Sorbents for CO2 capture from high carbon fly ashes

Fly ashes with high-unburned-carbon content, referred to as fly ash carbons, are an increasing problem for the utility industry, since they cannot be marketed as a cement extender and, therefore, have to be disposed. Previous work has explored the potential development of amine-enriched fly ash carbons for CO(2) capture. However, their performance was lower than that of commercially available sorbents, probably because the samples investigated were not activated prior to impregnation and, therefore, had a very low surface area. Accordingly, the work described here focuses on the development of activated fly ash derived sorbents for CO(2) capture. The samples were steam activated at 850 degrees C, resulting in a significant increase of the surface area (1075m(2)/g). The activated samples were impregnated with different amine compounds, and the resultant samples were tested for CO(2) capture at different temperatures. The CO(2) adsorption of the parent and activated samples is typical of a physical adsorption process. The impregnation process results in a decrease of the surface areas, indicating a blocking of the porosity. The highest adsorption capacity at 30 and 70 degrees C for the amine impregnated activated carbons was probably due to a combination of physical adsorption inherent from the parent sample and chemical adsorption of the loaded amine groups. The CO(2) adsorption capacities for the activated amine impregnated samples are higher than those previously published for fly ash carbons without activation (68.6 vs. 45mg CO(2)/g sorbent).     

Flue gas desulfurization gypsum and coal fly ash as basic components of prefabricated building materials

The manufacture of prefabricated building materials containing binding products such as ettringite (6CaO·Al₂O₃·3SO₃·32H₂O) and calcium silicate hydrate (CSH) can give, in addition to other well-defined industrial activities, the opportunity of using wastes and by-products as raw materials, thus contributing to further saving of natural resources and protection of the environment.Two ternary mixtures, composed by 40% flue gas desulfurization (FGD) gypsum or natural gypsum (as a reference material), 35% calcium hydroxide and 25% coal fly ash, were submitted to laboratory hydrothermal treatments carried out within time and temperature ranges of 2 h–7 days and 55–85 °C, respectively. The formation of (i) ettringite, by hydration of calcium sulfate given by FGD or natural gypsum, alumina of fly ash and part of calcium hydroxide, and (ii) CSH, by hydration of silica contained in fly ash and residual lime, was observed within both the reacting systems. For the FGD gypsum-based mixture, the conversion toward ettringite and CSH was highest at 70 °C and increased with curing time. Some discrepancies in the hydration behavior between the mixtures were ascribed to differences in mineralogical composition between natural and FGD gypsum.     

Synthesis of zeolite from waste fly ash for adsorption of CO2

Zeolitic sorbents for CO₂ adsorption were prepared from waste coal fly ash (FA) through hydrothermal treatment at various ratios of NaOH/FA and NaAlO₂/FA, including an initial alkali fusion step. The fusion step decomposed the fly ash into very small amorphous particulate zeolite forms. The fly ash was converted to Na-P1 type with a NaOH/FA ratio of 0.5 and Na-A type with a NaAlO₂/FA ratio of 0.53. The product properties were affected by the reaction temperature: Na-P1 and Na-A types were formed at 100°C. Temperatures above 140°C led to the formation of more sodalite because of the redissolving and recrystallization of zeolite crystals. Alkali metal and alkaline earth metal cations were impregnated in the synthesized Na-P1 and Na-A zeolite through an ion-exchange method. The completed zeolitic sorbents were applied to the adsorption of low-level CO₂. As a result of the experiments, calcium ions were found to be the best for CO₂ adsorption owing to their electrostatic behavior and acid-base interaction.     

水合改性对钙基吸收剂循环捕集CO2的影响

在钙基吸收剂捕集CO2的过程中,吸收剂转化率会随着循环次数的增加而迅速降低。钙基吸收剂的水合改性作为改善吸收剂循环转化率的主要方法之一受到了国内外学者的广泛关注。总结了目前国内外研究者对不同的吸收剂、循环捕集条件下的水合改性方法的研究成果。结果表明,在循环过程中的不同阶段对吸收剂进行水合处理后得到的效果不同。其中,在碳酸化阶段、煅烧阶段、循环捕集前预处理以及煅烧后对吸收剂水合改性．吸收剂捕集CO2的能力均得到了改善;碳酸化反应后对吸收剂进行水合处理是否对循环吸收有利还存在争议。目前,利用水合改性的方法提高钙基吸收剂循环捕集CO2能力的机理还存在争议,且水合改性后的吸收剂机械性能差以及此方法能耗高的问题尚待解决。     

