Numerical simulation of chemical looping combustion process with CaSO4 oxygen carrier

Chemical-looping combustion (CLC) is a promising technology for the combustion of gas or solid fuel with efficient use of energy and inherent separation of CO₂. The technique involves the use of an oxygen carrier which transfers oxygen from combustion air to the fuel, and hence a direct contact between air and fuel is avoided. A chemical-looping combustion system consists of a fuel reactor and an air reactor. A metal oxide is used as oxygen carrier that circulates between the two reactors. The air reactor is a high velocity fluidized bed where the oxygen carrier particles are transported together with the air stream to the top of the air reactor, where they are then transferred to the fuel reactor using a cyclone. The fuel reactor is a bubbling fluidized bed reactor where oxygen carrier particles react with hydrocarbon fuel and get reduced. The reduced oxygen carrier particles are transported back to the air reactor where they react with oxygen in the air and are oxidized back to metal oxide. The exhaust from the fuel reactor mainly consists of CO₂ and water vapor. After condensation of the water in the exit gas from the fuel reactor, the remaining CO₂ gas is compressed and cooled to yield liquid CO₂, which can be disposed of in various ways.With the improvement of numerical methods and more advanced hardware technology, the time needed to run CFD (Computational fluid dynamics) codes is decreasing. Hence multiphase CFD-based models for dealing with complex gas-solid hydrodynamics and chemical reactions are becoming more accessible. Until now there were a few literatures about mathematical modeling of chemical-looping combustion using CFD approach. In this work, the reaction kinetics model of the fuel reactor (CaSO₄ + H₂) was developed by means of the commercial code FLUENT. The bubble formation and the relation between bubble formation and molar fraction of products in gas phase were well captured by CFD simulation. Computational results from the simulation also showed low fuel conversion rate. The conversion of H₂ was about 34% partially due to fast, large bubbles rising through the reactor, low bed temperature and large particles diameter.     

Waste products from the steel industry with NiO as additive as oxygen carrier for chemical-looping combustion

Fe₂O₃-containing waste materials from the steel industry are proposed as oxygen carrier for chemical-looping combustion. Three such materials, red iron oxide, brown iron oxide and iron oxide scales, have been examined by oxidation and reduction experiments in a batch fluidized-bed reactor at temperatures between 800 and 950 °C. NiO-based particles have been used as additive, in order to examine if it is possible to utilize the catalytic properties of metallic Ni to facilitate decomposition of hydrocarbons into more reactive combustion intermediates such as CO and H₂. The experiments indicated modest reactivity between the waste materials and CH₄, which was used as reducing gas. Adding small amounts of NiO-based particles to the sample increased the yield of CO₂ in a standard experiment, typically by a factor of 1.5–3.5. The fraction of unconverted fuel typically was reduced by 70–90%. The conversion of CH₄ to CO₂ was 94% at best, corresponding to a combustion efficiency of 96%. This was achieved using a bed mass corresponding to 57 kg oxygen carrier per MW fuel, of which only 5 wt% was NiO-based synthetic particles. The different materials fared differently well during the experiments. Red iron oxide was fairly stable, while brown iron oxide was soft and subject to considerable erosion. Iron oxide scales experienced increased reactivity and porosity as function of the numbers of reduction cycles.     

160 h of chemical-looping combustion in a 10 kW reactor system with a NiO-based oxygen carrier

Chemical-looping combustion, CLC, is a technology with inherent separation of the greenhouse gas CO₂. The technique uses an oxygen carrier made up of particulate metal oxide to transfer oxygen from combustion air to fuel. In this work, an oxygen carrier consisting of 60% NiO and 40% NiAl₂O₄ was used in a 10 kW CLC reactor system for 160 h of operation with fuel. The first 3 h of fuel operation excepted, the test series was accomplished with the same batch of oxygen carrier particles. The fuel used in the experiments was natural gas, and a fuel conversion to CO₂ of approximately 99% was accomplished. Combustion conditions were very stable during the test period, except for the operation at sub-stoichiometric conditions. It was shown that the methane fraction in the fuel reactor exit gas was dependent upon the rate of solids circulation, with higher circulation leading to more unconverted methane. The carbon monoxide fraction was found to follow the thermodynamical equilibrium for all investigated fuel reactor temperatures, 660–950 °C. Thermal analysis of the fuel reactor at stable conditions enabled calculation of the particle circulation which was found to be approximately 4 kg/s, MW. The loss of fines, i.e. the amount of elutriated oxygen carrier particles with diameter <45 μm, decreased during the entire test period. After 160 h of operation the fractional loss of fines was 0.00022 h⁻¹, corresponding to a particle life time of 4500 h.     

