Elsevier

Fuel Processing Technology

Volume 171, March 2018, Pages 198-204
Fuel Processing Technology

Research article
The extent of sorbent attrition and degradation of ethanol-treated CaO sorbents for CO2 capture within a fluidised bed reactor

https://doi.org/10.1016/j.fuproc.2017.10.009Get rights and content

Highlights

  • Ethanol pre-treatment step to develop an optimised pore structure tested

  • Synthesised sorbents tested within a fluidised bed reactor

  • Attrition was significant cause for loss of sorbent mass and CO2 capture capacity.

  • Ethanol pre-treatment appeared to negatively impact attrition resistance.

Abstract

The application of an ethanol pre-treatment step on biomass-templated calcium looping sorbents resulting in an improved pore structure for cyclic CO2 capture was investigated. Three ethanol solutions of varying concentrations were used with an improved pore and particle structure, and thermogravimetric analyser CO2 carrying capacity arising with the 70 vol% ethanol solution. The extent of attrition of these sorbents was tested within a fluidised bed reactor and compared against an untreated sorbent and a limestone base case. It found that despite the ethanol-treated sorbents displaying an admirable CO2 carrying capacity within the thermogravimetric analyser even under realistic post-combustion conditions, this was not translated equivalently in the fluidised bed. Attrition and elutriation of the biomass-templated sorbents was a significant issue and the ethanol pre-treatment step appeared to worsen the situation due to the roughened surface and mechanically weaker structure.

Introduction

Climate change is forcing the global community to rethink the way energy and industrial products are generated. Carbon Capture and Storage (CCS) is one method of preventing CO2 emissions from carbon-intensive industries, whereby the CO2 is collected, transported and safely stored underground thus preventing its action as greenhouse gas in the atmosphere. CCS is also well regarded as being essential to cost-effectively combatting climate change [1], [2].

Calcium looping (CaL) is one form of CCS and can be operated as a post- or pre-combustion system [3]. The most familiar is a post-combustion setting consisting of two reactors: a carbonator - where CO2 is absorbed onto a CaO sorbent to form CaCO3, and a calciner – where CO2 is released from the sorbent generating a nearly pure stream of CO2 [4]. Typically, the CO2 sorbent used is CaO derived from natural limestone although many synthetic sorbents have also been manufactured with varying suitability for commercial application [5].

A major technical limitation to the full market deployment of CaL is the degradation in the sorbent's ability to cyclically capture and release CO2. Pure CaO has a maximum CO2 carrying capacity of 1 mol CO2 per 1 mol CaO, however due to high-temperature CO2/H2O sintering [6], attrition [7] and competitive sulphation reactions [8] this CO2 carrying capacity can decrease to just 0.1–0.2 mol/mol in a matter of a few cycles. Some previously demonstrated effective methods for minimising the degradation in the carrying capacity are the incorporation of a support structure within the particle (such as a calcium aluminate cement [9], polymorphic dicalcium silicate [10], or many others [11], [12], [13]), doping with HBr [14], steam or water hydration [15], thermal pre-activation [16], biomass templating [17], acid treatment [18], [19], [20] and ethanol treatment [21].

Ethanol treatment is where a solution of ethanol and water are allowed to soak a sorbent (whilst in its calcined form). The water then begins to react with the CaO to form disassociated Ca2 + and OH ions, in doing so opening up the pores [22]. The extent of the hydration reaction is limited by a high ethanol concentration and the strong polar bonds experienced between the water and ethanol molecules. Upon drying the sorbent to remove the solution the dissolved ions reform at the surface of the particle to form Ca(OH)2 leaving behind particles with increased porosity and a high surface area. This method has been demonstrated to be quite effective at increasing the long-term CO2 carrying capacity of the sorbent over multiple cycles, even under realistic calcination conditions [21].

Attrition in particular has been ignored in much of the research efforts in trying to maintain the original carrying capacity. Nevertheless, attrition is an important factor to consider as a sorbent that easily attrites will require a higher make-up rate of fresh sorbent thus increasing the operating cost [23]. Attrition is known to be caused by three main routes: [24]

  • Primary fragmentation – thermal shocks and over-pressurisation (due to near instantaneous reactions occurring within the particle) causing the particle to splinter/crack and break apart into finer particles

  • Secondary fragmentation – crushing and mechanical weakening of particles caused by mechanical contact between other particles and a reactor's internal features

  • Abrasion – the rubbing of particles against other particles and a reactor's internals providing a shear force at the exterior of the particles, gradually wearing away the surface

The aim of this investigation was to examine the extent of attrition on a simply manufactured sorbent with an additional ethanol pre-treatment step for pore structure enhancement. The sorbents tested within this work also benefited from an enhanced macro-pore structure due to biomass templating with wheat flour and the pores were developed further by the application of ethanol pre-treatment. Additional structural rigidity was given to the particles from the inclusion of a calcium aluminate cement support. The ethanol-treated sorbents were tested within a TGA and a fluidised bed reactor to determine their ability to resist attrition.

Section snippets

Sorbent preparation

To produce the sorbents utilised within this work Longcliffe limestone (supplied by Longcliffe Quarries Ltd., UK) was first calcined in a muffle furnace at 850 °C for 2 h under an air purge gas. The produced lime was transferred into a mixing vessel (Glatt TMG 1/6 granulator) in which a commercial calcium aluminate cement (CA-14, 73% Al2O3 and 27% CaO) and flour was added in order to form a powdered mixture of 80 wt% CaO, 10 wt% calcium aluminate cement and 10 wt% flour. After thoroughly dry mixing

TGA carrying capacity study

The results of the calcium looping cycling conducted within the TGA are displayed as Fig. 2. The CaO conversion displayed in Fig. 2 was calculated based on the mass change between the sample mass after the zeroth calcination and the sample mass at the end of each carbonation period. These mass changes were converted into moles by dividing by the relative molecular masses of each respective species, as shown in Eq. (1). The value of CaO purity utilised in Eq. (1) was determined through XRF and

Conclusions

The aim of this investigation was to examine the extent of attrition of an easy-to-manufacture sorbent with an enhanced macro-pore structure due to sacrificial biomass (flour) templating and the application of an ethanol pre-treatment step. Additional structural rigidity was provided by the incorporation of a calcium aluminate cementitious support structure. The sorbents were tested within a TGA and a fluidised bed reactor to determine their ability to resist attrition.

This work has highlighted

Acknowledgements

The research leading to these results has received funding from the European Community's Research Fund for Coal and Steel (RFCS) under grant agreement no. RFCR-CT-2014-00007.

References (37)

Cited by (9)

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