Influence of catalysts on hydrogen production from wastewater generated from the HTL of human feces via catalytic hydrothermal gasification

doi:10.1016/j.ijhydene.2017.05.083

International Journal of Hydrogen Energy

Volume 42, Issue 32, 10 August 2017, Pages 20503-20511

https://doi.org/10.1016/j.ijhydene.2017.05.083 Get rights and content

Highlights

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NaOH (46.9%) and Raney Ni (41.2%) yielded the highest individual H₂ productions.
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Ru/AC catalyst resulted in the highest reduction in the liquid COD (97.7%).
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A combination of Raney Ni and Ru/AC catalysts lead to an H₂ production of 56.3%.
•
Catalyst synergy may exist between Raney Ni and Ru/AC which enhances H₂ production.
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400 °C and a 60-min retention time maximized COD_r, energy recovery, and H₂/COD.

Abstract

The aqueous phase from the hydrothermal liquefaction of human feces is rich in organic matter. This study explored the usage of different catalysts on the catalytic hydrothermal gasification (CHG) of wastewater resulting from the hydrothermal liquefaction of human feces. The catalyst screening study revealed that NaOH (46.9%) and Raney Ni (41.2%) resulted in the highest H₂ composition, while Ru/AC resulted in the highest reduction in the liquid COD (97.7%). A catalyst mixture was then studied combining two heterogeneous catalysts, Raney Ni and Ru/AC, at different ratios to increase the H₂ composition. A weight ratio of 90% Raney Ni and 10% Ru/AC yielded a H₂ composition of 56.3%, a gas yield of 350.0 g/kg dry feed, and a H₂ yield of 10.61 mol/kg dry feed, indicating that catalyst synergy may exist between these two catalyst which further enhances H₂ production. Incorporation of a reaction coordinate diagram allowed for the direct comparison of energy recovery, COD_r, and H₂/COD value using a mixed Raney Ni and Ru/AC catalyst and varying temperatures and reaction times. Results showed that the optimal condition to maximize the H₂/COD content (9.5 mg H₂/g COD), the COD_r (64.3%), and energy recovery (30.4%) occurred at a temperature of 400 °C, a retention time of 60 min, and a catalyst to feedstock ratio of 0.1.

Introduction

Due to a lack of access to proper sanitation facilities, nearly 15% of the world's population currently defecates in the open [11]. Open defecation not only causes aquatic dead zones, but also leads to severe health risks resulting in significant financial burdens. In particular, throughout Southeast Asia, 13 million tons of human fecal matter are released into groundwater sources ever year, which incurs an economic burden of greater than $2 billion per year [14]. Therefore, properly handling fecal matter has become a pressing issue from a sanitation, environmental, and economic standpoint.

Hydrothermal liquefaction (HTL) is a thermochemical process that operates at elevated temperatures (∼260–350 °C) and pressures (∼10–20 MPa) in order to convert wet organic feedstocks into liquefied biocrude oil. HTL is a promising thermochemical technology because it requires a low total solid content (10–25%), and its by-products can be controlled by an alteration of the experimental parameters [30]. Previously, swine manure has been utilized as a feedstock for HTL purposes. Higher heating values (HHV) of biocrude oil have been reported of up to 38.8 MJ/kg with a biocrude oil yield of 39% [7]. Since human feces have a similar chemical composition to that of swine manure, it also has the potential to be a promising HTL feedstock [23]. However, one of the main drawbacks of the HTL process is that it produces a wastewater known as post-hydrothermal liquefaction wastewater (PHWW). This human feces PHWW (HF-PHWW) contains a large amount of organic compounds, including organic acids (acetic acid, pentanoic acid, etc.), nitrogenous compounds (2-piperidone, 2-6-dimethyl-3-pyridinol, etc.), and phenol derivatives (3-dimethylamino-phenol, etc.) [27]. The large content of nitrogenous compounds and ammonia present in the wastewater limits the self-sustaining nature of the HTL process. Therefore, finding ways to treat and utilize HF-PHWW is essential to minimize the environmental and economic impacts for the thermochemical conversion of human feces via HTL.

