Ilmenite as alternative bed material for the combustion of coal and biomass blends in a fluidised bed combustor to improve combustion performance and reduce agglomeration tendency
Introduction
For decades fossil fuels have been the main energy source worldwide, contributing to over 80% of the total primary energy use worldwide in 2019 [1]. Coal is still the main resource for electricity and heat generation in many parts of the world such as India, China, and the United States. Coal combustion in the world power and heat generation sector accounted for 30% of all energy-related CO2 emissions, exceeding 10 Gt CO2 in 2018 [2]. The continued dominance of fossil fuels, particularly coal, in the world energy mix is largely due to the slow uptake of low-carbon technologies [3,4]. According to IEA [2], CO2 emitted from coal combustion was responsible for over 0.3 °C of the 1 °C increase in global average annual surface temperatures above pre-industrial levels. This makes coal the single largest source of global temperature increase.
Co-combustion of coal and biomass is a promising alternative in the short-term for reducing the deleterious effect of coal use in the production of electricity and heat but it still faces unsolved challenges [5,6]. Co-combustion of biomass and coal in comparison to exclusive coal use leads to the reductions of greenhouse gas (GHG) and harmful emissions such as NOx, SOx, CO. Furthermore, co-combustion can lower the cost of biomass utilisation through adapting existing dedicated coal combustion facilities rather than building new dedicated biomass combustion facilities [7]. However, the maximum ratio of biomass in the fuel blend in most commercial co-combustion applications has been limited to 5–10% (on an energy basis), although 20% is currently feasible and more than 50% is technically achievable [8,9]. Despite the potential for greater reductions in CO2 and combustion-generated pollutant emissions with a higher biomass ratio in the biomass and coal co-combustion, the current inability to address operational challenges linked to the biomass properties and ash characteristics, for example, high levels of alkali and alkaline metals in the fuel ash and the lower ash melting point in comparison to coal ash, limits the increase of biomass ratio in co-combustion applications.
The combustion efficiency of solid fuels in a combustion process is largely depending on the contact of oxygen and fuels during the combustion process and the stability of the process [10]. The better the contact between the oxygen and fuels, the lower the excess air is required to achieve a high combustion efficiency. For a fluidised bed (FB) combustion process, better oxygen and fuel contact could also lead to even temperature distributions in the FB combustor/boiler, reducing the likelihood of hot spots and the risk of agglomeration, leading to further benefits of less emissions of NOx, SO2, CO and lower levels of unburnt carbon in the ash [[11], [12], [13]]. Recently, ilmenite, an iron-titanium oxide natural mineral, was used in the investigation of a novel concept called oxygen carrier aided combustion (OCAC) in fluidised beds. It had been shown to have the potential to improve the combustion efficiency while alleviating/solving the ash related issues that were originated from the alkali and alkaline metals in the biomass ash [12]. Replacing the bed material, totally or partially, with a solid oxygen carrier could enhance the contact between the oxygen and fuel. In the oxygen-lean regions within a FB combustor/boiler, the oxygen carrier material may provide the required oxygen for combustion. The oxygen carrier mainly reacts with the volatiles released from the solid fuels into CO2 and H2O [14]. Other reactions that may occur in a lesser proportion are between char and the oxygen carrier [15]. The iron oxide in ilmenite has several oxidation states. When considering the different reduction degrees, the Fe2O3–Fe3O4 step is faster than the steps of Fe3O4–FeO and FeO–Fe [16]. In general, the reaction rate of Fe2O3 with different fuels decreases with the following order: H2 > CO > CH4 > solid fuels [14]. The direct reduction of iron oxides by solid fuels is very slow and iron oxides cannot directly release gas phase O2 via oxygen uncoupling [12]. However, the reduction of Fe2O3 by solid fuels either in the presence or absence of CO2/H2O can be significantly enhanced by alkali metals [17]. Furthermore, more Fe2O3 can be reduced to FeO when Al2O3 is coupled to produce FeAl2O4, which has a high oxygen transport capacity [14]. Biomass often contains higher amounts of moisture and volatiles compared to coal. When burning high volatile fuels the lateral mixing may be insufficient when using conventional bed material such as silica sand, resulting in a requirement of a large amount of excess air [18]. An oxygen carrier has the possibility to not only facilitate the distribution of heat in the combustor, as silica sand, but also to even out the oxygen distribution. This can consequently result in a decreased amount of excess air required and increase the combustion efficiency [18].
Thunman et al. [13] carried out OCAC combustion experiments on a 12 MWth circulating fluidised bed, using ilmenite as the additive (up to 40 wt%) to the bed material of silica sand and burning woody biomass fuels. Their results showed CO and NOx emissions were reduced by 80% and 30%, respectively, in comparison with the case of silica sand as the bed material, and the authors attributed these reductions in CO and NOx emissions to the addition of ilmenite to the bed [13]. Furthermore, there was a reduction in the accumulation of deposits on the heat exchange surfaces. Hughes et al. [19] investigated the use of ilmenite as an additive and alternative bed material to improve the combustion performance and sulphur capture in a 50 kWth pilot-scale fluidised bed combustor under atmospheric oxy-fuel combustion conditions using two Canadian coals. Their results showed replacing the silica sand bed material with ilmenite led to a reduction of CO emissions by up to 30% and 13% corresponding to the two coals used. The CO reduction was found to be more prominent at low oxygen concentration in the flue gas. Furthermore, no agglomeration of the ilmenite bed material was found during the experiments. Wang et al. [10] studied the combustion performance and NO emissions of wood char combustion in a small fluidised bed reactor at different air to fuel ratios with four oxygen carriers (ilmenite, manganese ore and two by-product oxides from steel production) and quartz sand as the bed materials. Their results showed the use of oxygen carriers as the alternative bed material instead of the quartz sand led to an improvement in combustion efficiency and reduced CO emissions. The use of oxygen carriers as the bed material made it possible to decrease the excess air and, thereby, lower the NO emissions, while keeping the same level of CO emissions. The increase in combustion efficiency was attributed to the reactivity of the oxygen carriers with CO. Among the oxygen carriers under study, manganese ore showed better performance in improving combustion efficiency and reducing CO emissions. However, it agglomerated earlier than the other oxygen carriers during the combustion tests [10].