Investigation of NiO/NiAl2O4 oxygen carriers for chemical-looping combustion produced by spray-drying

Chemical-looping combustion is a novel combustion technology with inherent separation of the greenhouse gas CO₂. The technology uses circulating oxygen carriers to transfer oxygen from the combustion air to the fuel. In this paper, oxygen carriers based on commercially available NiO and α-Al₂O₃ were prepared using the industrial spray-drying method, and compared with particles prepared by freeze-granulation. The materials were investigated under alternating oxidizing and reducing conditions in a laboratory fluidized bed, thus simulating the cyclic conditions of a chemical-looping combustion system. The particles produced by spray-drying displayed a remarkable similarity to the freeze-granulated oxygen carriers, with high reactivity when the bed was fluidized and similar physical properties when sintered at the same temperature. This is an important result as it shows that the scaling-up from a laboratory production method, i.e. freeze-granulation, to a commercial method suitable for large-scale production, i.e. spray-drying, did not involve any unexpected difficulties. A difference noticed between the spray-dried and freeze-granulated particles was the sphericity. Whereas the freeze-granulated particles showed near perfect sphericity, a large portion of the spray-dried particles had hollow interiors. Defluidization was most likely to occur for highly reduced particles, at low gas velocities. The apparent density and crushing strength of the oxygen carriers could be increased either by increasing the sintering temperature or by increasing the sintering time. However, the fuel conversion was fairly unchanged when the sintering temperature was increased but was clearly improved when the sintering time was increased.     

Efficacy of alum and coal combustion by-products in stabilizing manure phosphorus

Animal manures contain large amounts of soluble phosphorus (P), which is prone to runoff losses when manure is surface-applied. Here we report the efficacy of alum and three coal combustion by-products in reducing P solubility when added to dairy, swine, or broiler litter manures in a laboratory incubation study. Compared with unamended controls, alum effectively reduced readily soluble P, determined in water extracts of moist manure samples with 1 h of shaking, for all three manures. The reduction ranged from 80 to 99% at treatment rates of 100 to 250 g alum kg(-1) manure dry matter. The fluidized bed combustion fly ash (FBC) reduced readily soluble P by 50 to 60% at a rate of 400 g kg(-1) for all three manures. Flue gas desulfurization by-product (FGD) reduced readily soluble P by nearly 80% when added to swine manure and broiler litter at 150 and 250 g kg(-1). Another by-product, anthracite refuse fly ash (ANT), was ineffective for all three manures. In all cases, reduction in readily soluble P is primarily associated with inorganic phosphorus (P(i)) with little change in organic phosphorus (P(o)). Sequential extraction results indicate that the by-product treatments shifted manure P from H2O-P into a less vulnerable fraction, NaHCO3 - P, while the alum treatment shifted the P into even more stable forms, mostly NaOH-P. Such shifts in P fractions would have little influence on P availability for crops over the long-term but would retard and reduce potential losses of P following manure applications.     

Electricity systems with near-zero emissions of CO2 based on wind energy and coal gasification with CCS and hydrogen storage

Emissions from electricity generation will have to be reduced to near-zero to meet targets for reducing overall greenhouse gas emissions. Variable renewable energy sources such as wind will help to achieve this goal but they will have to be used in conjunction with other flexible power plants with low-CO₂ emissions. A process which would be well suited to this role would be coal gasification hydrogen production with CCS, underground buffer storage of hydrogen and independent gas turbine power generation. The gasification hydrogen production and CO₂ capture and storage equipment could operate at full load and only the power plants would need to operate flexibly and at low load, which would result in substantial practical and economic advantages. This paper analyses the performances and costs of such plants in scenarios with various amounts of wind generation, based on data for power demand and wind energy variability in the UK. In a scenario with 35% wind generation, overall emissions of CO₂ could be reduced by 98–99%. The cost of abating CO₂ emissions from the non-wind residual generation using the technique proposed in this paper would be less than 40% of the cost of using coal-fired power plants with integrated CCS.     