Catalytic hydrothermal gasification (CHG) has been proposed as one way to treat high moisture content feedstocks, which is believed to be a promising process for fuel gas and hydrogen production [9]. The process of gasification involves the incorporation of catalysts with high reaction temperatures in order to chemically reform the bonds in organic feedstocks into basic gaseous constituents, including H₂, CH₄, CO₂, and CO [33]. CHG utilizes water as the reaction medium, which allows this process to bypass the energetically costly step that involves drying the organic feedstock [28]. CHG can also selectively produce syngas (H₂ and CO) in high concentrations, and the byproducts of this form of gasification are environmentally benign. The benefit of CHG over traditional gas and liquid thermochemical conversion technologies is that it has an elevated reaction rate and enhanced heat transfer properties due to the use of water as the reaction medium [18], [21], [24], [37]. Therefore, CHG shows potential as a method to treat the organic matter present in the PHWW in order to enhance the prospect of HTL and produce value-added gasses.

Due to the high moisture content and large organic matter content of HF-PHWW, it serves as an ideal gasification feedstock. Currently, much of the work in the field of supercritical water gasification includes the incorporation of model compounds, such as glucose [6], fructose [25], and mixtures of model compounds [15]. Although previous studies have focused on the CHG of wastewater from feedstocks such as algae [10], poly-vinyl alcohol [34], and the amino-acid production process [19], no studies have focused on the wastewater generated from the HTL of human feces. Further, no previous studies have focused on concomitantly analyzing CHG from the perspectives of energy recovery, H₂ production, and organic matter reduction (COD_r).

The objectives of this study are: (1) to investigate the use of gasification catalysts using low reaction conditions on a previously unutilized wastewater feedstock for the maximization of H₂ content and liquid effluent quality; (2) to study the impact of the mixing ratio of two combined catalysts on the gas and liquid characteristics; (3) to optimize the reaction conditions for the maximization of the H₂ content, COD_r, and energy recovery.

Section snippets

Feedstock characterization

Human feces samples were collected by volunteers at the University of Illinois at Urbana-Champaign. All samples were combined, mixed, and stored in an airtight receptacle in a 4 °C fridge. The moisture content of the human feces feedstock was determined as the solid residue remaining at 105 °C after 24 h. The elemental composition of human feces was determined using a CE 440 Element Analyzer (Exeter Analytical, Inc., North Chelmsford, MA), and the chemical composition was analyzed per the

Influence of catalysts on CHG liquid and gas effluent quality

Fig. 1 displays the gas and liquid characteristics resulting from the catalyst screening gasification study conducted at 400 °C, a 60-min retention time (RT), an initial pressure of 30 bar, and a catalyst/feedstock (C/F) ratio of 0.1. From the data presented in Fig. 1, NaOH produced the highest H₂ content (46.9%) and one of the lowest CO₂ contents (49.7%). This is due to the role of homogenous catalysts in promoting the adsorption of CO₂ to produce carbonate salts and water which enhance the

Conclusions

This study demonstrated that a mixture of Raney Ni and Ru/AC could produce more H₂ than either catalyst individually, indicating catalyst synergy may exist between these two catalysts. A Ni:Ru ratio of 0.9 at 400 °C and a 30-min retention time lead to a maximum H₂ composition of 56.3%, compared to 41.2% using pure Raney Ni. Similarly, increasing the Raney Ni content lead to enhanced gas characteristics. At a Ni:Ru ratio of 0.9, the H₂ yield, gas yield, and H₂/organic matter content yielded

Compliance with ethical standards

The authors declare that they have no conflict of interest. All authors have read and have abided by the statement of ethical standards.

Acknowledgments

This work was supported by the Bill & Melinda Gates Foundation (RTTC-C-R2-01-001).

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Influence of catalysts on hydrogen production from wastewater generated from the HTL of human feces via catalytic hydrothermal gasification

Highlights

Abstract

Introduction

Section snippets

Feedstock characterization

Influence of catalysts on CHG liquid and gas effluent quality

Conclusions

Compliance with ethical standards

Acknowledgments

Int J Hydrogen Energy

Appl Catal A Gen

Int J Hydrogen Energy

J Supercrit Fluids

Appl Energy

Bioresour Technol

Int J Hydrogen Energy

Fuel

Int J Hydrogen Energy

Energy Convers Manag

Appl Catal A General

Int J Hydrogen Energy

Bioresour Technol

Energy Convers Manag

Biomass Bioenergy

J Supercrit Fluids

Int J Hydrogen Energy

J Supercrit Fluids

Renew Sustain Energy Rev

Int J Hydrogen Energy