Article pubs.acs.org/EF
Role of Pulverized Coal Ash against Agglomeration, Fouling, and Corrosion in Circulating Fluidized-Bed Boilers Firing Challenging Biomass Vesna Barišić, Kari Peltola, and Edgardo Coda Zabetta* Foster Wheeler Energia Oy, Relanderinkatu 2, FI-78201 Varkaus, Finland ABSTRACT: The rising use of agricultural residues (agros) aggravates some of the well-known challenges of biomass combustion in plants, such as agglomeration, fouling, and corrosion (AFC). Several countermeasures have been devised to contain AFC problems in biomass plants, some of which are broadly effective but somewhat costly, while others may be ineffective or harmful if deployed on unsuitable agros. The use of additives often falls in the first category, having broad applicability, high efficiency, high reliability, but high procurement costs. However, ash discarded from combustion plants firing coal can be a convenient exception, because its cost is negligible or even negative. Ash coming from pulverized fuel plants firing coal (PC-ash) was tested in a bench-scale reactor and a 1 MWth circulating fluidized-bed (CFB) pilot to assess its beneficial effects against agglomeration when firing the most demanding agro biomass. PC-ash was procured from different plants based on different PC technologies and firing diverse coals. Process conditions, boiler design, and coal type influenced dramatically the performance of PC-ash, but even the worst PC-ash still improved the resistance of agglomeration in CFBs by a factor of 2, as compared to the sand regularly used as bed material. Such performance resulted from the synergy of physical and chemical interactions between ash-forming elements and PC-ash. Contrary to many other countermeasures and additives, the beneficial effects of PC-ash against agglomeration did not unveil major drawbacks on fouling and corrosion. Because fuel alkalis are captured by the PC-ash and chlorine is released as HCl, both fouling and corrosion can be kept at bay. Nonetheless, proper management of the PC-ash is imperative because fine fractions increase dust loading in the backpass and can increase fouling if uncontrolled. This paper summarizes the key observations from PC-ash testing and consolidates the role of PC-ash against AFC. Chemical/physical mechanisms are proposed and verified against test results.
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INTRODUCTION After the adoption of the Kyoto Protocol in 1997 and the Bali Road Map in 2007, most nations, including developing countries, will have binding obligations to reduce progressively their greenhouse gas (GHG) emissions within a very short time. In response, several countries have chosen to promote the use of biomass in partial replacement to fossil fuels in combustion plants. Some countries provide incentives to biomass-firing plants, while others release new permits only for “green” plants. Furthermore, some countries have set restrictions on eligible biomass types. Under such circumstances, the market for agricultural residues suddenly boomed. In some countries, the demand exceeded the domestic availability, and worldwide trading of pelletized biomass residues followed. From a technical standpoint, the rising use of agricultural residues (agros) aggravates some of the well-known challenges of biomass combustion in plants, such as agglomeration, fouling, and corrosion (AFC), which can lead to unscheduled shutdowns, prolonged downtime (loss of availability), increased costs for maintenance, and more frequent replacement of pressure parts. The agglomeration observed during fluidized-bed combustion of biomass progresses by a combination of two well-known mechanisms: “melt-induced” and “coating-induced”.1−7 In pilot-scale tests, the two mechanisms can be observed independently. In full-scale circulating fluidized-beds (CFBs), the two mechanisms coexist, but with most fuels, one © 2013 American Chemical Society
mechanism is dominant. Even though the alkali content plays the major role in agglomeration, the concentration and ratios of elements, such as Cl, P, Ca, etc., dictate the extent and mechanism of this process. The melt-induced mechanism includes direct adhesion of bed particles by partly molten fuel-derived ash particles/ droplets, often still attached to the burning char particle. Sticky fuel-derived ash may form in the case when fuel ash chemistry is governed by a high content of potassium and organically bound silicon combined with a high content of chlorine and a low content of other ash-forming elements, for example, with certain straw compositions.1−3,5,6 The coating-induced mechanism includes the formation of a sticky-coating layer on the surface of bed material particle as a result of (a) the direct contact with ash melts, (b) condensation/deposition, and/or (c) reaction with fuel-derived potassium compounds in the gaseous or aerosol phase. Such coating usually starts as potassium silicate, which, in the molten phase, can absorb/dissolve calcium, causing viscous flow sintering and agglomeration.2,3 The adhesive behavior of coatings increases with the increase of the potassium/calcium Special Issue: Impacts of Fuel Quality on Power Production and the Environment Received: April 2, 2013 Revised: July 24, 2013 Published: July 28, 2013 5706
dx.doi.org/10.1021/ef400547q | Energy Fuels 2013, 27, 5706−5713
Energy & Fuels
Article
coal boiler (PCBA). Among the tested materials, PCBA performed best, as seen in Figure 2. Contrary to natural
ratio. Biomass fuels that contain more calcium compared to potassium, combined with low amounts of silicon, will typically form thick coatings with relatively low agglomeration tendency, for example, woody biomass.1−3,5,6 On the other hand, fuels with high potassium and low calcium and silicon contents tend to develop sticky coatings, leading to high agglomeration and, if not controlled, defluidization at bed temperatures typical for fluidized-bed combustion, for example, olive waste.3−5 As the continuous deposition on bed particles proceeds through collision with burning char particles and accumulation of fuel-derived compounds from the gas phase, the inner layer of the coating is homogenized and strengthened via sintering. Further coarsening of the bed material, leading to defluidization, depends upon the temperature-controlled melting behavior and viscosity of the silicate layers superposed around bed particles2 (Figure 1).
