Article pubs.acs.org/EF
Ash Properties of Ilmenite Used as Bed Material for Combustion of Biomass in a Circulating Fluidized Bed Boiler Angelica Corcoran,* Jelena Marinkovic, Fredrik Lind, Henrik Thunman, Pavleta Knutsson, and Martin Seemann Department of Energy and Environment, Division of Energy Technology, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden ABSTRACT: Both agglomeration of bed material and corrosion of heat transfer equipment are issues related to combustion of biomass in a fluidized bed boiler. The biomass-ash component potassium is considered a major contributor for both phenomena. In this study, the conventionally used bed material, silica sand, was replaced with up to 40 wt % by the natural ore ilmenite in Chalmers 12 MWth circulating fluidized bed (CFB) boiler. In this study the purpose was to evaluate the physical and chemical changes ilmenite undergoes during this process. Close observations revealed that ilmenite underwent segregation of iron to the surfaces and an enrichment of titanium in the particle core. The ash formed a calcium-rich double layer on the particle, surrounding the iron layer. A diffusion of potassium into the particle core was also seen which led to the formation of KTi8O16. In addition to evaluating how ash components interact with the material, the ilmenite was leached and investigated as a possible potassium capturer. Leaching experiments on the used ilmenite showed that calcium and potassium were leachable to a very limited degree, namely, to less than 0.2 and 1 wt %, respectively, of the total content. The diffusion of potassium into the core of the particle could reduce both agglomeration and corrosion issues and could thereby be of great value for the improvement of the resistance of the bed material agglomeration in the fluidized bed boiler.
1. INTRODUCTION As awareness of climate change continuously increases, research which focuses on energy sources resulting in low climate impact has become of great importance. Biomass is considered to be a CO2 neutral and renewable energy source that has, however, high fuel variability and which makes its thermal conversion problematic. Some of the variables that most affect thermal conversion are the moisture content as well as the inorganic constituents of the biomass.1 An extensive overview of biomass, including a classification of composition, shows that the major elements in biomass are, in decreasing order of abundance; C, O, H, N, Ca, K, Si, Mg, Al, S, Fe, P, Cl, and Na.2 Due to the wide variety of biomass composition, fluidized bed boilers are a common technology used for its thermal conversion as it provides high fuel flexibility.3 The ash composition of biomass is associated with difficulties during combustion such as agglomeration and sintering of bed material.3−6 If the agglomeration becomes too extensive, it will result in defluidization and unplanned shut down of the plant. The main parameters that influence the agglomeration of bed materials are temperature and the content and composition of the ash. The amount of melted ash increases with rising temperatures and results in an increase of the adhesive tendency of bed particles, thereby causing the formation of agglomerates.4 Ash rich in potassium is considered to be particularly problematic in the agglomeration of silica sand due to the eutectic mixture with low melting point they form.5 As a result of the temperature and reducing conditions in the combustion zone, alkali metals such as potassium may be released from the fuel during vaporization, during which it is either captured by the sand or released to the gaseous phase. Consequently the high potassium content in the ash could, © 2014 American Chemical Society
apart from causing agglomeration, lead to severe fouling and corrosion of the heat transfer equipment. The vaporization of potassium is often enhanced by high chlorine content in the fuel which promotes the formation of alkali chlorides and hydroxides.1,7 As temperature decreases in the convection path, the gaseous potassium compounds condense on the walls of the heat exchangers, where the deposits then increase corrosion and reduce heat transfer.1,8 The vaporization of alkali compounds may be reduced by using the additives kaolin and bauxite9,10 or by lowering the operational temperature. The latter may, however, have a negative impact on the boiler efficiency. Wiinikka et al.8 have provided an alternative method to prevent the release of alkali metals. The authors proposed the introduction of TiO2 as an additive to prevent the vaporization of potassium. As a result, a reaction between potassium and TiO2 occurred, leading to the formation of the crystalline compound KTi8O16 which is found in the bottom ashes. The work here presents a method to capture potassium in the bottom ashes through the use of an unconventional bed material. The procedure is to fully or partly replace the used bed material, silica sand, with a bed material used in chemicallooping combustion (CLC) processesthe naturally occurring ore ilmenite, FeTiO3. The use of oxygen-carrying bed material is an accepted procedure11,12 and is based on its capability to absorb and release oxygen during the combustion process. Hence, oxygen is made accessible throughout the boiler. Not only does this promote complete combustion but also the Received: August 13, 2014 Revised: November 6, 2014 Published: November 10, 2014 7672