Chemical analysis of soil and leachate from experimental wetland mesocosms lined with coal combustion products

Small-scale (1 m2) wetland mesocosm experiments were conducted over two consecutive growing seasons to investigate the effects on soil and leachate chemistry of using a recycled coal combustion product as a liner. The coal combustion product used as a liner consisted of flue gas desulfurization (FGD) by-products and fly ash. This paper provides the chemical characteristics of mesocosm soil and leachate after 2 yr of experimentation. Arsenic, Ca, and pH were higher in FGD-lined mesocosm surface soil relative to unlined mesocosms. Aluminum was higher in the soils of unlined mesocosms relative to FGD-lined mesocosms. No significant difference of potentially phytotoxic B was observed between lined and unlined mesocosms in the soil. Higher pH, conductivity, and concentrations of Al, B, Ca, K, and S (SO4-S) were observed in leachate from lined mesocosms compared with unlined controls while Fe, Mg, and Mn were higher in leachate from unlined mesocosms. Concentrations of most elements analyzed in the leachate were below national primary and secondary drinking water standards after 2 yr of experimentation. Initially high pH and soluble salt concentrations measured in the leachate from the lined mesocosms may indicate the reason for early effects noted on the development of wetland vegetation in the mesocosms.     

Reduction and utilization of coal mine waste rock in China: A case study in Tiefa coalfield

In China, coal mine waste rock (CMWR) produced during coal mining and processing is still increasing significantly as a result of coal production which has huge environmental impact. CMWR reduction and utilization is a major issue for coal enterprises and government to reduce the surface footprint and the public environmental impact. Tiefa coalfield, an old coalfield with 60 years of coal exploitation, was selected as a case to study the methods to minimize the environmental impacts of CMWR piles in a short period. We argue that a systematic design on CMWR utilization is needed on the basis of a usage evaluation which takes consideration of CMWR source, compositions, and proximate analysis. Mine design is crucial and the base for reducing the CMWR generation at the headstream. Placing roadway into coal seam rather than rock, panel optimization, and parametric analysis for mining technique were conducted in Tiefa coalfield. A promising technology of CMWR backfill under the ground was employed with a resultant increase of coal recovery rate. The surface CMWR recycling depends on brick making, electricity generating, and rehabilitation of subsided land. The practice of the presented methods indicates that the CMWR piles on Tiefa coalfield may disappear in 3 years, which could significantly reduce the environmental impacts of CMWR dumps. The technologies conducted in Tiefa coalfield developed a model of CMWR reduction and utilization for Chinese coal mines.     

Impingement of threadfin shad: Effects of temperature and hydrography

A model is presented that allows testing of hypotheses concerning the effects of temperature and change in temperature on impingement. The model is evaluated using data from the Tennessee Valley Authority's Kingston Steam Plant, Watts Bar Reservoir, Tennessee, USA for two fish species impinged in large numbers in the United States: threadfin and gizzard shad,Dorosoma petenense andD. cepedianum. Hydrographic characteristics near the intake screens were mapped to help explain the possible role of hydrography in distributing fish across the screens. Understanding the role of temperature and hydrography in impingement of fish provides a basis for new intake designs that may reduce impingement and helps in the development of methods to reduce impingement at existing facilities. The temperature modeling approach and conclusions about hydrographic effects might be applied to other systems in which cold-stressed schooling fish are impinged.Research sponsored by the Office of Health and Environmental Research, US Department of Energy, under contract W-7405-eng-26 with Union Carbide Corporation.Publication No. 1872, Environmental Sciences Division, ORNL.     

温室效应与全球变化（四）：通过国际谈判与合作减少全球人为二氧化碳的排放

在限制人为二氧化碳的排放以控制全球变化的问题上，发达国家与发展中国家之间存在着尖锐的利益冲突。目前，大气中人为增加的二氧化碳绝大中份是发达国家在过去１５０年内排放的。它们目前的排放量仍占全球人为排放量的四分之三。发展中国家在经济发展过程中不可避免地要增加二氧化碳的排放量，这是它们正当的发展权利。作者讨论了各种限制人为二氧化碳排放的国际协定方案，认为可交易的二氧化碳排放许可证制度是比较现实的方案。     

Experimental validation of in situ CO2 capture with CaO during the low temperature combustion of biomass in a fluidized bed reactor

