The detailed chemical composition and particle size distribution data of ash particles from the ash formation model were incorporated into a computational fluid dynamics (CFD) code to get the arrival rate and retention ability of each ash particle onto the heat transfer surfaces.The CFD code along with the ash character from the ash formation model was able to simulate trends in ash deposition along the furnace length similar to those in the test furnace. The approach was also able to distinguish coals with high ash deposition potential from low ash deposition problems and the results agreed with those in test furnace.     

Glow discharge excited low temperature ashing: A new technique for separating mineral matter of coals

Low temperature ashing techniques are widely used to determine mineral matter content and in the analysis of the composition of the inorganic matter in coal and coal products. This Paper presents a new technique which makes it possible to ash coals in 4 (low-rank coals) to ≈8 (high-rank coals) h per gramme. The problem of the formation of LTA artefacts is described and differences between authigenic and artefactic minerals are indicated.     

Characterization and catalytic activity of coal mineral matter: 1. Effect of hydrogen pretreatment and exposure to sulphur and nitrogen compounds

Low temperature ash (LTA) samples prepared from nine US coals were characterized by X-ray diffraction. X-ray fluorescence, and surface area analyses. The results showed that illite, kaolinite, quartz and pyrite are major components in LTAs and that $\text{S}\text{Fe}$ ratios of some LTAs decreased significantly after H₂ treatment implying the occurrence of a partial reduction of pyrite during this treatment. Surface areas of LTAs increased drastically on H₂ treatment but decreased after exposure to sulphur and nitrogen compounds in activity testing. Correlations for the surface areas of LTAs before and after H₂ treatment were found in terms of clay content and element concentrations.     

Catalytic activity of mineral matter from western Kentucky coals for hydro-desulphurization and hydrodenitrogenation

A pulse reactor was used to study the catalytic activity and selectivity of mineral matter from Kentucky No.9 and No.11 seam coals and a low calorific value gasifier ash in the hydrodesulphurization (HDS) of thiophene and hydrodenitrogenation (HDN) of pyrrole, pyrrolidene and n-butylamine. Mineral matter in its least altered state was obtained by low temperature ashing. Thiophene conversion correlated well with the iron and zirconium contents (R > 0.95) of the catalysts, and at 450 °C all catalysts including a commercial Fe₂O₃/AI₂O₃ gave similar distributions of the butenes and <5% butane. Reactivities of N-compounds catalysed by mineral matter from Kentucky Nos.9 and 11 coals were in the order: n-butylamine > pyrrolidine > pyrrole ≈0. Nickel was found to be the important element n-butylamine conversion and 1-butene was the predominant product species for all the catalysts. Mineral acid treatment of the mineral matter from No.11 coal decreased its HDN activity but increased its HDS activity. Results of this study indicate that hydrogen consumption for removal of sulphur and nitrogen can be reduced by using dehydrogenation-type catalysts.     

Hydropyrolysis of a high-sulphur-high-calcite Italian Sulcis coal. 2. Importance of the mineral matter on the sulphur behaviour

The sulphur distribution among the char, oil and gas obtained after hydropyrolysis of a high-sulphur (4.3 wt%) and high-calcite (7.3 wt%) coal has been investigated. The chars were examined by scanning electron microscopy coupled with an energy dispersion analyser and by X-ray diffraction. The proportion of the combustible and non-combustible sulphur in the char has been determined. Hydrogen pressure promotes reaction with sulphur but the sulphur content of the chars increases from 3 to 4.5 wt% when the temperature is increased from 616 to 845 °C. This increase in sulphur is a consequence of the reaction between hydrogen sulphide, produced during hydropyrolysis of coal, with the alkaline-earth mineral matter to produce alkaline-earth sulphide. The SEM and X-ray diffraction images show that in the char formed at 780 °C the sulphur, calcium and magnesium are localized in the same compounds. This is not the case when hydropyrolysis is performed at lower temperature. Combustion of the chars produces only <0.6 S0₂ MJ^?1 compared to 2.2 g S0₂ MJ^?1 for untreated coal. X-ray diffraction has shown that the sulphur in the char is oxidized and fixed in the ashes mainly as CaS0₄ and also some as MgS0₄. Although sulphur remains partly in the chars after hydropyrolysis, 75% of it is non-combustible. The hydropyrolysis of a high-sulphur coal containing calcite, produces a char which may be used as a clean fuel.     

