Lufei Jia

Pacification of high calcic residues using carbon dioxide

The development of an alternative to hydration for the treatment and/or disposal of FBC ash was investigated. Carbonation (pacification) of the ashes was the option examined. The process is rapid at FBC operating conditions for dry ashes, but does not proceed below 400°C. Hydration of the ashes promotes the carbonation reaction below this temperature. Rapid carbonation of hydrated ashes occurs at 200–230°C. Carbonation will occur at ambient conditions for hydrated ashes, although the reaction is extremely slow. An unexpected finding in this research was that CaO, combined in the form of silicates, ferrites and aluminates (OCCs), also react with CO2. Specifically, OCCs, which are formed by reaction of the CaO in the sorbent and the fuel ash components, react with CO2 if the ashes are fully hydrated and liquid water is present. This has an impact on the disposal of FBC ash, as the reaction with CO2 will continue after placement in a disposal area. Control of the reaction would be advantageous in stabilization of the FBC ash. An effective limit to conversion of the free lime to carbonate, in the range of 60–70% was observed for the ash. The work showed the type of limestone is an important factor in the success of the recarbonation. Dolomitic limestone, a more porous sorbent, should have greater conversion rates, making the process more economic.

Reuse of landfilled FBC residues

Two aspects of the possible reactivation of fluidized bed combustion (FBC) residues are considered. One is the behaviour of aluminosulfates such as ettringite, which may form when excess water is used in hydration methods designed to reactivate the lime in the FBC residues. The second is the behaviour of compounds such as calcium aluminates, silicates or ferrites (‘other calcium compounds’ or OCC) that are produced by reactions in the fluidized bed. In both cases, the possibility of recovering the CaO contained in these materials for retention of SO2 when they are reintroduced into a combustor is assessed. It was found that ettringite is an excellent sulfur sorbent under FBC conditions, with reactivity superior to normal limestones. The CaO moiety in the OCC is not easily reactivated by hydration. Unhydrated, they do react in a sulfating atmosphere like that of the combustor, but their performance as sorbents is much poorer than limestone and varies with the nature of the compound.

Dioxin and Furan Formation in FBC Boilers

Fluidized bed combustion (FBC) is increasingly being used for municipal solid waste incineration and for various applications in which dioxin and furan (PCDD/F) formation are concerns. Despite the fact that fluidized bed combustion is a relatively low temperature system, current evidence shows that they perform relatively well. One hypothesis is that sulfur, which is contained in some of the fuels used in FBC (e.g. coal in cofiring situations), helps to reduce dioxin production. This paper endeavors to examine this speculation and finds that such benefits depend on the CI/S ratio and that at higher sulfur addition levels dioxins, furans, and polycyclic aromatic hydrocarbon (PAH) production may actually increase. This and the reasons for it are discussed.

An examination of the exothermic nature of fluidized bed combustion (FBC) residues

Circulating fluidized bed combustion (CFBC) ashes from nine operational periods at the 183 MWe CFBC boiler at Point Aconi were examined for exothermic behaviour. Bed ashes and fly ashes were investigated using a Parr 1455 solution calorimeter. Limited tests were also carried out with additional samples from Point Aconi and from the 160 MWe TVA Bubbling Fluidized Bed Combustion boiler to evaluate the effects of particle size and aging on exothermic behaviour. For the Point Aconi ashes, heat release from the bed ash ranged from 11 to 52 J/g, and the maximum heat release rates ranged from 0.06 to 0.17 J/g/s. For the fly ash heat release varied from 114 to 187 J/g and the maximum heat release rates ranged from 0.8 to 1.9 J/g/s. In the fly ash samples, 50% or more of available CaO was converted to Ca(OH)2, while for the bed ash a third or less of the CaO was converted to Ca(OH)2. The exothermicity of the bed ash is directly proportional to the CaO content of the ash. However, this is not true for the fly ash. The exothermic behaviour of fresh FBC ash appeared to be greatly reduced by exposure in air over a 48-h period. Another conclusion of this work is that particle size effects the exothermic behaviour.

Agglomeration and Fouling in Petroleum Coke-Fired FBC Boilers

Experiments have been done subjecting ashes from industrial-scale FBC boilers to sulphating conditions in an oven for up to 105 days. These show that sulphation by itself causes agglomeration in the virtual absence of V, K, and Na, the elements normally associated with ash softening and classical fouling. In addition, it has been demonstrated that sulphation goes to completion over long periods of time and, at a specific level which differs from one ash to another, results in agglomeration. These experiments have also shown that there is a size range (75-300 μm) in which the agglomeration is worst, and particles that are smaller or larger either do not agglomerate or agglomerate more weakly. Added inert coal-derived ash decreases or prevent s the agglomeration. However, this ash does not appear to chemically combine with the sulphate, but acts by mechanically separating the sulphating particles. Finally, if alkali metals are present they can cause agglomeration at levels lower than those at which either the alkalis or sulphation separately cause agglomeration, i.e., they operate synergistically to cause fouling. Current work is being directed at examining these phenomena at higher temperatures (900°C and above).

