Reactive jet and cyclonic attrition analysis of ilmenite in chemical looping combustion systems

Highlights

• Reactions affect oxygen carrier (OC) lifetime in chemical looping combustion systems

• Jet and cyclonic attrition testing conducted on ilmenite under high temperature redox conditions

• Chemical changes plus mechanical and thermal stresses contributes to OC attrition

• Showed relevance of multi-parameter attrition test units for OC performance analysis

Abstract

In chemical looping combustion (CLC), the chosen oxygen carrier’s (OC) reactivity maintenance and attrition propensity play key roles in determining its lifetime in such systems. Jet and cyclonic attrition are two main mechanisms of attrition in CLC systems, which typically use fluidized beds and cyclones. In contrast to standardized tests, that assess attrition characteristics under ambient and non-reacting conditions, a realistic evaluation must include the behavior under relevant high temperatures and cyclic oxidation-reduction. In this study, separate jet and cyclonic attrition test units were constructed to investigate the effects of temperature, fuel gas concentration and velocity on the performance of ilmenite, a natural ore-based OC in CLC. Testing showed that thermal and chemical effects were crucial to accurately determine the performance of ilmenite. For experiments conducted between 820°C and 970°C, the intermediate temperature of 895°C provided the best balance between high reactivity and material durability. Additionally, fuel gas concentration affected particle morphology evolution. Iron migration to the surface of the ilmenite particle was enhanced at higher fuel gas concentrations and resulted in reduced attrition rate, presumably due to sintering of the more highly reduced enriched iron phase on the particle surface. The laboratory jet and cyclonic attrition evaluation test units tested in this study can be used to investigate multiple parameters relevant for OC performance, including the degree of reduction, oxygen carrying capacity, attrition resistance, agglomeration, and optimum material operability conditions, as well as to extrapolate test data to determine OC replacement costs in a commercial CLC system.

Introduction

Chemical looping combustion (CLC) has emerged as an attractive alternative for CO₂ capture from carbonaceous fuel-fired systems, where a near-pure CO₂ stream is produced without the use of oxygen obtained from an air separation unit (ASU). In this type of system, a solid carrier is used to bring oxygen to the fuel to convert it to a pure CO₂ stream. The solid is then regenerated separately using air. Although promising, CLC has many challenges, including the need for high oxygen carrying capacity and physically/chemically durable oxygen carriers (OC).

The majority of CLC units use the configuration of two interconnected fluidized bed reactors, one being the fuel- and the other the air- reactor (Adánez et al., 2018). Interconnected fluidized bed systems can provide the necessary residence time and good gas/solids contacting for oxygen uptake and release, as well as enable efficient segregation of the solids from the gas streams using cyclones. However, the mechanical forces acting on the OC within and during transport between the fuel and air reactors will cause attrition. OC attrition is an important issue to address with interconnected fluidized bed reactors (Cabello et al., 2016; Lyngfelt and Linderholm, 2014). Eventually, the OC becomes so fine it cannot be retained in the process. In addition, the repeated oxidation and reduction cycles of the OC material causes structural changes (Knutsson and Linderholm, 2015, Wei et al., 2013). The addition of chemical and thermal effects may negatively affect OC reactivity and decrease the material’s mechanical strength. Loss of reactivity and material loss due to attrition will necessitate OC material replacement and a potentially significant operating cost burden on the CLC process.

The attrition behavior of OCs is a very important characteristic for their use in full-scale systems. In a circulating fluid bed (CFB) system there are several modes that contribute to the material undergoing attrition when placed under thermal, chemical, or mechanical stress (Bayham et al., 2017). All modes have proven to add to a material’s overall attrition, with some of these being more destructive than others. Bemrose and Bridgwater identified the regions of fluidized beds where attrition is most likely to occur (Bemrose and Bridgwater, 1987). Within the bed, attrition originates from bubbling, grid jets, and splashing of ejected particles. During material transport, cyclones are another region where attrition is likely due to particle impaction (Lyngfelt and Linderholm, 2014; Bemrose and Bridgwater, 1987; Galinsky et al., 2017; Kunii and Levenspiel, 2013). Materic et al., noted that there are three main attrition mechanisms, and that these mechanisms are activated based on the velocity of the system (Materic et al., 2014). Under low velocity particle-particle collisions, which are bubble induced, abrasion dominates and very fine particles are produced. These fine particles are typically elutriated from the bed. Under high velocity collisions – grid jet and cyclonic (material transport) – particle fragmentation is the main mechanism. With particle fragmentation, particles fracture, and the bed distribution has fewer coarse particles, and these fractures eventually lead to fines production (Brown et al., 2012).