Advanced ash management technologies for CFBC ash

The combustion of high-sulphur coal demands the reduction of sulphur emissions. The sorbent most often used in sulphur capture technology is calcium-based. Ashes from technologies such as circulating fluidized bed combustion (CFBC), therefore, contain high calcium levels. The use and disposal of these ashes poses challenges, because of highly exothermic reactions with water, high-pH leachates, and excessive expansion of solidified materials. This paper looks at the potential of two post-combustion ash treatment processes, CERCHAR hydration and AWDS disposal, in solving these challenges. A high-sulphur coal-derived CFBC ash is examined, after CERCHAR hydration treatment, in conjunction with a conventionally hydrated ash, in a range of chemical, geotechnical and utilization scenarios. The ashes are used to make no-cement and roller-compacted concrete as well as Ash Water Dense Suspensions (AWDS). The solidified mortar paste from no-cement concrete is subjected to an extensive geochemical examination to determine how solidification progresses and strength develops, from a chemical point of view.     

The hydration characteristics and utilization of slag obtained by the vitrification of MSWI fly ash

This study investigated the effects of slag composition on the hydration characteristics of slag blended cement (SBC) pastes. Synthetic slag samples were prepared by melting CaO-modified and Al(2)O(3)-modified municipal solid waste incinerator (MSWI) fly ash. MSWI fly ash was mixed with 5% CaO and 5% Al(2)O(3) (by weight), respectively, resulting in two fly ash mixtures. These mixtures were then melted at 1400 degrees C for 30 min to produce two types of slag with different contents, designated at C-slag and A-slag. Both the C-slag and A-slag samples exhibited a pozzolanic activity index higher than the unmodified slag sample. The results show that the synthetic slags all met the Taiwan EPA's current regulatory thresholds. These synthetic slags were then blended with ordinary Portland cement (OPC) at various weight ratios ranging from 10 to 40%. The 28-day strength of the C1 paste was higher than that developed by the OPC paste, suggesting that the C-slag contributed to the earlier strength of the SBC pastes. At curing times beyond 28 days, the strength of the A1 paste samples approached that of the OPC paste samples. It can be seen from this that increasing the amount of calcium and aluminum oxide increases the early strength of SBC. The C-slag blended cement paste samples showed an increase in the number of fine pores with the curing time, showing that the C-slag enhanced the pozzolanic reactions, filling the pores. Also, the incorporation of a 10% addition of C-slag also tended to enhance the degree of hydration of the SBC pastes during the early ages (3-28 days). However, at later ages, no significant difference in degree of hydration between the OPC pastes and the SBC pastes was observed with the 10% C-slag addition. However, the incorporation of A-slag did decreased the degree of hydration. A slag blend ratio of 40% significantly decreased the hydration degree.     

Legislative and environmental issues on the use of ash from coal and municipal sewage sludge co-firing as construction material

For the economy of any co-firing process, it is important that the common waste management options of ash remain practical. Ash from bituminous coal combustion is typically handed to the construction industry. This paper describes the current European legislation on use of ash for construction purposes. Also, it presents an experimental study on the suitability of fly ash from combustion of mixtures of bituminous coal and municipal sewage sludge as additive to cement and concrete, and for use in open-air construction works, based on the ash chemical composition and the characteristics of the extract of the ash. Presently, two European standards forbid the use of ash from co-firing as additive to cement or concrete. This study shows that ash derived from coal and sewage sludge co-firing contains generally less unburned carbon, alkali, magnesium oxide, chlorine, and sulfate than coal ash. Only the concentration of free lime in mixed ash is higher than in coal, even though, at least up to 25% of the thermal input, still below the requirements of the standards. This ash also meets the requirements for the use of fly ash in open-air construction works--concentration and mobility of few elements--although this management option is forbidden to ash from co-firing. The leaching of Cd, Cr, Cu, Ni, Pb and Zn was investigated with three leaching tests. The concentration of these metals in the extracts was below the detection limit in most cases. The concentration of Cu and Zn in the extract from fly ash was found to increase with increasing share of sewage sludge in the fuel mixture. However, the concentration of these two metals in the extract is not regulated. This study indicates that excluding a priori the use of ash from co-firing as a suitable additive for construction material could cause an unnecessary burden on the environment, since probably ash would have to be disposed of in landfill. However, allowing this requires the modification of current European standards to include limitations on all elements and compounds, absent in coal but which might be present in other fuels, that are deleterious for the quality of construction materials.     