Behaviour of coal mineral matter in sintering and slagging of ash during the gasification process

The mineral matter in typical feed coals used in South African gasification processes and the ash derived from gasifying such coals have been investigated using a variety of mineralogical, chemical and electron microscope techniques. The mineral matter in the feed coals consists mainly of kaolinite, with minor proportions of quartz, illite, dolomite, calcite and pyrite plus traces of rutile and phosphate minerals. The calcite and dolomite occur in veins within the vitrinite macerals, and are concentrated in the floats fraction after density separation. Some Ca and Ti also appear to be present as inorganic elements associated with the organic matter.Electron microscope studies show that the gasification ash is typically made up of partly altered fragments of non-coal rock, bonded together by a slag-like material containing anorthite and mullite crystals and iron oxide particles, with interstitial vesicular glass of calcic to iron-rich composition. Ash formation and characteristics thus appear to be controlled by reactions at the particle scale, allowing the different types of particles within the feed coal to interact with each other in a manner controlled mainly by the modes of mineral occurrence. Integration of such techniques provides an improved basis for evaluating ash-forming processes, based on quantitative phase identification, bulk and particle chemistry, and the geometric forms in which the different phases occur.     

The roles of lime and iron oxide on the formation of ash and deposits in PF combustion

This paper presents the results of a study to assess the slagging propensities of a suite of UK, Spanish and South African coals, ranging from lignites to anthracites. Laboratory deposits were collected on ceramic deposition probes at gas temperatures of ∼1250°C, using an entrained flow reactor that simulates the time-temperature conditions experienced by pulverised coal particles in a large utility boiler. The degree of sintering and consolidation of the deposits would not have been predicted from bulk ash chemistry, indicating the importance of mineral matter distributions in the pulverised coal. Deposits with similar base to acid ratios and Fe₂O₃ contents displayed a range of slagging propensities on CCSEM analysis, consistent with the visual ranking. CCSEM analysis of the fly ashes collected from the combustion gases revealed a similar chemical composition to the coal ash and ash collected at the base of the EFR. CaO was observed to have readily assimilated into the aluminosilicate fly ash particles. On deposition, the CaO distribution largely remained unchanged. Fe₂O₃ was redistributed on forming a deposit possibly aided by CaO already dissolved in the aluminosilicates. The study provides an insight into the observations made by boiler operators burning coals with high CaO and Fe₂O₃ ashes.     

The relationship between excluded mineral matter and the abrasion index of a coal

Predictions of the wear rates of components in grinding mills at pulverised coal-fired power stations are currently made using empirical relationships based on the ash content of the coals. However, modern coal characterisation techniques now allow the mineral inclusions in a coal that are responsible for the abrasive nature of the coal to be accurately characterised. Hence, there is scope to make improved predictions of wear based on a detailed knowledge of the mineral matter in a particular coal. It is first necessary, however, to understand the nature of the minerals and properties of the minerals in a coal that would contribute to abrasive wear. In this study known quantities of quartz, pyrite and slate have been added to a washed coal and the Abrasion Indices of the coal/mineral mixtures have been measured. The results show how the size, shape and hardness of excluded mineral matter contribute to the abrasive properties of a coal.     

Determination of the ash yield and the mineral matter content of brown coals by density measurements

A method of measuring the density of bed-moist brown coals based on their plastic properties is described and the feasibility of using the method for determination of the ash yield of the coal is evaluated.     

Mineral matter effects on the rapid pyrolysis and hydropyrolysis of a bituminous coal. 1. Effects on yields of char,tar and light gaseous volatiles

The effects of minerals on product compositions from rapid pyrolysis of a Pittsburgh Seam bituminous coal were investigated. Whole, demineralized, and mineral treated samples of pulverized coal were heated in 100 KPa helium or 6.9 MPa hydrogen at 1000 K s^?1 to temperatures of up to 1300 K. Yields of char, tar and individual gaseous products were determined as a function of time-temperature conditions. Clays, iron-sulphur minerals, and quartz had few effects on pyrolysis in helium. Calcium minerals decreased yields of hydrocarbon products and increased yields of CO in helium pyrolysis. Calcite and clays decreased yields of CH₄ from hydropyrolysis, whereas iron-sulphur minerals had little effect on pyrolysis at 6.9 MPa H₂. Whole coal results were similar to demineralized coal results under all conditions.     