Agglomeration and defluidisation are the well-known operational issues associated with biomass FB combustion boilers/combustors [20]. The alkali metals in biomass ash can be readily vaporised due to being present in ionic forms or organically bound rather than associated with minerals [21]. The released alkali metals can interact with the bed material during thermal degradation. When silica sand is used as the bed material, the alkali metals from the biomass ash can react with silica forming low-melting silicates characterised by lower melting point temperatures than the individual components [22]. As a result, the sand particles are coated with a sticky surface that then grows to larger agglomerates [23,24]. Agglomeration with the bed material is a major problem that can be difficult to detect and can propagate to the whole bed resulting in an unscheduled shutdown for replacement of the bed material, adversely affecting the cost and reliability of the process [[25], [26], [27]]. The use of oxygen carrier materials as bed materials or additives in OCAC applications may impact the agglomeration and defluidisation tendency of the fuels to be burned. To the best of authors’ knowledge, few have specifically investigated the agglomeration and defluidisation behaviour of a fluidised bed combustor co-firing coal and biomass blends using an oxygen carrier (e.g. ilmenite) as the bed material or as an additive to the bed material of silica sand.
So far, few have paid particular attention to the potential capture of alkali and alkaline metals by oxygen carriers in OCAC combustion investigations. Corcoran et al. [28] studied the physical and chemical changes in ilmenite under OCAC woody biomass combustion conditions. Their results revealed the segregation of iron to the surface and the enrichment of titanium in the particles core along with the inward migration of K into the particles. In addition, the ash formed a calcium-rich double layer on the particle surface, surrounding the iron layer, and there was the formation of KTi8O16 as a consequence of the diffusion of K into the core of the particle. Through more recent combustion experiments, Corcoran et al. [25] also found that longer process times led to the formation of a calcium layer around the particle surface and the migration of calcium into the particle.
The focus of this study was the investigation of the effect of ilmenite on the most representative flue gas emissions (i.e. CO, and NOx), combustion performance and agglomeration tendency when it was used as the alternative bed material in the co-combustion of coal and biomass in a pilot-scale fluidised bed combustor. For comparison purposes, tests of co-combustion of the same coal and biomass blend were carried out with silica sand as the bed material.
Section snippets
Fuels and materials
The bituminous coal used in this study was supplied by the Newark Factory, British Sugar plc in the UK as ‘washed singles’ with 90 wt% of the particle size within the range of 12.5 mm and 28 mm. The same coal was used by the Newark's industrial-scale fluidised bed combustion boiler for producing animal feed by drying sugar beet pulp subsequent to the sugar extraction process. The wheat straw was supplied by Agripellets Ltd (UK) in the form of pellets with the average diameter of 6 ± 0.25 mm and
Effect of ilmenite as alternative bed material on the axial combustor temperature profile
Fig. 2 shows the axial temperature profile along the combustor height when ilmenite was used as bed material compared to silica sand. The results show that ilmenite enhanced fuel conversion in the dense bed region and less combustion took place in the freeboard in comparison to the silica sand bed. The use of an oxygen carrier as bed material can facilitate the combustion in the dense part of the bed and to some extent in the freeboard. Therefore, a more pronounce temperature drop for the
Conclusions
The effect of ilmenite on the gas emissions (CO, and NOx), combustion performance and agglomeration tendency when used as alternative bed material under co-combustion conditions of coal blended with wheat straw pellets at 40 wt% was investigated in a 30 kWth pilot scale BFB combustor. The experiments were conducted at atmospheric pressure and 900 °C. For comparison purposes, silica sand was used as the baseline bed material. The agglomerates and the cyclone ash particles obtained from the
Credit author statement
Eduardo Garcia: Investigation, Formal analysis, Validation, Conceptualization, Methodology, Writing – original draft. Hao Liu: Conceptualization, Methodology, Writing- Reviewing and Editing, Supervision, Project administration.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The research was supported by a Doctoral Research Fellowship from the Administrative Department of Science, Technology, and Innovation of Colombia (COLCIENCIAS) - Newton-Caldas Fund - ‘Convocatoria 679–2014’.
The authors would like to thank the Newark Factory of British Sugar plc (https://www.britishsugar.co.uk/), Agripellets (https://www.agripellets.com/), and Titania A/S (https://kronostio2.com/en/manufacturing-facilities/hauge-norway) for the support on the tested coal and silica sand,
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2023, FuelCitation Excerpt :In the past decade, OCAC technology is developed rapidly and has attracted the attention of industry and academia. Major research institutions include Chalmers University of Technology [6,10–20], University of Cambridge [21], Tsinghua University [22], CanmetENERGY [8,23–25], University of Nottingham [26], Southeast University [9]. They verified the effectiveness of OCAC technology by studying the influences of the types of OCs and fuels, oxygen concentrations and bed temperatures in FB combustors with different scales.