Algal biomass to hydrogen: optimising carbon negative hydrogen production from the supercritical water gasification of microalgae

Heeley, Kieran ORCID: 0000-0003-2534-1648 (2025). Algal biomass to hydrogen: optimising carbon negative hydrogen production from the supercritical water gasification of microalgae. University of Birmingham. Ph.D.

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Abstract

Supercritical water gasification (SCWG) of microalgae is a potentially viable alternative for hydrogen production that can simultaneously remove carbon from the atmosphere, without the environmental challenges associated with other biomass processes. However, due to the high costs of algal production, achieving these environmental benefits requires limiting the feedstock requirement. The work herein addresses this by optimising the hydrogen yield, while ensuring that significant carbon capture is achieved from the SCWG of the microalga Chlorella vulgaris.

A continuous SCWG rig was designed, constructed, commissioned and utilised to investigate the influence of operating conditions, catalysts and oxidant to find the optimal values. An initiative of using the algal growth water as the reaction media was adopted, as industrial applications would utilise this as the reaction medium in practice. A significant difference was observed upon using growth water compared to distilled water. Most notable was the decrease in carbon monoxide and increase in hydrogen produced, which was attributed to the elevated pH. Consequently, the algal growth water was utilised as reaction medium in subsequent experiments in this work.

To maximise the hydrogen production, it is important to find the ideal and cost-effective catalyst. Some literature outlined FeCl3 as potentially the most effective catalyst for achieving high hydrogen yields, yet further literature investigating this was scarce. Hence, this work investigates FeCl3 as a potential catalyst for the SCWG of Chlorella vulgaris, for a range of temperatures (400 - 600°C) and biomass concentrations (1 – 3wt.%). A significant decrease in hydrogen yield, energy efficiency and the amount of the feedstock carbon that is converted to gas was observed. This was attributed to the reduced pH caused by the Lewis acid activity of FeCl3 which suppressed the water gas shift reaction and increased polymerisation reactions. Accordingly, FeCl3 was deemed an unsuitable catalyst. The more established catalysts of potassium hydroxide (KOH) and ruthenium (Ru/C) were
found to be effective and were therefore used for subsequent work in this thesis.

An in-depth study of the influence of temperature (400 - 600°C), biomass concentration (1 - 3wt.%), KOH concentration (0 – 1wt.%), Ru/C catalyst (present or not present) and oxidant (oxidant coefficient 0 - 0.5), including any interactions, was completed. High temperatures, low biomass concentrations, high KOH concentrations and Ru/C catalyst were all found to be favourable for high hydrogen yields. Oxidant enhanced hydrogen yields up to 20% of the stoichiometric concentration for complete oxidation, after which hydrogen was oxidised causing a reduction in hydrogen product yield. Additionally, several significant interactive effects were observed between the catalysts and operating conditions. Hence, in the reaction alone, the greatest hydrogen yield was predicted to be23 mg/g at 1wt.% biomass, 600°C, 1wt.% KOH, Ru/C and an oxidant coefficient of 0.2.

A whole system model was developed in ASPEN plus ®, which incorporated the experimental work performed as part of this research, to find the true optimum conditions when system losses are considered. The use of experimental data to program the reactor in this model revealed significant deviations from equilibrium reactor models used in literature, which emphasises the importance of incorporation of experimental data in SCWG system analysis. The model of the whole system showed that, as with the reaction alone, higher temperatures and the presence of Ru/C favoured hydrogen yield and efficiency. Oxidant coefficient also showed a similar trend however, greater concentration of oxidant was preferred in the model, due to a larger reaction enthalpy. Higher heating requirements and pumping power required at lower biomass concentrations and poor reaction performance at higher biomass concentrations meant that a moderate 2wt.% was the optimal concentration. Also, KOH was found to be most effective at minimum or maximum concentration ( 0
or 1wt.%), which differed from the reaction alone. As a result, the optimum hydrogen yield predicted was 12.7mg/g at 2wt% biomass, 600°C, 1wt.% KOH, Ru/C and an oxidant coefficient of 0.3. At these conditions, 68% of the feedstock carbon was captured as CO2 (38%) or biochar (30%), making the process significantly carbon negative at those conditions.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):