Figure 2. Comparison of defluidization times for different materials (modified with permission after Hiltunen and Almark9).
rocks and other synthetic materials with low quartz content, which repel the potassium out of bed material, PCBA seemed to trap potassium in a less reactive form. Possible use of ash coming from pulverized fuel plants firing coal (PC-ash) as an alternative bed material and/or additive in CFB combustion of high alkali biomass fuels was further studied in this work. Both PC bottom ash and PC fly ash were tested in a bench-scale reactor and a 1 MWth CFB pilot to assess its beneficial effects against agglomeration when firing most demanding agro biomass. This paper summarizes the key observations from PC-ash testing and consolidates the role of PC-ash against AFC. Chemical/physical mechanisms are proposed and verified against test results.
Figure 1. Agglomeration mechanisms in biomass combustion (modified with permission after Ö hman et al.2).
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Fouling and corrosion of convective superheaters in biomass combustion is typically connected to the deposition of sticky fly ash and vapors rich in alkali chlorides. The formation of such deposits interferes with heat-exchange efficiency and, in the worst case, can plug the flue gas passages. In addition, deposits rich in chlorine can induce corrosion of superheater tube metal. The mechanisms of fouling and corrosion in biomass combustion have been extensively studied for a long time, and although the basic aspects are well-accepted, the complexity of the details are not yet fully understood.8,9 Several countermeasures have been devised to contain AFC problems in biomass plants, some of which are broadly effective but somewhat costly, while others may be ineffective or harmful if deployed on unsuitable agros. The use of additives often falls in the first category, having broad applicability, high efficiency, high reliability, but high procurement costs. However, ash discarded from combustion plants firing coal can be a convenient exception, because its cost is negligible or even negative. To counteract agglomeration, i.e., formation of sticky coatings on quartz particles, an alternative material with low or almost no quartz can be used as bed materials.5 In an earlier study, Foster Wheeler tested the effectiveness of different alternative bed materials to counteract the agglomeration induced by high-alkali biomass.9 Tests were performed in a defluidization reactor, which was also used in the study described in this paper. All tests were conducted with the same fuel, aspen bark with high potassium content (Table 2). The study included several natural materials, including mineral ores and natural rocks, as well as bottom ash from a pulverized-fired
MATERIALS AND METHODS
The effectiveness of PC-ash to reduce AFC induced by biomass fuels rich in alkali and chlorine was pretested in a bench-scale defluidization reactor (100 kWth) and demonstrated in a large-scale pilot (1 MWth). These two well-established setups provided complementary information about the mechanism and impact of PC-ash on tested phenomena. The defluidization reactor consists of a riser tube divided into bottom (inner diameter of 36 mm and height of 225 mm) and upper (inner diameter of 53.1 mm and height of 669 mm) sections, a bottom grid to allow fluidization air, and feeding lines to load bed material and fuel. The reactor is first heated, and then bed material is added and allowed to attain the target temperature while fluidized with air. The target bed temperature in this work was 850 °C to simulate the bed temperature of a commercial CFB unit. Finally, fuel is fed at a controlled rate until a defluidization signal is detected as sudden changes in pressure and temperature across the bed. The time elapsed between the initial start of fuel feeding and the occurrence of the defluidization signal is defined as the defluidization time, and it quantifies the agglomeration tendency of the fuel/bed pair. In all tests, the bed mass was limited to 26 g (height of 20−40 mm at the stationary state), the fluidization velocity was maintained at ∼0.55 m/ s, and the oxygen level was kept between 6 and 10 vol % (dry). The bed material had size limited between 250 and 500 μm. Fuel was crushed and sieved between 0.25 and 2 mm prior to feeding, and the fuel feeding rate was 1.35−2.04 g/min. The reactor was equipped with conventional (O2, CO2, CO, SO2, and NO) and Fourier transform infrared spectroscopy (FTIR) (H2O, CO2, CO, SO2, NO, N2O, and HCl) online flue gas analyzers; however, here, only the results from SO2 and HCl measurements of FTIR analyzers will be shown. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) of the bed material before and after the 5707
dx.doi.org/10.1021/ef400547q | Energy Fuels 2013, 27, 5706−5713
Energy & Fuels
Article
defluidization tests provided additional information of the occurring phenomena. The defluidization reactor was used for a preliminary evaluation of the effectiveness of several bed materials and additives against agglomeration, including PC-ash.8 In this paper, PC-bottom ash (PCBA) was tested as an alternative bed material, while PC-fly ash (PCFA) was tested as an additive to either sand or PCBA. PC-ash was procured from different plants based on different PC technologies and firing diverse coals. The properties of tested PC-ashes are given in Table 1. Moreover, the defluidization reactor was used to screen the
Table 1. Properties of Tested PC-Ashes
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bulk density (g/dm , dry) moisture (wt %, as received) C (wt %, as received) Na2O (wt %ash, dry) K2O (wt %ash, dry) CaO (wt %ash, dry) MgO (wt %ash, dry) Al2O3 (wt %ash, dry) Fe2O3 (wt %ash, dry) SiO2 (wt %ash, dry) TiO2 (wt %ash, dry) P2O5 (wt %ash, dry) SO3 (wt %ash, dry) Cl (wt %ash, dry) d10 (mm) d50 (mm) d90 (mm)
Table 3. Bench-Scale Experiments bed materiala
fuel aspen bark
sand PCBA: sand PCBA: sand sand PCBA: PCBA: sand PCBA: sand PCBA:
olive 1 olive 2
a
PCFA two samples
straw 1
sand
PCBA four samples
1570 90b 13 34 11 13 25−62 >80b 9 14 76 110−123
PCFA: 2
1 2
Figures 2 and 5 Figure 5 Figure 5 Figures 5 and 11 Fugures 5 and 8 Figures 5 and 11 Figures 5 and 11 Figure 5 Figures 5 and 6 Figures 5 and 10
a