A novel concept for capturing CO₂ from biomass combustion using CaO as an active solid sorbent of CO₂ is discussed and experimentally tested. According to the CaO/CaCO₃ equilibrium, if a fuel could be burned at a sufficiently low temperature (below 700 °C) it would be possible to capture CO₂ “in situ” with the CaO particles at atmospheric pressure. A subsequent step involving the regeneration of CaCO₃ in a calciner operating at typical conditions of oxyfired-circulating fluidized combustion would deliver the CO₂ ready for purification, compression and permanent geological storage. Several series of experiments to prove this concept have been conducted in a 30 kW interconnected fluidized bed test facility at INCAR-CSIC, made up of two interconnected circulating fluidized bed reactors, one acting as biomass combustor-carbonator and the other as air-fired calciner (which is considered to yield similar sorbent properties than those of an oxyfired calciner). CO₂ capture efficiencies in dynamic tests in the combustor-carbonator reactor were measured over a wide range of operating conditions, including different superficial gas velocities, solids circulation rates, excess air above stoichiometric, and biomass type (olive pits, saw dust and pellets). Biomass combustion in air is effective at temperatures even below the 700 °C, necessary for the effective capture of CO₂ by carbonation of CaO. Overall CO₂ capture efficiencies in the combustor-carbonator higher than 70% can be achieved with sufficiently high solids circulation rates of CaO and solids inventories. The application of a simple reactor model for the combined combustion and CO₂ capture reactions allows an efficiency factor to be obtained from the dynamic experimental test that could be valuable for scaling up purposes.     

Effects of moisture and temperature on carbon and nitrogen mineralization in mine tailings mixed with sewage sludge

Digested sewage sludge mixed with copper mine tailings was incubated for 3 mo at 16 combinations of temperature (-1, 5, 10, and 15 degrees C) and soil moisture content (2, 8, 14, and 24% by weight). Carbon dioxide evolution and net N mineralization were measured at increasing time intervals. A two compartment first-order kinetic model (refractory and labile C) was fitted to the time series of measured CO2 fluxes using nonlinear regression analysis. The dependencies of the rate constants on moisture and temperature could be well described by log-linear functions. The estimated Q10 value (the factor by which the rate is increased as temperature is increased 10 degrees C) was 2.55. Within the range of temperature and moisture considered here, which correspond to conditions occurring naturally in Sweden, CO2 evolution was more strongly controlled by moisture than by temperature. Less mineral N accumulated during the experiment at the lowest moisture or temperature. However, the dependency of net N mineralization on moisture and temperature in the remaining treatments was less clear than for C evolution, presumably due to denitrification at the higher temperatures and moisture contents. Nitrate was formed after around 2 wk but only at 10 and 15 degrees C.     

Estimation of annual CH4 and N2O emissions from fluidised bed combustion: An advanced measurement-based method and its application to Finland

The aim of this study was to develop and apply an advanced, measurement based method for the estimation of annual CH₄ and N₂O emissions and thus gain improved understanding on the actual greenhouse gas (GHG) balances of combustion of fossil fuels, peat, biofuels and REF. CH₄ and N₂O emissions depend strongly on combustion conditions, and therefore the emission factors used in the calculation of annual emissions contain significant uncertainties. Fluidised bed combustion (FBC) has many good properties for combustion of different types of fuels and fuels of varying quality, e.g., biofuels and wastes. Therefore, it is currently increasing its market share. In this study, long term measurements (up to 50 days) were carried out at seven FBC boilers representing different size classes, loadings and fuel mixes. Both decreasing load and increasing share of coal in fuel mix increased N₂O emissions. Measurement results from different loading levels were combined with the common loading curves of similar plants in Finland to estimate annual emissions. Based on the results, recommendations for emission factors for the Finnish GHG emission inventory are given. The role of FBC as a potential technology for the utilisation of biofuels and wastes with future GHG reduction requirements is discussed.     