dx.doi.org/10.1021/ef501810u | Energy Fuels 2014, 28, 7672−7679
Energy & Fuels
Article
temperature gradients generally smooth out.13 Hence, as ilmenite improves the distribution of oxygen in the furnace, it reduces the risk for hot spots14 and the agglomeration of bed material. Apart from its oxygen-carrying capacity, ilmenite has been shown to be an attractive alternative with regard to its bulk price which is comparable to that of silica sand.11 The current work examines ilmenite particles used as bed material in a 12 MWth circulating fluidized bed (CFB) boiler fired with biomass and focuses on the chemical and physical changes of ilmenite after its exposure as bed material. Initial components of the mineral as well as components originated from the biomass ashes have been examined. The aforementioned positive effects on the operation are presented by Thunman et al.14
In the literature, various bed materials, additives, and ash components have been investigated using different fluidizing technologies. Apart from studies on ilmenite, the mineral olivine, (Mg,Fe)2SiO4, has been included here as it shows similarities in its composition and behavior toward ash. Olivine is a mineral which naturally contains iron and is often used in gasification processes due to its catalytic activity regarding tar reduction.19,20 The catalytic active component in olivine for tar conversion has both been connected to the iron in olivine21 as well as layer formation of calcium from the ash.22 Layers formed on ilmenite and olivine that have been observed containing calcium or iron are presented as follows. Iron Layer. A resulting iron layer has been found on the bed materials ilmenite and olivine, which both naturally contain iron, in studies using fluidizing technologies. In an attempt to study the catalytic activity of olivine in a fluidized bed biomass gasifier, iron-rich areas were found on the surface of the particles after calcination. The migration observed was nonuniform and increased with the calcination time for up to 10 h where the content was as much as 34 wt % in some positions.23 In further CLC studies by Cuadrat et al.12 a migration of iron was not found when ilmenite was calcined for 24 h. Results demonstrated that the same batch of an activated ilmenite revealed an iron layer and titanium enrichment in the core of the particle. The segregation increased with the number of redox cycles and was noted by the increase of free TiO2 in the particle core. This was confirmed by Adánez et al.18 in a similar study with ilmenite. The performance of ilmenite as an oxygen carrier was tested in a laboratory scale fluidized bed reactor by Azis et al.24 with the separate addition of two different ash types. The ashes used in the study were Chinese bituminous coal ash and German lignite coal ash. The bituminous ash, which contained SiO2 and Fe2,946O4, caused a segregation of iron to the surfaces of the ilmenite particle. The lignite ash, which consisted of SiO2, Fe2O3, CaO, CaSO4, CaFe3O5, Ca3Al2O6, and MgO, did, however, not form an iron layer on the ilmenite.24 Hence, an iron layer was observed on calcined olivine and on activated ilmenite with and without the addition of ash containing quartz and an iron oxide. Conversely, the iron layer was neither observed on calcined ilmenite nor on activated ilmenite with ash containing quartz, an iron oxide and, notably, several calcium compounds. Calcium Layer. Ash layers formed on bed materials are extensively represented in literature. This section will consider the findings of layers containing the ash component calcium. For example, two calcium-rich layers were found in a study where olivine was used as bed material in a dual fluidized bed steam gasification plant. 22 The inner layer was more homogeneous while the outer layer had a composition similar to the one found in the fly ash from biomass. It was also noticed that the core of the particle showed an increase of potassium. According to Kirnbauer and Hofbauer22 the layer formation followed the third mechanism described earlier. The homogeneous layer with a constant thickness was formed by a reaction of the ash components and the bed particle where a layer was grown into the particle. The outer layer, on the contrary, was inhomogeneous and did not reveal constant thickness, implying that it had grown outward from the particle surface.22 Furthermore, during a study of agglomeration characteristics on olivine in a bench scale bubbling fluidized bed reactor, a similar double layer of calcium was found on the olivine particles.15 It was also found that the inner layer was
