Oxygen carrier aided combustion: Implementation of oxygen carriers to existing industrial settings

Utilization of biomass and waste to produce heat and power is necessary for a sustainable future energy mix. Thermal conversion of biomass is considered to yield CO2 neutral emissions and the utilization of waste reduces its volumes in a world struggling to dispose of ever-increasing amounts. However, these fuels can be difficult to combust as they are complex in their composition. One technology allowing for conversion of both biomass and waste is fluidized bed conversion. Commonly, fluidized bed boilers are operated with an excess of air which lowers the efficiency of the plant. Replacing the quartz sand used as bed material during biomass and waste conversion with an oxygen carrier is referred to as oxygen carrier aided combustion (OCAC). By this replacement, oxygen availability is increased throughout the combustion chamber with increased boiler efficiency as a consequence.This thesis presents the implementation of oxygen carriers to existing industrial units. The development has been rapid due to the possibility of conducting research through integration of scales. Experiments on semi- and full industrial scale validated the improved distribution of oxygen. Bed materials extracted from the industrial units were analyzed and tested for their oxygen transferring capacity on laboratory scale. The evaluation of bed materials provided for an understanding of how oxygen carriers can be utilized for the concept. The oxygen carriers included in this work are two types of ilmenite: sand and rock, and a manganese ore. This work provides a comprehensive understanding of how the bed material develops regarding oxygen transfer, as well as chemical and mechanical resistance. Sand and rock ilmenite show different characteristics when exposed to the process. When following their progression of iron and structural development after being subjected to OCAC, sand ilmenite develops cavities inside the particles to which iron migrates and which further causes mechanical instability and shattering of particles over time. Iron migrates to surfaces on rock ilmenite particles, which are decomposed by splitting. The materials interact similarly with main ash constituents of the fuel. A heterogenic outer layer is formed, consisting mainly of Ca but also traces of other elements from the fuel ash. Ca and K diffuse inward and are incorporated in the ilmenite structure. The ash interactions are not found to directly inhibit the oxygen carrying capacity, however, a decline in capacity is noticed as ash layers build up and become thicker. This work shows that the oxygen carrier ilmenite can be implemented in existing industrial settings, without reconstruction of the current system. Optimization measures are proposed where magnetic separation allows for reuse of bed material that still contain oxygen transferring capacity and the regeneration of bed material can be decreased in comparison to quartz sand. Thus, the results of this thesis suggest that OCAC is a feasible concept for conversion of complex fuels

A Scale-Up Project for Operating a 115 MWth Biomass-Fired CFB boiler with Oxygen Carriers as Bed Material

Oxygen Carrier Aided Combustion (OCAC) is a concept that uses an oxygen-active bed material to increase the overall efficiency in fluidized bed (FB) combustors. The introduction of oxygen carriers (OCs) in existing FB plants is an attractive opportunity to investigate OCs under industrially-relevant conditions at a low economic risk. In this way, important experience and knowledge of the physiochemical properties of OCs can be gained during long-term operation, which in turn can be used for the scale-up of oxygen-looping techniques, e.g. chemical-looping combustion. The overall aim of this experimental study was to investigate, develop and collect data to increase the knowledge of how OCs can be deployed in commercial FB boilers, while at the same time granting the plant operator benefits from an increased revenue. This paper summarizes the first results from an experimental campaign performed during three weeks of OCAC operation in a 115 MWth commercial CFB boiler, which is fired with a mix of recycled waste-wood and wood chips. During the campaign, the silica-sand bed material was gradually replaced by the mineral ilmenite. It was shown that ilmenite operation, in comparison to operation with silica-sand, facilitated a reduction in the air surplus by as much as 30 %, while increasing the boiler load from 115 MWth to 123 MWth. During ilmenite operation no problems related to the external or internal bed-material logistics nor in the overall boiler operation. However, during ilmenite operation slightly higher emissions of NOx and consumption of ammonia were detected in comparison to operation with silica-sand

Biomass ash interactions with a manganese ore used as oxygen-carrying bed material in a 12 MWth CFB boiler

