Interaction of oxygen carriers with common biomass ash components

Highlights

• Investigation of solid-state interaction between three oxygen carriers and three biomass ash components

• Thermodynamic calculations were performed and compared with experimental results.

• Limited interaction with the oxygen carriers was observed for systems with SiO₂ and CaCO_2.

• Systems of oxygen carriers with K₂CO₃ and K₂CO₃–SiO₂ formed new phases which were accompanied by severe agglomeration.

Abstract

Carbon capture and storage (CCS) has been proposed as a bridging technology between the current energy production and a future renewable energy system. One promising carbon capture technology is chemical-looping combustion (CLC). In CLC the reactors are filled with metal oxide bed material called oxygen carriers. The interaction between oxygen carriers and biomass ashes is a poorly explored field. To make CLC a viable process, and thereby creating carbon emission reductions, more knowledge about the interactions between biomass ashes and oxygen carriers is needed.

This study investigated solid-state reactions of three promising oxygen carriers, hematite, hausmannite and synthesised ilmenite with different biomass ash components. Oxygen carriers were exposed with the ash components: calcium carbonate, silica and potassium carbonate at 900 °C and at different reducing potentials. Crystalline phases of the exposed samples were determined using powder x-ray diffraction (XRD).

Results showed that the oxygen carriers hausmannite and hematite interact to a higher extent compared to synthesised ilmenite regarding both physical characteristics and detectable phases. Synthesised ilmenite formed new phases only in systems including potassium.

Thermodynamic calculations were performed on the multicomponent system and compared with experimental results. The results suggest that optimisation of systems involving manganese and potassium should be performed.

Introduction

The concentrations of greenhouse gases in the atmosphere are increasing as the result of anthropologic activities, where the extensive combustion of fossil fuels is the main reason for the increase. A quarter of the total emissions of greenhouse gases originate from the production of electricity and heat, where the main energy source is fossil fuels [1,2].The increasing global mean temperature, with subsequent climate change, are the severe consequences of these activities. In fact, a recent report from the UN indicates that it is almost impossible for the world to meet the limits set in the Paris agreement, as the carbon budget for limited warming is almost exhausted [3]. This means that it may be necessary to remove carbon dioxide from the atmosphere in order to limit warming.

Carbon capture and storage (CCS) has been proposed as a bridging technology between the current energy production and a future renewable energy system. The CCS concept consists of the capture of carbon dioxide from point sources such as power plants, followed by compression, transportation and finally deposition at a storage site [4]. Bioenergy with carbon capture and storage (BECCS) could even make it possible to achieve negative greenhouse gas emissions, which is now necessary if the climate targets are to be reached [5,6].

Chemical-looping combustion (CLC) is one technology proposed for capturing carbon dioxide from combustion facilities [7]. It is based on the use of metal oxides with the ability to be oxidised or reduced at combustion conditions depending on the surrounding oxygen partial pressure [8]. Thus, the need for gas separation can be avoided, and carbon dioxide may be captured at a much lower cost than with other technologies [9]. Numerous metal oxides, referred to as oxygen carriers, have been operated in chemical-looping units ranging from small lab-scale units up to a 1 MW_th reactor [10]. The use of biomass in CLC is increasing, which can clearly be seen in the recent review article by Adánez et al. [11]. For example, a 1 MW_th chemical-looping pilot plant has been operated with a fuel mix of hard coal and torrefied biomass [12]. The manganese ore examined in this paper has also been used as oxygen carrier in two pilot scaled chemical-looping pilots, 10 kW_th and 100 kW_th, with wood char and pellets as fuels [13]. To make CLC a viable process for BECCS, more knowledge about the interactions between biomass ashes and oxygen carriers is needed, as it is well known that biomass ash can be aggressive and corrosive during thermal conversion at high temperature.

In addition to CLC, there are other technologies utilising oxygen carriers for fuel conversion. For example, the use of oxygen carriers in conventional combustion in fluidised bed boilers has been proposed as a measure to even out the oxygen availability and temperature in the boiler. This technology has been referred to as oxygen carrier aided combustion (OCAC) and promising results have been reported [[14], [15], [16]]. The concept has been successfully operated for >12,000 h using ilmenite as bed material in full industrial scale [17]. OCAC is currently being commercialized [17].

In any technology utilising oxygen carriers with biomass, one important issue is whether ash components in the fuel will interact with the oxygen carrier and how this will affect the performance of the system. Consequences of interactions between conventional bed materials in fluidised beds and biomass ashes have been widely studied [[18], [19], [20], [21]]. A few studies have been made concerning biomass ash interaction with the oxygen carriers ilmenite, iron ore and manganese ore [15,22,23]. This is however still a poorly explored field of research.

The objective of this study is to systematically investigate interactions between three commonly used oxygen carriers and three biomass ash components containing calcium, silicon and potassium respectively, all elements known for interacting with bed materials during combustion.