Teresa Mendiara

Biomass chemical looping gasification (BCLG) represents an innovative process that allows the generation of non-nitrogen-diluted synthesis gas with low tar content and the potential to avoid CO₂ emissions. In this work, a 1.5D macroscopic model for the fuel reactor of a BCLG unit was developed and validated to simulate the performance of the system under different operating conditions. The model was developed as simple as possible in order to have a powerful tool to simulate a large number of conditions in a relatively short period of time with low computing effort. However, it has the required complexity to consider the main processes affecting the reaction of the biomass and the oxygen carrier, such as reactor fluid dynamics and the reaction pathway of biomass in the fuel reactor. The main outputs of the model are presented and validated against results from a 20 kW_th BCLG unit with two biomasses, namely, pine forest residue and wheat straw pellets. The effects of several operating conditions (temperature, solid circulation rate, solid inventory, and gas flow) on the syngas yield and composition were successfully predicted by the model.

The cornerstone in the development of different applications of the Chemical Looping (CL) technology is the oxygen carrier material that transfers the oxygen. We evaluate the performance of manganese oxide doped with TiO₂ to reinforce the release of molecular oxygen and regeneration at high temperatures (850–950 °C). Different manganese-titanium mixed oxides with mass fractions of titanium oxide from 0 to 50 wt.% were considered and prepared by granulation. The sample with 20 wt.%. TiO₂ (MnTi_20) showed better oxygen release/regeneration capabilities than pure manganese oxide under similar conditions and was selected for further development. To evaluate the performance of MnTi_20, long tests (200 cycles) were carried out in a TGA to evaluate the chemical and mechanical stability of the sample. Additionally, MnTi_20 was tested in a batch fluidized bed reactor to evaluate its oxygen uncoupling capability and its reactivity with the main combustion gases (H₂, CO, and CH₄) as well as its fluidization properties. MnTi_20 is capable of regenerating Mn₃O₄ to Mn₂O₃ at 950 °C and maintains its reactivity with the redox cycles. No fluidization problems were encountered during almost 60 h of continuous fluidization, and a lifetime of 2500 h was estimated for this oxygen carrier.

Chemical looping with oxygen uncoupling (CLOU) is an advanced CO₂ capture technology where an oxygen carrier supplies the gaseous oxygen required for combustion. The technology has experienced a significant development degree in the most recent decades, up to TRL 4. This study presents results that further prove the viability of this technology to TRL 5 in a continuous CLC unit with two interconnected circulating fluidized beds acting as fuel and air reactors. The oxygen carrier used in the experiments was a magnetic copper-based oxygen carrier denoted as Cu30MnFe. It was used in the combustion of three types of biomass and a bituminous coal (Taldinsky coal). The effects on char conversion, CO₂ capture efficiency, and total oxygen demand of different parameters such as excess air ratio, fuel reactor temperature, residence time in fuel reactor, gas velocity in the carbon stripper, solid circulation rate, and solid inventory in fuel reactor were investigated. CO₂ capture efficiency of 93% was achieved at 886 °C with pine sawdust combustion, while the total oxygen demand was 4.6%. In the experiments with bituminous coal, the effectiveness of the carbon stripper in improving the CO₂ capture efficiency was demonstrated. During the highest load of coal, low total oxygen demands were achieved even at temperatures below 900 °C.

The unmixed combustion of biomass by chemical looping (Bio-CLC) has outstanding properties to produce energy by thermochemical conversion with CO₂ capture at low economic and energetic costs as well as minimizing NOx emissions to the atmosphere. A detailed analysis of the experimental results during the biomass combustion in chemical looping combustion (CLC) units is useful to evaluate the Bio-CLC potential for future industrial up-scaling. This work compiles the results obtained during more than 500 h of combustion in the previous years at the Instituto de Carboquímica (ICB-CSIC). Different biomasses were burnt with several oxygen carriers in two singular CLC units. Critical key performance indicators, such as the CO₂ capture rate and combustion efficiency, have been evaluated considering the combustion mode and the fluidization regime in the fuel reactor. Regarding the combustion mode, results by in situ gasification (iG-CLC) vs chemical looping with oxygen uncoupling (CLOU) are compared. As for the fluidization regime, it was bubbling in a 0.5 kW_th CLC unit and circulating in a 50 kW_th CLC unit. A methodical comparison of the results allows us to understand the fundamentals of the processes and to evaluate the different behaviors observed in each case. The operating conditions having major effects on the Bio-CLC performance were identified, as well as the operating conditions necessary to optimize the CO₂ capture and the combustion efficiency. In addition, other issues are evaluated, such as the fate of fuel-N in a CLC process, the presence of tar compounds in the CO₂ stream, or the interaction of ash compounds with the oxygen carrier particles. Finally, possible future actions are discussed to improve the performance of the Bio-CLC process as well as the possible development paths of new oxygen carriers.

Chemical-looping combustion (CLC) stands out as a promising method for carbon capture and storage for the purpose of mitigating climate change. The process involves the conversion of fuel facilitated by an oxygen carrier, with the resulting CO₂ inherently separated from other air components. Notably, when applied to biomass combustion this process offers a pathway to achieving negative CO₂ emissions. However, a significant challenge for CLC, particularly in its application to biomass, is the management of interactions between ash and oxygen carriers. Biomass-derived ashes typically contain substantial quantities of reactive ash-forming substances, such as alkaline and alkali earth elements. These interactions can impact the performance and longevity of the oxygen carrier, necessitating careful consideration and mitigation strategies in CLC systems utilizing biomass feedstocks. This study examined the interaction between biomass ash components and two recently developed oxygen carriers, Cu30MnFekao7.5 and Cu30MnFe, during combustion in a 1.5 kW_th continuous unit. Both oxygen carriers achieved 100% combustion efficiency and a CO₂ capture efficiency of 95% at 900 °C. Although the copper in both oxygen carriers did not exhibit any noticeable interaction with ash components, the accumulative presence of potassium and magnesium in Cu30MnFekao7.5 was identified by inductively coupled plasma and scanning electron microscopy with energy dispersive X-ray analysis, indicating an increase in the amount of both elements in the particles after combustion operation. No problems of agglomeration or fluidization were observed in any of the experiments.