Application of calcium looping for heat generation and CO₂ enrichment in greenhouses

Greenhouses typically employ conventional natural gas burners to meet the heat demand especially during cold winter nights and make use of additional carbon dioxide (CO₂) to increase the CO₂concentration above the atmospheric levels, thereby, accelerating the photosynthesis process and plant growth. As an alternative, this thesis describes a novel Greenhouse Calcium Looping (GCL) process which offers a cost effective solution for satisfying both heat and CO₂ demands of modern greenhouses. More specifically, the GCL process can produce significant quantity of heat during night-time through an exothermic carbonation reaction between calcium oxide (CaO) particles and the CO₂ emitted by growing plants. The resulting calcium carbonate (CaCO₃) particles are then calcined during day-time releasing their CO₂ content so that the photosynthesis process can be enhanced. The carbonation-calcination cycle is then repeated again. Interestingly, the greenhouse calcium looping (GCL) process is found to have better efficiencies and nearly zero CO₂ emission profile compared to other conventional processes. However, so far, the optimum conditions, i.e. temperature and pressure, for providing the required heat and CO₂ to the greenhouses in the GCL process are not clear. This includes also finding out how much CaO inventory is required for fulfilling the requirements of greenhouses? Accordingly, the main objective of this study is to find out the feasibility of the GCL process using Aspen Plus simulator software and establish the optimum conditions of the GCL process using Aspen Plus and experimental data. The conceptual design of greenhouse calcium looping process was carried out in the Aspen Plus® v 7.3 simulator. The process simulations carried out in Aspen indicated that the GCL process theoretically contributes to net zero emission of carbon dioxide given that the CO₂ released by plants over the night cycle are absorbed again during the day cycle through the photosynthesis. Moreover, in a scenario modelling study compared to the conventional natural gas burner system, the heat duty requirements in the GCL process were found to reduce by as much as 72%. Unique conditions of the GCL process (low temperature and low CO₂ partial pressure) led to determination of the intrinsic carbonation reaction kinetic parameters in a temperature range of 400 to 500 ^oC and CO₂ partial pressures of 0.05 to 0.1% (500 to 1000 ppm) which were carried out experimentally via a thermogravimetric analyser (TGA). Various gas-solid reaction mechanisms were considered to determine the best reaction mechanism for the carbonation reaction. Moreover, the activation energy and pre-exponential factor of the carbonation reaction were established. The derived kinetic parameters were used in Aspen Plus® to establish the optimum carbonator size considering an RPlug unit. The required size of the reactor decreased with an increase in the operating temperature of the reactor. Exergy analysis revealed that the carbonation process in the GCL technology boasted an overall exergetic efficiency of 80%. The calcination reaction part of the GCL process was also studied in a TGA to determine its intrinsic kinetic parameters. The experimental results showed that the calcination reaction in unique operational conditions of the GCL process (e.g. a temperature range of 600 to 800 ^oC and CO₂ partial pressure of 400 to 1600 ppm) follows zero order. Similar to the carbonation part, various models were analysed to find out the most appropriate model predicting the calcination reaction in the GCL process. The derived kinetic parameters were used in Aspen Plus® v 7.3 to establish the optimum carbonator volume considering an RPlug unit which increased with an increase in air circulation rate and size of the greenhouse. To obtain a practical overview of the energy efficiency of the calcination reaction in the GCL process, an exergy analysis was conducted which highlighted an overall exergetic efficiency of 85%. The full cyclic carbonation/calcination reactions were also studied in a lab-scale fluidised bed reactor to verify the technical viability of the complete process (rather than half cycles) but more importantly to determine the apparent reaction kinetic parameters needed in the design of large-scale systems. It was found that after 3 cycles, the CO₂capture capacity of CaO sorbents in the GCL process were more than that of those used for post-combustion capture of CO₂in typical fossil fuelled power plants. At the 10th cycle, the reaction conversion in the GCL process was about 40% more than that used in typical fossil fuelled power plants. SEM photos also confirmed this higher maintenance capacity of CaO sorbent in the GCL process compared to the sorbents used in typical fossil fuelled power plants. The kinetic parameters of the carbonation reaction occurred in a small FBR were also determined in operational conditions of the GCL process (e.g. a low temperature range of 400 to 500 ^oC and a low CO₂ partial pressure range of 600 to 2000 ppm). As expected there was a discrepancy between the activation energy and pre-exponential factor of the carbonation reaction derived in lab-scale FBR and TGA experiments. This is related to the fact that because of small test samples the TGA data are independent of heat and mass transfer (diffusional) limitations whereas the FBR represent a more realistic situation where data is in fact influenced by diffusional heat and mass transfer processes.