Assessment of Sorbent Reactivation by Water Hydration for Fluidized Bed Combustion Application

Disposal of fluidized bed combustion (FBC) solid residues currently represents one of the major issues in FBC design and operation, and contributes significantly to its operating cost. This issue has triggered research activities on the enhancement of sorbent utilization for in situ sulfur removal. The present study addresses the effectiveness of the reactivation by liquid water hydration of FB spent sorbents. Two materials are considered in the study, namely the bottom ash from the operation of a full-scale utility FB boiler and the raw commercial limestone used in the same boiler. Hydration-reactivation tests were carried out at temperatures of 40{sup o}C and 80{sup o}C and for curing times ranging from 15 minutes to 2d, depending on the sample. The influence of hydration conditions on the enhancement of sulfur utilization has been assessed. A combination of methods has been used to characterize the properties of liquid water-hydrated materials

Reactivation of fluidised bed combustor ash for sulphur capture

Abstract The efficiency of limestone utilisation for sulphur capture in fluidised bed combustors (FBC) is low due to incomplete sulphation of CaO. The disposal of the spent sorbent is problematic and costly. Reutilisation of the spent sorbent is highly desirable both in terms of improving overall plant economics and process efficiency. In this work, wet grinding of the bottom ash from a CFBC unit was used to reactivate the spent ash. The ash was then reinjected into a CFBC pilot plant and the performance of sulphur capture was compared with that of limestone. The activated ash showed higher sulphur capture efficiency, which is attributed to the more porous structure or larger specific surface area of the particles of the ash brought about by hydration of CaO and the subsequent dehydration of Ca(OH)2. The kinetics related to sulphur capture and the wet grinding process are also discussed.

Effect of Operating Conditions on SO2 and NOx Emissions in Oxy-Fuel Mini-CFB Combustion Tests

Anthropogenic CO2 production is caused primarily by fossil fuel combustion. In consequence, it is increasingly necessary to find ways to reduce these emissions when fossil fuel is used. CO2 capture and storage (CCS) appears to be among the most promising. All of the CCS technologies involve producing a pure stream of CO2 either by concentrating it from the flue gases, or by using pure oxygen as the combustion gas. The latter option, oxy-fuel combustion, has now been well studied for pulverized coal combustion, but hasreceived relatively little attention to date in the case of oxy-fuel circulating fluidized bed combustion. Recently, oxy-fuel CFBC hasbeen examined ina 100 kW pilot plant operating with flue gas recycle at CanmetEnergy. The results strongly support the view that this technology offers all of the advantages of air-fired FBC, with one possible exception. Emissions such as CO or NOx are lower or comparable to air firing. It is possible to switch from air-firing to oxy-firing mode easily, with oxygen concentrations as high as 60–70%, and flue gas recycle levels of 50–60%. Only sulphur capture is poorer. However, this result is not in good agreement with other studies, and the reasons for this discrepancy need further exploration. Here, longer tests have confirmed previous findings from CanmetEnergy with two coals and a petroleum coke. It also appears that changing from direct to indirect sulphation with the petroleum coke improves sulphur capture efficiency, although a similar effect could not be confirmed with coal from these results.

Combustion Of Poultry-Derived Fuel in a CFBC

Poultry farming generates large quantities of waste. Current disposal practice is to spread the poultry wastes onto farmland as fertilizer. However, as the factory farms for poultry grow both in numbers and size, the amount of poultry wastes generated has increased significandy in recent years. In consequence, excessive application of poultry wastes on farmland is resulting in more and more contaminants entering the surface water. One of the options being considered is the use of poultry waste as power plant fuel. Since poultry-derived fuel (PDF) is biomass, its co-firing will have the added advantage of reducing greenhouse gas emissions from power generation. To evaluate the combustion characteristics of co-firing PDF with coal, combustion tests of mixtures of coal and PDF were conducted in CanmetENERGY’s pilot-scale CFBC. The goal of the tests was to verify that PDF can be co-fired with coal and, more importantly, that emissions from the combustion process are not adversely affected by the presence of PDF in the fuel feed. The test results were very promising and support the view that co-firing in an existing coal-fired CFBC is an effective method of utilizing this potential fuel, both resolving a potential waste disposal problem and reducing the amount of CO2 released by the boiler.

Treatment of Sydney Tar Pond Sludge in CFBC

Test burns of mixtures of Sydney tar pond sludge and coal were carried out using CETC’s mini-circulating fluidized bed combustor (mini-CFBC). The goal was to determine if CFBC technology could be used to treat the tar pond sludge. During the tests, CO2 , O2 , CO, SO2 , and NOx in the flue gas were monitored continuously. Stack gas sampling was carried out for HCl, metals, particulate matter, VOCs, total hydrocarbons, semi-volatile organic compounds, dioxins and furans and PCBs. Results showed that HCl, Hg, particulate matter, PCDD/Fs and metal concentrations were all below both the current limits and the gas release limits to be implemented in 2008 in Canada. Sulphur capture efficiency was about 89–90%. The percentage of fuel nitrogen converted to NOx was of the order of 4.7 to 6.1, which is significantly lower than that of conventional pulverized coal-fired boilers and well within normal range of FBC boilers. PCB and PAH emissions levels were comparable or lower than levels reported in the literature for industrial-scale FBCs. VOC concentrations were low except for benzene, for which the concentration was higher than that reported for utility-scale FBC and pulverized coal-fired boilers. In addition, CO concentration was high at 1200 to 2200 ppm. However, these CO concentrations are typical of CETC’s mini-CFBC firing coal. The trials showed that, for 10% by weight tar pond sludge mixed with 90% by weight coal, the combustion was both stable and efficient. The tests demonstrated that CFBC technology could be an environmentally sound option for eliminating wastes from the Sydney tar pond.Copyright