2. THEORY Agglomeration of bed material, which is a major cause of defluidization, is initiated by the formation of layers around the bed material particles. Several studies5,6,15 show, for sand used as bed material, that potassium compounds most probably initiate the formation of these layers around silica sand particles. The low melting point of the potassium compounds formed during combustion allow for evaporation at an early stage of combustion.7 Evaporated alkali salts react with the bed material particles and form eutectic mixtures of potassium silicates on the surfaces of sand particles. The potassium silicates are sticky and accelerate the process of coating by including other ash components such as calcium on the sticky surface. According to Zevenhoven-Onderwater et al.6 there are three ways for a layer to form: (1) The bed particle is considered to be inert and only acts as a carrier of the layer. In this case the layer grows outward beyond the particle and consists of elements originating from the fuel. (2) The formation is caused by a reaction between the bed particle and elements from the fuel or additives. In this case the layer grows inward into the particle. (3) The layer can also be formed by a combination of (1) and (2). The enumerated mechanisms are defined for a case where silica sand is used as bed material but can still be applied to any other material. In all of these instances, the authors conclude that the layer formation is limited by erosion of the layer and/ or diffusive possibilities. An iron-rich layer, which could not be included in the formation mechanisms mentioned earlier, has been observed on ilmenite particles that have undergone repeated redox reactions in CLC processes. According to Rao and Rigaud,16 the separation of iron and titanium in ilmenite is due to oxidizing conditions. The authors found that Fe2O3 was formed as a layer on the surface of ilmenite, through which oxygen diffuses and reacts further with ilmenite at the FeTiO3/Fe2O3 interface during oxidation below 770 °C. It was also concluded by the authors that pseudorutile (Fe2Ti3O9) was formed during oxidation of ilmenite between 770 and 900 °C, which is the operating temperature span for a fluidized bed boiler. It was suggested that iron ions are mobile through pseudorutile. Thus, segregation takes place where iron ions migrate to the surfaces of the ilmenite particle where Fe2O3 can be formed. This has been confirmed by more recent studies17,18 in which the iron layer formed is enhanced by the number of redox cycles the ilmenite undergoes.18 7673
dx.doi.org/10.1021/ef501810u | Energy Fuels 2014, 28, 7672−7679
Energy & Fuels
Article
Figure 1. Schematic representation of the 12 MWth CFB boiler at Chalmers University of Technology.
Table 2. Ash Composition of Woodchipsa
more homogeneous while the outer layer was granular and similar in composition to the fly ash.15
ash content wt % in ash obtained at 550 °C
3. EXPERIMENTAL PROCEDURE The boiler in which the experiments were conducted was the CFB boiler/gasifier reactor system at Chalmers University of Technology. A basic scheme of the system is shown in Figure 1 and is described in detail by Thunman et al.14 The 12 MWth CFB boiler is used to produce hot water for district heating and is operated from November to April. The district heating from the boiler is predominantly used by Chalmers facilities. The combustion chamber, which has a crosssection of 2.25 m2 and a height of 13.6 m, is fed with biomass via the fuel chute on top of the bottom bed. Ordinarily the boiler is operated with 2000 kg of silica sand as bed material. The boiler is equipped with a number of extraction ports, H1−H13 in Figure 1, where samples of bed material and bottom ashes can be extracted at H1−H3 while gas samples can be extracted at H4−H13. Furthermore, fly ash samples can be extracted from the secondary cyclone and as finer fractions from the textile filter, referred to as 6 and 7, respectively, in Figure 1. In the conducted experiment, ilmenite was co-used up to 40 wt % (800 kg) with the silica sand. The fuel used in the CFB boiler during the experiment was woodchips with the composition on a dry basis shown in Table 1. The ash composition of the woodchips is shown in Table 2. The used ilmenite was supplied by Titania A/S (Hauge, Norway). It was roughly 95% pure and concentrated from a natural
wt % dry basis
C O H N S Cl Ash
49.7 44.0 5.9 0.12
