| dc.description.abstract | Hydrogen can help decarbonize and bring a paradigm shift in the global energy mix. Green hydrogen, with its potential to mitigate carbon emission, offers an inexhaustible supply – free from the constraint of finite reserves. The aim of the present work is to generate hydrogen rich gas from biomass for green hydrogen and chemicals. This thesis presents an experimental investigation of the oxy-steam gasification of biomass in order to study the non-catalytic in-situ reforming of CO for the enrichment of hydrogen in product synthesis gas. The in-situ reforming of CO has been aimed by utilizing the steam in the oxidizing media to favor the water gas shift reaction. The work also presents the effect of steam-to-biomass ratio (SBR) and the injection temperature of oxy-steam mixture in the product syngas yield and hydrogen enrichment.
The current state of literature discusses several studies on a bench scale or gasification with external energy support or studies with lower SBR operations or recommended external catalytic reactors to enhance hydrogen concentration (Hamad, Radwan, Heggo, & Moustafa, 2016; Khan, Yusup, Kamble, Naqvi, & Watson, 2018; Peng, et al., 2017; Ozbas, Aksu, Ongen, Aydin, & Ozcan, 2019; Li, et al., 2020; Yao, et al., 2016; Fremaux, Beheshti, Ghassemi, & Shahsavan-Markadeh, 2015). A study with external energy support for gasification has reported a maximum yield of 92 g/kg of biomass by using Ni catalyst (Yao, et al., 2016) while another autothermal oxy-steam gasification study has reported a maximum hydrogen yield of 104 g/kg (Kumar, 2016).
The literature review has highlighted several critical research needs in the field of biomass gasification for hydrogen production. There is a pressing demand for studies on autothermal gasification systems, which could reduce reliance on external energy sources. In-situ reforming, particularly those utilizing the water gas shift reaction, require further investigation to enhance hydrogen concentration in the product gas. Research into higher Steam-to-Biomass Ratio (SBR) operations is necessary to optimize the gasification process. The catalytic effects of hot char beds derived from biomass on the water gas shift reaction need to be explored, potentially eliminating the need for external catalytic reformers. Studies should address multi-fuel compatibility in gasification reactors to accommodate diverse feedstocks. The impact of gasifying media temperature on the process requires thorough examination. Importantly, there is a significant need for large-scale experimental investigations, as most current studies are limited to bench-scale setups. To advance biomass gasification as a viable method for green hydrogen production, this study aims to address these research needs using a scaled-up setup capable of processing approximately 14 kg/h of biomass.
The present work aims to address the needs identified in the literature, and has been carried out in three parts: (1) study of the oxy-steam gasification at lower injection temperature (T = 375 - 550 °C), (2) study of the reduction zone of the gasification system by simulating the environment in a separate experimental set-up of char bed, and (3) study of oxy-steam gasification at higher injection temperature (T = 550 - 750 °C).
In the present study, three kinds of biomass were used as the solid fuel for the gasification experiments namely, casuarina wood chips, coconut shells and corncob with higher heating values 18.3, 20.5 and 17.5 MJ/kg, respectively. The biomass feeding took place in batch mode with an average feed rate of about 5 to 15 kg/h. The steam and oxygen flow rate varied from about 9 to 25 kg/h and 3 to 5 kg/h, respectively.
The present study was conducted with a closed top downdraft gasification system with thermal rating of about 14 kW. The gasification system is a cylindrical reactor with 2 m in height and 200 mm diameter.
The first phase of the experimental study demonstrates the attainment of stable oxy-steam gasification operations surpassing the previously established carbon boundary point of SBR 1.5 through the introduction of oxy-steam mixtures at elevated temperatures. Results show that with an increase in SBR, hydrogen fraction increases with a corresponding decrease in CO, substantiating the occurrence of the water gas shift reaction (WGSR). However, beyond an SBR threshold of approximately 3, the progression of the water gas shift reaction does not manifest in the gas composition data.
The low-temperature injection experiments encompassed an SBR range of 1.3 to 4.7 and equivalence ratio (ER) from 0.25 to 0.5. The highest hydrogen concentration in the dry gas mixture was recorded for casuarina wood chips at 49.5% (83.4 g/kg biomass) at an SBR of 3.6 and ER of 0.34, yielding an overall efficiency of 50.6% and hydrogen efficiency of 35.7%. Similarly, the maximum hydrogen yield reached 96.6 g/kg (47.6%) of biomass at an SBR of 4.2 and ER of 0.36, with an overall efficiency of 55.9% and hydrogen efficiency of 37.8% with casuarina wood chips as feed. Overall efficiency considers the heating value of the cold syngas as the output while hydrogen efficiency considers the heating value of hydrogen in the cold syngas as the output. Notably, H2O conversion peaked at the lowest SBR of 1.3, reaching 43%, before gradually declining with increasing SBR. Beyond an SBR of 2, H2O conversion remained relatively stable, fluctuating between 11% and 20%, with an average of 15% considering both kinds of biomass.