Effect of gas composition in Chemical-Looping Combustion with copper-based oxygen carriers: Fate of sulphur

Chemical-Looping Combustion (CLC) is an emerging technology for CO₂ capture because separation of this gas from the other flue gas components is inherent to the process and thus no energy is expended for the separation. Natural or refinery gas can be used as gaseous fuels and they may contain different amounts of sulphur compounds, such as H₂S and COS. This paper presents the combustion results obtained with a Cu-based oxygen carrier using mixtures of CH₄ and H₂S as fuel. The influence of H₂S concentration on the gas product distribution and combustion efficiency, sulphur splitting between the fuel reactor (FR) and the air reactor (AR), oxygen carrier deactivation and material agglomeration was investigated in a continuous CLC plant (500 W_th). The oxygen carrier to fuel ratio, ?, was the main operating parameter affecting the CLC system. Complete fuel combustion were reached at 1073 K working at ? values ≥1.5. The presence of H₂S did not produce a decrease in the combustion efficiency even when working with a fuel containing 1300 vppm H₂S. At these conditions, the great majority of the sulphur fed into the system was released in the gas outlet of the FR as SO₂, affecting to the quality of the CO₂ produced. Formation of copper sulphide, Cu₂S, and the subsequent reactivity loss was only detected working at low values of ? ≤ 1.5, although this fact did not produce any agglomeration problem in the fluidized beds. In addition, the oxygen carrier was fully regenerated in a H₂S-free environment. It can be concluded that Cu-based oxygen carriers are adequate materials to be used in a CLC process using fuels containing H₂S although quality of the CO₂ produced is affected.     

Effect of gas composition in Chemical-Looping Combustion with copper-based oxygen carriers: Fate of light hydrocarbons

Chemical-Looping Combustion (CLC) is an emerging technology for CO₂ capture because separation of this gas from the other flue gas components is inherent to the process and thus no energy is expended for the separation. Natural or refinery gas can be used as gaseous fuels and they may contain different amounts of light hydrocarbons. This paper presents the combustion results obtained with a Cu-based oxygen carrier using mixtures of CH₄ and light hydrocarbons (LHC) (C₂H₆ and C₃H₈) as fuel. The effect on combustion efficiency of the fuel reactor temperature, solid circulation flow rate and gas composition was studied in a continuous CLC plant (500 W_th). Full combustions were reached at 1073 and 1153 K working at oxygen to fuel ratios, ? higher than 1.5 and 1.2 respectively. Unburnt hydrocarbons were never detected at any experimental conditions at the fuel reactor outlet. Carbon formation can be avoided working at 1153 K or at ? values higher than 1.5 at 1073 K. After 30 h of continuous operation, the oxygen carrier exhibited an adequate behavior regarding attrition and agglomeration. It can be concluded that no special measures should be taken in a CLC process with Cu-based OC with respect to the presence of LHC in the fuel gas.     

Reduction of Waste and Electricity Demand in The Netherlands: A Hypothetical Intervention

An important question in the present reorganization and liberalization of infrastructure networks in many countries is what kind of regulatory regimes can provide incentives for demand management and reduction in a future situation. In this paper, the present situation in the Dutch waste and electricity sectors is analysed. It is concluded that a quantity-based tariff system and the entwined interests of organizations are key impediments to effective demand management in these two sectors. A hypothetical intervention is proposed which aims to remove these two fundamental drawbacks. Some likely consequences of the suggested intervention are discussed.     

Effects of thermal barrier coating on the performance and combustion characteristics of a diesel engine fueled with biodiesel produced from waste frying cottonseed oil and ultra-low sulfur diesel

In this study, the top surfaces of piston and valves of a four-strokes and direct-injection diesel engine have been coated—with no change in the compression ratio—with a 100 μm of NiCrAl lining layer via plasma spray method and this layer has later been coated with main coating material with a mixture of 88% of ZrO₂, 4% of MgO and 8% of Al₂O₃ (400 μm). Then, after the engine-coating process, ultra-low sulfur diesel (ULSD) as base fuels and its blend with used frying cottonseed oil derived biodiesel in proportion of 20%, volumetrically, have been tested in the coated engine and data of combustion and performance characteristics on full load and at different speeds have been noted. The results, which were compared with those obtained by uncoated-engine operation, showed that thermal efficiency increased, and engine noise reduced. Cylinder gas pressure values obtained from the diesel engine which has been coated with thermal barriers have been found to be somewhat higher than those of the uncoated-engine. Also, maximum pressure values measured in both engines and under the same experimental conditions through the use of test fuel have been obtained after TDC. Moreover, heat release rate and heat release have occurred earlier in the coated-engine. NOx emissions were increased while CO and HC emissions were remained almost the same with a little bit decrease.