Oxygen carrier aided combustion (OCAC) is a combustion concept which utilises oxygen carriers as bed material in existing fluidised bed boilers. In this study, a manganese ore was used in a 12 MWthCFB boiler. During the experimental session with the manganese ore, the boiler was operated with wood chips as fuel for more than a week without replacement of the bed material. Bed samples were extracted each day in order to investigate interactions between the manganese ore and the wood ash components. The samples were examined with SEM/EDX to follow the chemical distribution of ash elements in the bed particles. Physical properties such as density, size distribution and attrition resistance were followed as well. The impact on the reactivity of the oxygen-carrier bed particles was examined in a batch fluidised bed reactor at laboratory scale with gaseous fuels. Elemental composition analysis of the samples showed that common ash elements such as silicon, calcium, potassium, magnesium and sulphur had been accumulated in the manganese ore. Silicon, calcium and potassium were found throughout the particles as well as in formed surface layers. Sulphur was only found at the surface of the particles. The reactivity of the oxygen-carrying particles was affected during operation and showed a continuous decrease with increasing residence time in the boiler. The decrease in reactivity could be coupled to the layers of ash formed. Thus, this is an important issue when developing novel combustion concepts, such as OCAC and chemical-looping combustion (CLC), for biomass fuels

Magnetic separation of ilmenite used as oxygen carrier during combustion of biomass and the effect of ash layer buildup on its activity and mechanical strength

Utilization of oxygen carriers in existing circulating fluidized bed (CFB) units is referred to as oxygen carrier aided combustion (OCAC) and is employed to achieve enhanced combustion of heterogeneous fuels, such as biomass and waste. Although oxygen carriers have successfully been implemented to industrial settings, the materials are more expensive than conventional alternatives. The oxygen carrier ilmenite has shown to be magnetically separable from the residual ash stream providing a potential to reuse the material and mitigate cost. For this purpose, it is imperative to understand how properties of the material such as its oxygen carrying capacity and mechanical strength are developed with increased residence time in the system. In this study, rock ilmenite was evaluated after being used during OCAC in two separate industrial facilities. In Chalmers semi-industrial (12 MWth) CFB boiler, a set of samples were extracted on daily basis. These provide insight to how the material changes over time. In the utility company Kraftringen\u27s 115 MWth CFB boiler, one sample was retrieved and divided by a magnetic separator. To evaluate the oxygen transferring capacity, the materials were tested in a batch reactor with syngas as fuel. The present study shows that ash layers are not necessarily inhibitory to the oxygen transferring capacity, but long-term operation with thick layer buildup as a result causes reduction of the materials oxygen transferring capacity. Ash layer formation can instead improve mechanical strength of bed particles. Optimization measures are suggested where activated material is separated by magnet and reused in the system

Comparing the structural development of sand and rock ilmenite during long-term exposure in a biomass fired 12 mWthCFB-boiler

Oxygen Carrier Aided Combustion (OCAC) is a novel combustion concept and a spinoff from Chemical-Looping Combustion (CLC). The purpose of the concept is to increase the overall efficiency in conventional circulating fluidized bed (CFB) boilers by replacing the commonly used silica-sand bed material with an oxygen carrier (OC). The conceptual idea is to utilize the fluid dynamics in a CFB and the inherent oxygen transport supported by the OC to increase the oxygen distribution within the furnace in time and space. This is achieved as the OC can buffer oxygen in oxygen rich regions and release oxygen in oxygen poor regions of the furnace, resulting in less emissions of harmful and unburnt species as well as operation at lower air-to-fuel ratios. The OCAC concept has been successfully proven in laboratory and pilot plants and further demonstrated in full scale operation (75 MW) during more than 10\u27000 hours. However, as far as known to the authors no studies have been reported on the evolution in mechanical stability of an OC during continuous operation in a scale larger than 100 kW. This work aims to make a first evaluation of how ilmenite particles used as OC are affected with regard to mechanical resistance during long-term exposure to combustion conditions in Chalmers semi-industrial scaled (12 MW) CFB-boiler. The mechanical stability of two different types of ilmenite with similar composition, a sand ilmenite and a rock ilmenite, are evaluated in experiments conducted in the Chalmers boiler. Samples of the fresh materials and samples collected during operation in the Chalmers boiler are investigated with regard to their morphology, size distribution but also to attrition in a laboratory test rig. The study shows that the two materials differ in how the mechanical degradation occurs with exposure time. Cavities are formed inside the sand ilmenite particles which are held together by an ash layer before they are shattered into numerous pieces, whereas the rock ilmenite develops distinct cracks that cause splitting of the particles