Utilization of industrial by-products for the production of controlled low strength materials (CLSM)

Industrial by-products were used for the production of controlled low-strength material (CLSM). CLSM, also known as 'flowable fill' is used as a replacement of compacted soil in cases where the application of the latter is difficult or impossible. The low mechanical requirements (compared with structural concrete) enable the use of industrial by-products for the production of CLSM. In this study cement kiln dust, asphalt dust, coal fly ash, coal bottom ash and quarry waste were tested for the possibility of producing CLSM with large proportions of those wastes. The results showed that in most cases, CLSM with good properties could be made with significant amounts of dust (25-50%w), especially when the dust has some cementing or pozzolanic potential as do fly ash and cement kiln dust.     

Remediation of fly ash landfills through plantation

In India, a significant area of land is occupied by preexisting coal‐fired thermal power plants (TPPs) for the storage of fly ash slurry in ash ponds. However, the area available for storage of fly ash at these TPPs is limited. In addition, this type of fly ash disposal poses a problem due to restricted land availability and potential contamination issues. A viable alternative is the reclamation of fly ash ponds by plantation. A study at the Ramagundam Super Thermal Power Station (RSTPS) in Andhra Pradesh, India, on reclamation of a portion of an ash‐filled, low‐lying area has been performed. This article describes the characteristics of the RSTPS pond ash, ash leachates, and improvements in the fertility status of the reclaimed area over a three‐year period. Furthermore, morphometric observations of different planted species indicate that these types of ash‐filled, low‐lying areas can be suitably reclaimed and the nutrient‐rich leachate from ash‐filled areas potentially can be used for irrigation purposes. © 2008 Wiley Periodicals, Inc.     

钛硅分子筛催化双氧水氧化水中间甲酚

探索催化双氧水氧化去除间甲酚对开发炼油厂碱渣废水处理新技术意义重大。采用钛硅分子筛催化双氧水氧化水中间甲酚,考察了反应时间、反应温度、双氧水加入量、催化剂加入量和初始溶液pH对间甲酚去除率的影响。实验结果表明：钛硅分子筛对双氧水氧化间甲酚具有显著的催化作用;在反应时间为90 min、反应温度为80 ℃、n（H₂O₂）∶n（间甲酚）为4、催化剂加入量为1.5 g/L、初始溶液pH为1.0~11.0的条件下,间甲酚去除率约为94%,间甲酚溶液的BOD₅/COD从氧化前的0.26提高到氧化后的0.38,可生化性显著提高。     

Polycyclic aromatic hydrocarbon (PAH) emission from co-firing municipal solid waste (MSW) and coal in a fluidized bed incinerator

Polycyclic aromatic hydrocarbons (PAH) emissions from a commercial municipal solid waste incinerator (MSWI) were studied. A MSW–coal mixture and coal only were used as fuel for the fluidized bed incinerator. Seven sampling points were chosen according to the classified four PAH emission pathways: flue gas, residue, ash and water. The mixture of MSW and coal resulted in PAH emission more than that of coal only, and PAH emission increased with increasing MSW mass percentage. Calcium oxide (CaO) or calcium carbonate (CaCO₃) was added as a desulfurizer. PAH emission also changed with different desulfurizers because of their different influences on heat balance. The PAH toxic equivalent (TEQ) of all operating conditions was also examined, showing that total daily PAH emission from MSWI can be determined.     

Production of cement clinkers from municipal solid waste incineration (MSWI) fly ash

This communication reports the laboratory scale study on the production of cement clinkers from two types of municipal solid waste incineration fly ash (MSW ash) samples. XRD technique was used to monitor the phase formation during the burning of the raw mixes. The amount of trace elements volatilized during clinkerization and hydration, as well as leaching behaviours of the clinkers obtained from optimum compositions, were also evaluated. From the results it is observed that all of the major components of ordinary Portland cement (OPC) clinkers are present in the produced clinkers. Results also show the volatilization of considerable amounts of Na, K, Pb, Zn and Cd during the production of clinkers. However, major parts of the toxic elements remaining in the clinkers appear to be immobilized in the clinkers phases. Hydration studies of the clinkers obtained from optimum compositions show that the clinkers prepared from raw MSW ash are more reactive than the washed MSW ash based clinkers. TG/DTA analyses of the hydrated pastes show the formation of hydration products, which are generally found in OPC and OPC derived cements. The initial study, therefore, shows that more than 44% of MSW ash with the addition of very small amounts of silica and iron oxide can be used to produce cement clinkers. The amount of CaCO3 necessary to produce clinkers (approximately 50%) is also smaller than the same required for the conventional process (more than 70%).     