The second part of the study investigates in-situ catalysis by the char bed in a gasification system. To investigate the impact of temperature and SBR on reactions occurring within the reduction zone, separate experiments were conducted in a scaled down char bed. The experimental setup was a ceramic cylindrical reactor with a diameter of 50 mm and height of 400 mm externally heated to maintain desired temperature. To account for SBR, H2O/CO ratio was varied on the basis of prevailing H2O/CO in a gasification system. Results show that rates of water gas shift and steam char reactions increased with an increase in temperature from 520 to 760 °C at H2O/CO = 2 when reactant mixture contained H2O and CO. These reactions contribute to the enrichment of the product gas mixture with hydrogen. Notably, the temperature range of 720–760°C was identified as crucial for char conversion by steam and increased hydrogen production. Further, the effect of H2O/CO was studied by fixing the temperature at 750 °C. It was seen that char consumption increased (8.1 to 16.4 mmol/h-gchar) when H2O/CO increased from 2 to 10 while rate of water gas shift reaction decreased. The decrease in water gas shift reaction rate can be attributed to CO addition from steam-char reaction which decreases the effective H2O/CO in the system. Experiments with producer gas-H2O mixture as the reactant mixture at H2O/CO from 2 to 10 at temperature 750 °C showed that increase in H2O/CO favors hydrogen enrichment by water gas shift reaction and steam-char reaction, however, the rates of reactions were lower than in case of H2O-CO mixture in the feed. At H2O/CO = 10, with H2O-CO mixture, the water gas shift reaction was 2.3 times faster than with producer gas – H2O mixture, for steam-char reaction, this factor was 2.4 at the same H2O/CO. This decrease could be attributed to dilution effect of other components in the reactant mixture which contains CO2, H2, CO, CH4 and N2. The inhibition effect due to the presence of H2 to water gas shift reaction is also a reason for the decrease in reaction rates of water gas shift reaction and char reaction. The catalytic effect of the char bed was also found to be favoring the water gas shift reaction due to presence of inorganics and higher surface area, as validated by results from experiments with an inert bed under similar conditions. The presence of potassium in char has been shown to enhance the activity of char towards both the reactions.
On understanding the effect of H2O/CO and temperature over char bed separately conducted for reactant mixtures (CO+H2O and producer gas+H2O), oxy-steam gasification experiments were conducted at higher injection temperatures (550 °C to 750 °C) of oxy-steam mixture in the SBR range of 1.3 to 3.7. Elevating the injection temperature led to an improved hydrogen yield of 110 g/kg-biomass (53.4%) at an SBR of 2.4 and ER of 0.24 for casuarina wood chips, achieving an overall efficiency of 69.8% and hydrogen efficiency of 47.8% when the oxy-steam injection temperature reached 734°C. Furthermore, increasing the injection temperature resulted in a product gas mixture containing higher hydrogen concentration of 56.6% (vol.) at 750˚C for casuarina wood chips while at a lower injection temperature of 445˚C it was 46.7% (vol%.) at an SBR of 3.1. Results show higher temperature of injection results in better yield at lower SBR. The study recommends operating the gasification system at SBR of 2.3±0.1 with injection temperature more than 700 °C. Present work also establishes the multifuel capability of the gasification system where biomass like casuarina wood chips, coconut shells and corncob can be used to generate syngas.
Further, exergy analysis was done to identify the inefficiencies by quantifying the conversion efficiency at various subsystems of the gasification system. Exergy analysis reveals the maximum work obtainable by a process. This analysis takes into account thermodynamic irreversibilities, which result in energy degradation. In the context of the present study, solid biomass undergoes thermochemical conversion in a fixed bed downdraft reactor to produce synthesis gas. The gasification process occurs within the reactor, while subsequent cooling and cleaning stages remove sensible heat and moisture from the product gas mixture.
The exergy analysis of the process reveals that with an increase in injection temperature, the thermochemical conversion efficiency increased, with an average conversion efficiency of 77%, whereas steam generation for the process incurred the highest exergy loss. The conversion efficiency considered the exergy of biomass, steam and oxygen as the input and exergy of the hot gas mixture after gasification as the output. The equilibrium studies show that lower injection temperature favors higher H2 yield as dictated by thermodynamics. While SBR improves hydrogen composition in the output, the rate of increase plateaus at higher SBRs (>2.5), explaining the observation recorded in the first part of the study. | en_US |
