The main contaminants in biogas namely carbon dioxide, hydrogen sulfide, and siloxanes limit its application especially in engines by reducing energy density and causing corrosion on machine parts. Furthermore, the carbon dioxide in biogas contributes highly to global warming. Therefore, biogas should be upgraded to increase its safe application. Most commercial methods of biogas cleaning are expensive for small and medium scale digesters. There is no review documentation detailing the usage of natural materials in the purification of biogas. The aim of the current study was to systematically and critically review recent developments in the applications of natural materials in the purification and upgrade of biogas. Documented literature indicate that the low-cost natural adsorbents have potential in purification of biogas. The adsorption capacity of biogas contaminants for most materials can be enhanced by physical and chemical activation. The two mechanisms through which these material eliminate carbon dioxide in biogas namely surface adsorption and wet carbonation process have been discussed. In addition, the study looked at the factors that affect the removal of carbon dioxide from biogas using natural materials. The adsorption capacity of biogas contaminants for different natural and modified materials were reviewed. Furthermore, a summary of the merits and demerits of different natural adsorbents for biogas purification is presented. Future studies should investigate the methane loss during upgrading process using low-cost adsorbents. Comparative investigations of the process cost-effectiveness of using natural materials to upgrade biogas should be carried out to determine their suitability against the commercial processes.

A review of recent developments in application of low cost natural materials in purification and upgrade of biogas

On the state of the art of crystalline structure reconstruction of coal fly ash: A focus on zeolites

Nowadays, the massive consumption of fossil fuels and the resulting excessive emission of carbon dioxide (CO2) have broken the original carbon balance of nature, resulting in global warming and the consequent detrimental environmental impacts. To address these issues, various initiatives have been proposed, among which CO2 capture and utilization (CCU) is considered as the direct way to mitigate the accumulation of CO2 in the atmosphere. Although a plethora of CO2 capture reagents and utilization routes have been developed, the cost of CO2 capture reagents as well as the amount of CO2 utilization still encounters limitations. Recently, CCU with solid wastes have attracted sustained attention due to its ability to synchronize CO2 fixation with solid wastes utilization. Especially, the huge amount and low cost of the solid wastes can promote the economic fixation of CO2 and thus contribute significantly to the carbon sink. Given the tremendous utility of this strategy, this review article summarizes the state-of-the-art of CO2 capture and utilization with solid wastes such as steel slag, concrete waste, fly ash, red mud, calcium carbide residue, and biomass etc. And three parts including CO2 mineralization, solid waste-based catalyst promoted CO2 transformation, and collaborative transformation of CO2 and solid waste are introduced according to the roles of solid waste in CO2 utilization. We hope this review can arouse broad concern and spur further development in this field. 

CO2 capture and utilization with solid waste

Coal combustion accounts for a large proportion of the global energy supply due to its low cost and high heat value; however, pollution from the large amounts of coal fly ash (CFA) generated during...

High-End Reclamation of Coal Fly Ash Focusing on Elemental Extraction and Synthesis of Porous Materials

The rapid cleanup of endogenous CO2 has become a necessity in environmental control and life support systems (ECLSSs), to ensure crew safety and long-term task execution. The extensively used non-regenerable LiOH and soda lime adsorbents, although exhibit high CO2 storage capacities and fast kinetics, can hardly fulfill the demands of reduced launch weight and storage volume in the space- and load-limited ECLSSs. New CO2 adsorbents with desirable attributes including high CO2 uptakes, good selectivity, facile regeneration, fast adsorption and desorption kinetics, and good multicycle stability are urgently needed. We developed amine-functionalized adsorbents by loading tetraethylenepentamine (TEPA) on mesoporous supports of activated carbon (AC), aluminum oxide (AO) and silica gel (SG). The adsorbents were characterized by N2 adsorption-desorption, thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR), and scanning electron microscope (SEM) to study their microstructural properties. CO2 adsorption capacities and kinetic performance of the adsorbents with different supports and various amine loadings (10–50 wt%) were evaluated in 2%CO2 at 20 °C. The effects of support and amine loading on the structure-performance relationships of the adsorbents were demonstrated. The TEPA-SG-20 adsorbent (20 wt% TEPA loaded on silica gel support) exhibits the highest CO2 adsorption capacity of 1.90 mmol CO2/g and the maximum amine efficiency of 0.48 mmol CO2/mmol N. TEPA-SG-20 also exhibits fast CO2 adsorption kinetics, and the Avrami fractional order kinetic model provides a satisfactory correlation of the experimental CO2 uptakes. Furthermore, the TEPA-SG-20 adsorbent can be efficiently regenerated at 110 °C with a great regeneration efficacy of 97%. The desired adsorbent also exhibits good working stability with a low loss-in-capacity of 4.32% in 10 consecutive cycles. The good CO2 adsorption performance of TEPA-SG-20 is associated with the excellent microstructural properties such as high surface area, great pore volume and uniform dispersion of amine species. Overall, the desired TEPA-SG-20 adsorbent shows promise for low-concentration CO2 removal in ECLSSs. 

Experimental and kinetics investigations of low-concentration CO2 adsorption on several amine-functionalized adsorbents

Solid amine adsorbents are among the most promising CO2 adsorption technologies for biogas upgrading due to their high selectivity toward CO2, low energy consumption, and easy regeneration However, in most cases, these adsorbents undergo severe chemical inactivation due to urea formation when regenerated under a realistic CO2 atmosphere Herein, we demonstrated a facile and efficient synthesis route, involving the synthesis of nano-Al2O3 support derived from coal fly ash with a CO2 flow as the precipitant and the preparation of polyethylenimine (PEI)-impregnated Al2O3-supported adsorbent The optimal 55%PEI@2%Al2O3 adsorbent showed a high CO2 uptake of 139 mg·g-1 owing to the superior pore structure of synthesized nano-Al2O3 support and exhibited stable cyclic stability with a mere 029% decay per cycle even under the realistic regenerated CO2 atmosphere The stabilizing mechanism of PEI@nano-Al2O3 adsorbent was systematically demonstrated, namely, the cross-linking reaction between the amidogen of a PEI molecule and nano-Al2O3 support, owing to the abundant Lewis acid sites of nano-Al2O3 This cross-linking process promoted the conversion of primary amines into secondary amines in the PEI molecule and thus significantly enhanced the cyclic stability of PEI@nano-Al2O3 adsorbents by markedly inhibiting the formation of urea compounds Therefore, this facile and efficient strategy for PEI@nano-Al2O3 adsorbents with anti-urea properties, which can avoid active amine content dilution from PEI chemical modification, is promising for practical biogas upgrading and various CO2 separation processes

Biogas Upgrading via Cyclic CO2 Adsorption: Application of Highly Regenerable PEI@nano-Al2O3 Adsorbents with Anti-Urea Properties.

Highly efficient and stable PEI@Al2O3 adsorbents derived from coal fly ash for biogas upgrading

Efficient one-pot synthesis of ethyl levulinate from carbohydrates catalyzed by Wells-Dawson heteropolyacid supported on Ce–Si pillared montmorillonite

Fan Qu

Papers

Biogas Upgrading via Cyclic CO2 Adsorption: Application of Highly Regenerable PEI@nano-Al2O3 Adsorbents with Anti-Urea Properties.

Highly efficient and stable PEI@Al2O3 adsorbents derived from coal fly ash for biogas upgrading

Efficient one-pot synthesis of ethyl levulinate from carbohydrates catalyzed by Wells-Dawson heteropolyacid supported on Ce–Si pillared montmorillonite