Hydrothermal gasification of human feces as an alternative means of waste mitigation and hydrogen production

Hydrothermal gasification;Catalyst synergy;Hydrogen production;Waste mitigation

Watson, Jamison ; Sharma, Brajendra Kumar, ; ,Davidson, Paul ; Zhang ; Yuanhui

关键词: Hydrothermal gasification;    Catalyst synergy;    Hydrogen production;    Waste mitigation;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/99083/WATSON-THESIS-2017.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Non-catalytic and catalytic gasification have been recently studied as a means of converting waste biomass into value-added gaseous products while concomitantly minimizing the emission of harmful pollutants. With the incorporation of a homogeneous or heterogeneous catalyst, gas products with a hydrogen volume concentration of up to 32-55% and a carbon conversion efficiency (CCE) between 67-93% have been achieved. Due to gasification being an environmentally benign option to convert waste biomass into valuable products, it is a direct competitor to other current waste disposal technologies, such as incineration, combustion, and landfilling. This study deals with providing an overall outlook on the current state of noncatalytic and catalytic gasification. Further, this study also aims to produce hydrogen and methane gas while detailing the benefits and limitations of different gasification processes, especially for biowaste. The first section of this study compares the different gasification agents used in the gasification process. Specifically, steam is chosen as a particularly promising gasification agent and is compared with other gasification agents (oxygen and air) to understand the specific effects of these agents on the resulting gas quality and quantity. Second, the advantages and disadvantages of current reactor configurations for gasification are summarized. This section specifically focuses on which reactor configurations are best suited for different biowaste feedstocks and how these configurations work to minimize the production of inhibitory by-products. Third, influencing process factors (temperature, steam to biomass (S/B) ratio, and catalyst selection) are evaluated in terms of their impact on the resulting H 2 /CO ratio, lower heating value (LHV), gas yield, tar yield, and energy recovery. Finally, the current challenges facing the field of steam gasification and the future outlooks for this field are presented.This study also explores a relatively novel area of gasification, catalytic hydrothermal gasification (CHG). This specific type of gasification deals with utilizing the moisture in the feedstock as the primary gasification agent and sidesteps the energetically costly step of dewatering the feedstock. This portion of the study focuses on the usage of different catalysts on the gasification of wastewater resulting from the hydrothermal liquefaction (HTL) of human feces. The catalyst screening study revealed that NaOH (46.9%) and Raney Ni (41.2%) resulted in the highest H 2 composition, while Ru/AC resulted in the highest reduction in the liquid CODr (97.7%). A catalyst mixture was then studied combining two heterogeneous catalysts, Raney Ni and Ru/AC, at different ratios. A weight ratio of 90% Raney Ni and 10% Ru/AC yielded a H 2 composition of 56.3% indicating that catalyst synergy may exist between these two catalysts which further enhances H 2 production beyond the ability of each catalyst individually. Incorporation of a reaction coordinate diagram allowed for the direct comparison of energy recovery, COD r , and H 2 composition using a mixed catalyst and varying temperatures and retention times (RT). Results showed that the optimal condition to maximize the COD r (64.3%), energy recovery (30.3%), and H 2 production (37.9%) occurred at a temperature of 400ºC, a RT of 60 minutes, and a catalyst to feedstock ratio of 0.1.The final portion of this study involves conducting a preliminary examination of the impacts of temperature and RT on the non-catalytic CHG of human feces. This portion of the study determined that while temperature had a large impact on the gas composition and gas quality. Specifically, at 600ºC the CH 4 content and CCE were maximized at values of 44.3% and 51.9%, respectively. The temperature also had a positive influence on the conversion of carbon to the gaseous fraction and the conversion of energy to the gas fraction. As the temperature increased from 400ºC to 600ºC, the carbon content in the gaseous fraction increased from 26.5% to 51.8%, respectively, and the energy content in the gaseous fraction increased from 2.4% to 32.8%, respectively. On the other hand, the RT only had an impact on the organic matter conversion from the feedstock to gaseous products and did not have a large influence on the gaseous product composition. As the RT increased from 15 minutes to 120 minutes, the CCE increased from 20.2% to 56.8%. Overall, gasification shows both promise as a means of producing H 2 and CH 4 gases and efficiently converting organic matter in the feedstock into the gaseous product fraction. However, catalysts need to be employed in gasification processes to further enhance the production of valuable gaseous products, improve the efficiency of the conversion of organic compounds in the feedstock to gaseous products, and allow for the use of less extreme reaction conditions.