Utilization of blended fluidized bed combustion (FBC) ash and pulverized coal combustion (PCC) fly ash in geopolymer

In this paper, synthesis of geopolymer from fluidized bed combustion (FBC) ash and pulverized coal combustion (PCC) fly ash was studied in order to effectively utilize both ashes. FBC-fly ash and bottom ash were inter-ground to three different finenesses. The ashes were mixed with as-received PCC-fly ash in various proportions and used as source material for synthesis of geopolymer. Sodium silicate (Na₂SiO₃) and 10 M sodium hydroxide (NaOH) solutions at mass ratio of Na₂SiO₃/NaOH of 1.5 and curing temperature of 65 °C for 48 h were used for making geopolymer. X-ray diffraction (XRD), scanning electron microscopy (SEM), degree of reaction, and thermal gravimetric analysis (TGA) were performed on the geopolymer pastes. Compressive strength was also tested on geopolymer mortars. The results show that high strength geopolymer mortars of 35.0–44.0 MPa can be produced using mixture of ground FBC ash and as-received PCC-fly ash. Fine FBC ash is more reactive and results in higher degree of reaction and higher strength geopolymer as compared to the use of coarser FBC ash. Grinding increases reactivity of ash by means of increasing surface area and the amount of reactive phase of the ash. In addition, the packing effect due to fine particles also contributed to increase in strength of geopolymers.     

Preparation of activated carbons from heavy-oil fly ashes

The use of heavy oil fly ash with high ash content (45 wt.%) as a precursor for the preparation of activated carbons has been investigated. The raw fly ash and the fly ash with lower ash content, obtained by a HCl/HF washing treatment, have been pyrolyzed at 900 degrees C and then activated with CO(2) in the temperature range of 800-900 degrees C for different times. The activated carbons have been characterised as regards the surface area and the pore volume. The evolution of the porosity has been related to the burn-off degree.     

Improvements of nano-SiO2 on sludge/fly ash mortar

Sewage sludge ash has been widely applied to cementitious materials. In this study, in order to determine effects of nano-SiO(2) additives on properties of sludge/fly ash mortar, different amounts of nano-SiO(2) were added to sludge/fly ash mortar specimens to investigate their physical properties and micro-structures. A water-binding ratio of 0.7 was assigned to the mix. Substitution amounts of 0%, 10%, 20%, and 30% of sludge/fly ash (1:1 ratio) were proposed. Moreover, 0%, 1%, 2%, and 3% of nano-SiO(2) was added to the mix. Tests, including SEM and compressive strength, were carried out on mortar specimens cured at 3, 7, and 28 days. Results showed that sludge/fly ash can make the crystals of cement hydration product finer. Moreover, crystals increased after nano-SiO(2) was added. Hence, nano-SiO(2) can improve the effects of sludge/fly ash on the hydration of mortar. Further, due to the low pozzolanic reaction active index of sludge ash, early compressive strengths of sludge/fly ash mortar were decreased. Yet, nano-SiO(2) could help produce hydration crystals, which implies that the addition of nano-SiO(2) to mortar can improve the influence of sludge/fly ash on the development of the early strength of the mortar.     

Microbial fuel cell (MFC) for bioelectricity generation from organic wastes

Microbial fuel cells (MFCs) have gained a lot of attention recently as a mode of converting organic matter into electricity. In this study, a compost-based microbial fuel cell that generates bioelectricity by biodegradation of organic matter is developed. Grass cuttings, along with leaf mold, rice bran, oil cake (from mustard plants) and chicken droppings (waste from chickens) were used as organic waste. The electric properties of the MFC under anaerobic fermentation condition were investigated along with the influence of different types of membranes, the mixing of fly ash, and different types of electrode materials. It is observed that the maximum voltage was increased by mixing fly ash. Cellophane showed the highest value of voltage (around 350 mV). Bamboo charcoal is good for anode material; however carbon fiber is better for the cathode material in terms of optimization of power generated. This developed MFC is a simple cell to generate electricity from organic waste.     