Mineral matter effects on the rapid pyrolysis and hydropyrolysis of a bituminous coal: 2. Effects of yields of C3-C8 hydrocarbons

Effects of minerals on yields of C₃-C₈ volatiles from rapid pyrolysis of a Pittsburgh Seam bituminous coal were investigated. Whole, demineralized, and mineral-treated samples of pulverized coal were heated in 101 kPa He or 6.99 MPaH₂ at 1000 K s⁻¹ to temperatures of up to 1300 K. Yields of C₃, C₄–C₆, and C₆–C₈ hydrocarbon gases were determined as a function of time-temperature history. Calcium minerals decrease yields of all three fractions in pyrolysis under He atmosphere but have little effect on hydropyrolysis. Kaolinite reduces yields in pyrolysis, but increases them in hydropyrolysis. Other minerals, notably FeSO₄, have varying effects on product yields depending on run conditions.     

A new approach for the combined chemical and mineral classification of the inorganic matter in coal. 1. Chemical and mineral classification systems

This work introduces and evaluates a new approach for the combined chemical and mineral classification of the inorganic matter in coal. Thirty-seven coal samples from Australia, Bulgaria, USA, Japan, Canada, South Africa, China, Spain, and Ukraine, which differ considerably in their geology, rank, age, ash yield, chemistry and mineralogy, were used to establish the classifications. The chemical classification system was organized according to the contents and significant positive or negative correlations of ash-forming elements in coal ashes using three composition-based criteria, namely: (1) sum of Si, Al, K, and Ti oxides; (2) sum of Ca, Mg, S, and Na oxides; and (3) Fe oxide. This approach resulted in four chemical coal ash types (sialic, calsialic, ferrisialic, and ferricalsialic) further divided into seven subtypes (with high, medium and low acid tendencies) based on the sum of Si, Al, K, and Ti oxides. The more important mineral classification system was organized according to the contents, genesis, and behaviour of mineral classes and species in coals also using three composition-based criteria, namely: (1) silicates + oxyhydroxides; (2) carbonates; and (3) sulphides + sulphates + phosphates. This approach resulted in four mineral coal types (silicate, silicate-carbonate, silicate-sulphide, and silicate-sulphide-carbonate or mixed) further divided into seven subtypes (with high, medium and low detrital tendencies) based on the sum of silicates and oxyhydroxides. The chemical and mineral coal types and subtypes are characterized and relationships and distinctions between them also are described herein. The benefit of this new classification approach is the use of significant correlations and actual element associations, and well-defined and genetically described mineral classes and species in coal. Potential applications of the classification schemes are described in part 2 of the present work.     

富氧燃烧颗粒物生成和灰沉积特性研究进展

近年来,世界经济不断发展,人口不断增加,能源消耗随之持续增加,其中化石能源的使用使得全球二氧化碳排放总量居高不下。为了缓解全球碳排放上升趋势,中国承诺在2030年二氧化碳排放达峰并在2050年实现净零排放。为了实现这一目标,需要发展化石能源的清洁高效利用,其中富氧燃烧技术是最有前景的燃煤电厂碳减排技术之一,系统综述了富氧燃烧中的颗粒物生成和灰沉积现象,介绍了氧气浓度、压力及燃料特性对颗粒物生成和灰沉积特性的影响机理。富氧燃烧中生成的颗粒物主要可分为亚微米和微米颗粒,其中亚微米颗粒主要由灰分经气化、冷凝和凝聚过程形成,微米颗粒则主要伴随焦炭破碎过程形成。氧气浓度升高导致火焰温度升高,一方面更多的灰分在高温下气化,促进了亚微米颗粒的生成,另一方面高温加剧了碱金属蒸气和硅铝酸盐粗颗粒的交互反应,导致亚微米颗粒中碱金属含量降低。而压力的上升可能会抑制亚微米颗粒的生成。但氧气浓度和压力对微米颗粒的产率和成分影响均不显著。积灰主要通过惯性碰撞、热泳力、冷凝和化学反应形成。富氧燃烧中氧气浓度上升会促进积灰的生成,一方面因为烟气温度升高,颗粒物黏度降低而在碰撞到壁面后更易黏结形成积灰;另一方面因为亚微米颗粒增多,更多细颗粒可以通过热泳力迁移到换热器表面形成积灰。虽然氧气浓度对外层积灰的成分没有明显影响,但紧贴壁面的内层积灰由于含有较多亚微米颗粒,其成分变化与亚微米颗粒趋势相似。压力升高时,积灰中的硫含量增高而氯含量降低,但对积灰生成速率的影响暂不明确。     

