Article pubs.acs.org/EF
Reuse of Spent Sorbents from FBC for SO2 Capture by Simultaneous Reactivation and Pelletization Yinghai Wu, Vasilije Manovic, Ian He, and Edward J. Anthony* CanmetENERGY, Natural Resources Canada, 1 Haanel Dr., Ottawa, ON K1A1M1, Canada ABSTRACT: Sorbent utilization of CaO-based sorbents for in situ SO2 capture in fluidized bed combustion (FBC) is far from quantitative, and in consequence FBC residues contain a significant amount of unreacted CaO. Treatment of bed materials by hydration with liquid water or steam can reactivate the spent sorbent for further SO2 capture, although, to date, the costs of such processes have deterred its practical use. By contrast, fly ash is already very reactive, but given its short residence time in the combustor, direct reuse of fly ash appears to be an ineffective strategy. This is significant as fly ash often accounts for the majority of solid waste streams discharged from FBC systems. This paper describes a new technique for reactivation of FBC spent sorbent and preparation of pellets suitable for SO2 capture, which can also incorporate the fly ash into the pellets so that it has an adequate residence time in the primary combustion loop of a CFB to realize improved sulfur capture. Reactivation and pelletization of the spent sorbent were achieved simultaneously in a mechanical pelletizer with the addition of spray water. Four types of pellets were prepared with various proportions of bed ash and fly ash. Quick lime (CaO) powders were also tested as a useful additive for the pelletization process. The effectiveness of the reactivation technique was tested by the nitrogen physisorption, which confirmed that a more suitable pore surface area and pore volume distribution for sulfation were developed. The SO2 capture potential of the pellets was also examined in a thermogravimetric analyzer. The reactivated pelletized sorbents showed an improved sulfation rate in comparison to both the original sorbent and the spent sorbent, particularly during the diffusion-controlled reaction stage. and NaCl.16,17 The basic reaction during hydration of partially sulfated sorbent occurs as follows:
1. INTRODUCTION Fluidized bed combustion (FBC) is an established combustion technology for solid fuels (including high sulfur coals and petroleum coke). Calcium-based sorbents (calcitic limestone and dolomite) are typically used for SO2 capture in FBC systems burning high-sulfur fuels.1,2 One of the major limitations of the technology is the relatively low utilization of sorbent (typically, 30−40%). As a result, excess limestone sorbent is required to achieve an acceptable SO2 capture efficiency, and a typical Ca/S molar ratio of 2−3 is normal for an industrial FBC boiler for >90% SO2 removal. Excessive sorbent use adversely affects the economics of the FBC technology, as the cost for both limestone use and ash disposal for the spent sorbent increase. Moreover, the CO2 emission is slightly increased because extra limestone has to be used and calcined in the combustor.3 Another critical issue associated with the use of excessive sorbent quantities is a high level of unreacted calcium oxide (20−30% CaO) contained in the FBC solid residues (i.e., bed ash and fly ash) especially when firing high-sulfur fuels. Hence, FBC ash can produce highly exothermic reactions when in contact with water, creating various problems during handling and disposal with potential safety issues, and further, the ash produces high-pH leachate from the landfill, which must also be treated. In addition, the presence of significant quantities of CaO in the landfill ashes can also cause expansion due to the formation of ettringite, which further increases the cost for ash disposal.2 The most thoroughly investigated method to improve sorbent utilization is hydration by steam or liquid water,4−10 which may be enhanced by grinding11 and sonication12,13 and possibly improved via carbonation14,15 or addition of Na2CO3 © 2012 American Chemical Society
CaO + H 2O → Ca(OH)2
(1)
Water (or steam) reacts with CaO in the core and cracks the sulfated shell, because Ca(OH)2 has a larger molar volume (33 cm3/mol) than that of CaO (17 cm3/mol). The unreacted core is then exposed so that the capacity for additional sulfur removal can be substantially restored for the hydrated sorbent once it is reintroduced into the combustor. Although hydration is an effective reaction strategy for the bed material, there are two issues to be addressed. First, the hydrated material tends to be fragile since the hydrated particles show severe cracks throughout their structure, which means that such materials will have a reduced lifetime in an FBC boiler due to fragmentation or attrition.18,19 Second, it appears that, typically fly ash is not reactivated by hydration,20 as fly ash does not have a core−shell structure, which seems necessary for effective reactivation with water or steam.21 This, in itself, may not be important, given that such material is already very reactive; however, fly ash has an insufficient residence time in the primary reaction loop of a CFB to allow it to achieve adequate sulfation, as such particles are too small (d50 = 30−40 μm). Therefore, the main challenge in improving its sulfation characteristics is to increase its residence time in the combustor.20 This leaves pelletization of this material as a reasonable solution. Furthermore, it would be of economic Received: August 10, 2012 Revised: November 13, 2012 Published: November 14, 2012 82
dx.doi.org/10.1021/ef301328h | Energy Fuels 2013, 27, 82−86
Energy & Fuels
Article
interest to reuse fly ash for sulfation because fly ash often accounts for the bulk of the wastes formed from FBC. It should be noted that providing O2 is in excess; char carbon does not itself appear to influence sulfation, although, under reducing conditions, it causes CaSO4 to decompose.20 In this work, a novel reactivation method that combines hydration and pelletization in one process is explored. Both bed ash and fly ash were used, and a pelletized-type of synthetic sorbent are produced with or without extra CaO additive. The additional SO2 capture by produced pellets was investigated using a thermogravimetric analyzer (TGA).
Four different pellet types were produced, and their composition is given in Table 2. These pellets are classified into two types: one set of
Table 2. Component in Pellets (wt %)
2. EXPERIMENTAL SECTION
Table 1. Elemental Composition, in Oxide Form (wt %), of Limestone (KR), Bed Ash (BA), and Fly Ash (FA) KR
BA
FA
SiO2 Al2O3 Fe2O3 CaO MgO SO3 K2O Na2O loss on fusion
2.95 0.74 0.32 52.43 0.47 0.30 0.21