Optimizing Cr(VI) adsorption on activated carbon produced from heavy oil fly ash

In order to explore the beneficial utilization of heavy oil fly ash (HOFA) generated in the power plants, the present study is intended to optimize the chromium(VI) [Cr(VI)] adsorption on activated carbon produced from HOFA. The raw HOFA obtained from a power plant was washed by nitric/hydrochloric acid and activated at 800 °C with a holding time of 60 min to produce fly ash activated carbon (FAC). Phosphoric acid was used as a chemical agent to improve the surface characteristics of the HOFA during the activation process. Batch adsorption experiments were employed to evaluate the effects of different parameters such as initial Cr(VI) concentration, pH, and FAC dose on the removal of Cr(VI) from aqueous solution. A total of 17 adsorption experimental runs were carried out employing the detailed conditions followed the response surface methodology based on the Box–Behnken design. The results indicate that developed FAC has the potential for removing Cr(VI) from wastewater. Under the test conditions, a maximum of 91.51 % Cr(VI) removal efficiency was achieved.     

Accelerated carbonation of municipal solid waste incineration fly ashes

As a result of the EU Landfill Directive, the disposal of municipal solid waste incineration (MSWI) fly ash is restricted to only a few landfill sites in the UK. Alternative options for the management of fly ash, such as sintering, vitrification or stabilization/solidification, are either costly or not fully developed. In this paper an accelerated carbonation step is investigated for use with fly ash. The carbonation reaction involving fly ash was found to be optimum at a water/solid ratio of 0.3 under ambient temperature conditions. The study of ash mineralogy showed the disappearance of lime/portlandite/calcium chloride hydroxide and the formation of calcite as carbonation proceeded. The leaching properties of carbonated ash were examined. Release of soluble salts, such as SO4, Cl, was reduced after carbonation, but is still higher than the landfill acceptance limits for hazardous waste. It was also found that carbonation had a significant influence on lead leachability. The lead release from carbonated ash, with the exception of one of the fly ashes studied, was reduced by 2-3 orders of magnitude.     

Mercury Capture on Fly Ash and Sorbents: The Effects of Coal Properties and Combustion Conditions

The US fleet of coal-fired power plants, with generating capacity of just over 300 GW, is known to be a major source of domestic mercury (Hg) emissions. To address this, in March 2005, the Environmental Protection Agency (EPA) promulgated the Clean Air Mercury Rule (CAMR) to reduce emissions of mercury from these plants. It is generally believed that most of the initial (Phase I) mercury reductions will come as a co-benefit of existing controls used to remove particulate matter (PM), SO₂, and NO _X . Deeper reductions in emissions (as required in Phase II of CAMR) may require the installation of mercury-specific control technology. Duct injection of activated carbon sorbents is the mercury-specific control technology that has been most widely studied and has been demonstrated over a wide range of coal types and combustion conditions. The effectiveness of the mercury control options (both “co-benefit control” and “mercury-specific control”) is significantly impacted by site-specific characteristics such as the combustion conditions, the configuration of existing air pollution controls, and the type of coal burned. This paper identifies the role of coal properties and combustion conditions in the capture of mercury by fly ash and injected sorbents.     

Influence of flue-gas desulfurization systems on coal combustion by-product quality at kentucky power stations burning high-sulfur coal

Two Kentucky power plants burning similar blends of high-sulfur western Kentucky and southern Indiana coal provide a unique opportunity to examine the variations in coal combustion by-products due to differences in the method of wet flue-gas desulfurization (FGD). One plant employed carbide lime-based scrubbing for two units and a dual-alkali process for the third unit. The second plant employed a Mississippian limestone from Kentucky for all four units. This study provides an example of optical and SEM petrographic techniques, supplemented by chemical analyses, applied to the study of, at least from the geologic perspective, non-traditional materials. The coal sources comprise a blend of high volatile C and B bituminous, high vitrinite (85–90%, mmf), high-sulfur (> 3%, dry) coals. The fly ash is dominated by glassy phases (70–80%) with about 5–10% spinel (predominately magnetite), 3–10% quartz, and 4–10% isotropic coke comprising the remaining portion of the ash. SEM observations indicate that the glassy particles exhibit a bimodal size distribution with sub-micron glass spheres and a population of larger (several 10s of microns) spheres. The bottom ash has higher proportions of spinels and mullite, with negligible carbon forms, compared to the fly ash from the same units. Fly ashes were observed to be lower in Fe and higher in Al, Si, and S compared to the bottom ashes. Carbide lime, a by-product of acetylene manufacture, soda ash, and limestone were the reagents used in the flue-gas desulfurization processes. The primary FGD by-product is a calcium sulfite slurry which is vacuum filtered and mixed with fly ash and, usually, lime, to form a stable product for disposal. The FGD by-products have some potential, as yet unrealized, for utilization.