Ash chemistry and mineralogy of an Indonesian coal during combustion: Part 1 Drop-tube observations

The paper reports a systematic and comprehensive laboratory investigation into the ash chemistry and mineralogical changes undergone by a low-rank Indonesian coal during combustion. Combustion experiments conducted in a drop-tube furnace included ash formation experiments (using cyclone and filter arrangement) under closely controlled conditions in the range of 1200–1400 °C and deposition experiments at a probe temperature of 750 °C. Tests conducted with raw coal, coal/additive mixtures and washed coal indicated significant changes in ash characteristics. Of the ash formation and deposit samples examined, the raw coal + bauxite showed the lowest glass content and high contents of corundum indicating low ash deposition propensities. When compared to the ash formation samples, the deposit samples showed even significantly lower glass contents and were enriched in quartz. With the exception of the raw coal + bauxite sample, all are characterized by high silica and iron and moderate aluminium contents. In contrast, the raw coal + bauxite sample have low silica and much higher alumina contents which is in agreement with XRD observations. QEMSCAN™ results showed that the ash particles are sparsely distributed suggesting lack of a deposit initiation layer. Experimental observations suggest that the use of raw coal with bauxite would appear to offer the best performance with respect to handling ash-related issues. Present findings are of practical significance to power utilities employing Indonesian coal as there is no comprehensive work reported in the literature on ash chemistry and mineralogy of such coals.     

A new approach for the combined chemical and mineral classification of the inorganic matter in coal. 2. Potential applications of the classification systems

Part 1 of the present work introduced and evaluated a new approach for the combined chemical and mineral classification of the inorganic matter in coal. The benefit of these classification systems is the use of significant correlations and actual element associations, and well-defined and genetically described mineral classes and species in coal. Potential applications of the chemically and mineralogically categorized coal types and subtypes are discussed in the present part 2. The data show that various technological problems, environmental risks and health concerns of coal use are related directly or indirectly to specific mineral and chemical coal types and subtypes. Furthermore, a concept of “self-cleaning fuels” also is introduced and developed herein based on mineral coal types. The application of these chemical and mineral classification systems and concept is proposed to both the scientific and industrial community.     

Social, economic and policy dimensions of soil organic matter management in sub-Sahara Africa: challenges and opportunities

In recent years and in some situations the status of soil organic matter (SOM) has deteriorated considerably due to long periods of continuous cultivation and limited external inputs in the form of mineral fertilizers. Deterioration of SOM varies by agro-ecological zones, by soil types and by cropping patterns. It is more intense in East Africa, followed by coastal West Africa and Southern Africa and least intensive in the Sahel and Central Africa. It is also more serious in areas under low-input agriculture irrespective of the prevailing cropping system. The major consequence of the decrease in SOM in the tropics is lower agricultural productivity with a direct negative effect on food security. While biophysical dynamics of SOM have been extensively covered in the literature, social considerations have not received similar attention. This paper examines the social, economic and policy factors associated with the management of tropical soil organic matter. Empirical data from a range of environments in Africa show that SOM improvement options yield a positive return to land as well as labour. However, there are a number of constraints. Social constraints are related to the large quantities of organic matter that are required (case of farmyard manure), the competitive uses for the material (case of crop residues), land and labour requirements, and gender-related issues. From a policy stand point, unsecured tenure rights together with price distortions and other market failures may be important constraints. Challenges for sustainable management of SOM are identified. These include management conflicts, land tenure arrangements, the divergence in goals between individuals and society, land and labour requirements, inadequate support systems for land users, profitability issues, the role of subsidies, and the absence of national action plans. A number of opportunities are identified that could enhance the improvement or maintenance of SOM. These include: exploring the need and potential role of community-based SOM management practices; development of an integrated plant nutrient management strategy involving both organic and inorganic inputs; and development of concrete national action plans. It is argued that because externalities of SOM improvement or maintenance extend beyond the farmer's fields, SOM investment may require cost sharing between individuals and the society. Policies on subsidies need to be reconsidered. Research priorities are identified that require closer collaboration between scientists from a variety of disciplines. This revised version was published online in June 2006 with corrections to the Cover Date.     