Cyclonic attrition is an important mode to consider during the OC transport between the fuel reactor and the air reactor. Cyclones enable gas-solid carrier separation during this transport between the coupled reactor beds. Under cyclonic separation, particles are accelerated, promoting high velocity particle-particle collisions and particle-wall impacts, as well as tangiential particle-wall abrasive forces.

This study focuses on the investigation of two attrition modes that are induced at high velocity – grid jet attrition common to the fluidized bed reactor, as well as cyclonic attrition commonly observed in material transport.

The standard attrition test that would be the basis for the evaluation of OCs is the ASTM D5757 test method for determining the attrition characteristics of powdered catalysts by air jets (ASTM D5757-95, 1995). This apparatus was designed for attrition to be tested at ambient temperatures and in a non-reactive environment. In a properly constructed jet system, the interaction between particles is dominant. This test determined the relative attrition characteristics of powders by means of gas jets to provide information about the ability of powders to resist particle size reduction in a fluidized bed environment.

The ASTM air-jet test method has important advantages and disadvantages. The test requires about fifty grams of sample per test run; this affords a small sample investment for the screening process. On the other hand, the ASTM test is designed to be operated at low temperature and in a non-reactive environment. These conditions are not representative of CLC systems. To accurately mimic the attrition of OCs under typical CLC process conditions, it would be necessary to operate the attrition test at realistic operating temperatures and in continuously cycled oxidizing/reducing atmospheres (Rydén et al., 2014).

While prior work related to attrition testing has been valuable, jet cup experiments need to be conducted at more realistic CLC operating conditions to capture the impacts of high temperature reactions on particle structure, strength and attrition characteristics. Rydén et al. (2014)) and Cabello et al. (2016) have conducted experiments using fresh and used OC obtained from other test units. The attrition of the materials were determined from ambient jet cup attrition units. The researchers conclude that chemical and thermal effects should be built into these test units since significant differences were observed between the attrition rates of used versus fresh OCs (Rydén et al. (2014); Cabello et al. (2016)).

Through a baseline screening of various OCs in unpublished work by these authors, natural ores, in many cases, have stronger material structures than engineered materials. These structures have shown to better withstand the stresses brought on by the CLC process conditions. Ilmenite typically has a positive balance of relatively low attrition and high reactivity compared to the other carriers tested. Ilmenite has also been considered by other researchers as a viable OC (Leion et al., 2008; Azis et al., 2010; Chen et al., 2017). This is also in part due to its low cost. Chen et al. (2017) described ilmenite ore as found naturally reduced and primarily composed of FeTiO₃. Its fully oxidized form consisted of FeTiO₅, TiO₂, and a small amount of Fe₂O₃. When utilized as an OC, its active oxidized phases are Fe₂TiO₅ and Fe₂O₃ (Cuadrat et al., 2012). Cuadrat et al. (2012) found that to achieve its maximum oxygen carrying capacity, it must be activated by subjecting it to multiple redox cycles; this step creates pores, increasing reactivity (Cuadrat et al., 2012). Once activated, the OC reactivity stabilizes.

This study focuses on detailed testing of ilmenite as an OC, further evaluating its strengths and sensitivities under various operating conditions. During this study, ilmenite was acquired from several sources in various size ranges. The performance of the individual ilmenite samples differed from each other in terms of reactivity and attrition performance. The comparisons of ilmenite performance with different particle sizes were conducted using screened particles of the same ilmenite source to eliminate variation between other sources of ilmenite.