Impact of planted fallows and a crop rotation on nitrogen mineralization and phosphorus and organic matter fractions on a Colombian volcanic-ash soil

Soil fertility replenishment is a critical factor that many farmers in the tropical American hillsides have to cope with to increase food crop production. The effect of three planted fallow systems (Calliandra houstoniana-CAL, Indigofera zollingeriana-IND, Tithonia diversifolia-TTH) and a crop rotation (maize/beans-ROT) on soil nitrogen mineralization, organic matter and phosphorus fractions was compared to the usual practice of allowing natural regeneration of native vegetation or natural fallow management (NAT). Studies were conducted on severely degraded Colombian volcanic-ash soils, 28 months after fallow establishment, at two on-farm experimental sites (BM1 and BM2) in the Cauca Department. Tithonia diversifolia had a significantly higher contribution to exchangeable Ca, K and Mg as well as B and Zn; the order of soil nutrient contribution was TTH > CAL > IND > NAT > ROT. On the other hand, lND had significantly higher soil NO₃⁻–N at both experimental farms as compared to all the other fallow system treatments. For the readily available P fraction, CAL and ROT had significantly higher H₂O–Po and resin-Pi, respectively, in the 0–5 cm soil layer; whereas TTH showed significantly higher values for both H₂O–Po and resin-Pi in the 5–10 cm soil layer. Significant effects were observed on the weights of the soil organic matter fractions which decreased in the order LL (Ludox light) > LM (Ludox intermediate) > LH (Ludox heavy). Indigofera zollingeriana showed greater C, N and P in the soil organic matter fractions than all the other fallow treatments, with NAT having the lowest values. It is concluded that planted fallows can restore soil fertility more rapidly than natural fallows.     

Magnetic effects in particle adhesion. Part II. The influence of particle composition and size on deposition in a magnetic field

The effects of a magnetic field on the deposition of particles of various compositions, sizes, shapes (spherical and rod-like) on steel beads of different kinds and sizes in an aqueous environment are described. In the systems studied, the particles and the collector bear a negative charge. If both interacting bodies have a sufficiently high magnetic moment, the magnetic force causes an enhancement in the particle attachment. The process is very sensitive to the size of the depositing solids; larger particles adhere much faster. Interpretation of the results is based on the shape of the total interaction energy function consisting of electrostatic, dispersion, and magnetic contributions. The major influence of the magnetic field is in the formation of a deep secondary minimum in which the particles, moving toward the surface, are accumulated. The magnetic force enhances the flux of these particles and deepens the minimum, causing an increase in the retention efficiency.     

Synthesis of zinc oxide nanostructures on graphene/glass substrate by electrochemical deposition: effects of current density and temperature

The electrochemical growth of zinc oxide (ZnO) nanostructures on graphene on glass using zinc nitrate hexahydrate was studied. The effects of current densities and temperatures on the morphological, structural, and optical properties of the ZnO structures were studied. Vertically aligned nanorods were obtained at a low temperature of 75°C, and the diameters increased with current density. Growth temperature seems to have a strong effect in generating well-defined hexagonal-shape nanorods with a smooth top edge surface. A film-like structure was observed for high current densities above -1.0 mA/cm² and temperatures above 80°C due to the coalescence between the neighboring nanorods with large diameter. The nanorods grown at a temperature of 75°C with a low current density of -0.1 mA/cm² exhibited the highest density of 1.45 × 10⁹ cm^-2. X-ray diffraction measurements revealed that the grown ZnO crystallites were highly oriented along the c-axis. The intensity ratio of the ultraviolet (UV) region emission to the visible region emission, I_UV/I_VIS, showed a decrement with the current densities for all grown samples. The samples grown at the current density below -0.5 mA/cm² showed high I_UV/I_VIS values closer to or higher than 1.0, suggesting their fewer structural defects. For all the ZnO/graphene structures, the high transmittance up to 65% was obtained at the light wavelength of 550 nm. Structural and optical properties of the grown ZnO structures seem to be effectively controlled by the current density rather than the growth temperature. ZnO nanorod/graphene hybrid structure on glass is expected to be a promising structure for solar cell which is a conceivable candidate to address the global need for an inexpensive alternative energy source.