Utilisation of Liquefied Biomass in Water Co-Electrolysis for the Production of Synthesis Gas ^†

Abstract

This paper presents a study on the addition of liquefied biomass of different lignocellulosic forest residues as a means to enhance the co-electrolysis process leading to the production of synthesis gas, composed of H₂, CO, CO_2, and O₂, also known as syngas_, with the aim of a subsequent conversion into methane and methanol. Tests were made on a 1 kW prototype unit and showed that the use of liquefied biomass clearly enhances the reaction leading to syngas production. The optimisation study performed showed that the best results are obtained with an addition of 2.5% mass of liquefied biomass obtained from Acacia melanoxylon and operating conditions of a pressure of 4 bar gauge and a temperature of 110 °C.

1. Introduction

Today, an important amount of the World’s energy consumption is still based on fossil fuels, and thus, the harmful impacts of the combustion of fossil fuels are increasing in magnitude [1], which is devastating the environment and damaging the quality of life. Therefore, global energy transformation is taking place, which has been accelerated by the rapid development of using renewable energy [2]. To enhance this drive and to reduce atmospheric emissions, H₂ has been considered a promising energy carrier, as generating electricity from H₂ using a fuel cell or in combustion systems causes no local pollution as the only by-product is pure water. Hydrogen/synthesis gas can be produced locally, which also contributes to reducing countries’ dependence on external energy suppliers, and can be extracted from an extensive range of substances, such as oil, gas, and water. Decomposing water by electrolysis offers promising opportunities in terms of promoting synergies with renewable energies. H₂ and synthesis gas can be produced and stored before their use due to the intermittent nature of renewable energy sources, so it is suitable for distributed production and centralised production connected specifically to remote renewable resources. Advances in integrating H₂ and syngas in power systems have been made in recent years ranging from production and storage to re-electrification and safety measures [3]. Nevertheless, several integration issues still require adequate answers calling for applied research to solve and optimise possible solutions in order to achieve cost-effective approaches within the scope of a green fuel economy [4].

Therefore, water electrolysis is an important technology for producing clean hydrogen fuel and reducing our reliance on fossil fuels [5,6]. There are currently several challenges to the widespread adoption of green hydrogen, including the high cost of production and the lack of infrastructure for storing and distributing hydrogen. However, as renewable energy prices continue to decline and technology improves, green hydrogen is expected to become more competitive with fossil fuels and could play a major role in the energy mix of the future [7]. Apart from hydrogen, synthesis gas, which is a mixture of hydrogen H₂, carbon monoxide (CO), and, often, carbon dioxide (CO₂), can be used as an intermediate for the production of chemicals, fuels, and other products, such as methane, methanol, hydrogen, synthetic natural gas, ammonia, acetic acid, and liquid fuels through the Fisher–Tropsch process (such as diesel and gasoline) [8]. Of the technologies used to produce syngas through carbon-based substances [8,9], the co-electrolysis of water is particularly interesting. Water co-electrolysis is an electrochemical process by which carbon dioxide and steam are transformed into syngas through the use of a solid oxide cell, similar to normal Solid Oxide Electrolysis Cells (SOECs). Unlike normal SOECs or other methods of electrolysis that use only water, co-electrolysis can be used to recover and reuse CO₂ produced from other processes [10,11]. Co-electrolysis is an attractive process for syngas production that uses excess generated electricity. In extended applications, syngas produced from co-electrolysis can be used for various applications like methane and/or methanol production [12,13]. There has been increasing research interest in topics related to power-to-methane conversion and co-electrolysis, which has increased its feasibility and promoted its possible use in electric power management and CO₂ abatement. However, there have been no comprehensive studies focusing on power-to-methane conversion using co-electrolysis from a process systems engineering viewpoint. In order to achieve actual construction and operation at a commercial level, it is necessary to progress from the current development stage [14,15] and surpass the actual challenges preventing commercialisation.

Since 2014, this research team has been investigating this particular field following the design of a process capable of generating synthesis gas, composed of CO₂, CO, O₂, and H₂, with traces of CH₄, from alkaline water electrolysis using graphite electrodes [16]. During the first financed project, the laboratory proof of concept was made [17], which allowed proceeding to further developments under other funded projects, which resulted in the testing and optimised operation of a 1 kW pilot rig producing synthesis gas coupled with a heterogeneous tubular reactor able to produce methane and methanol [18,19]. More recently, another nationally funded project [20] aims to promote the development of a pilot (patented) system to produce syngas without separation of the generated gases. This novel process, intended as a positive contribution to reducing the dependence on fossil fuels, combines alkaline water electrolysis with the addition of a carbon source in the form of graphite composing the disks inside the stack and, also, the addition of liquified biomass obtained from forest residues [20]. Syngas has several potential uses for modern industry, as it can be converted into other synthetic fuels, both gaseous fuels like methane or liquid fuels such as gasoline and diesel, through processes such as the Fischer–Tropsch process or methanol-to-gasoline conversion [21]. It can also be used as a feedstock for the production of a wide range of chemicals, including methanol, ammonia, and acetic acid. In addition, syngas can be burned to generate electricity in gas turbines or internal combustion engines and can function as a substitute for more expensive or scarce feedstocks in refinery processes, such as the hydrogen production taking place in petroleum refining [6].

2. Materials and Methods

As displayed schematically in Figure 1, the prototype unit consists of an electrolyte reservoir, a set of pressure and temperature sensors, a pump, a radiator with a fan (in order to control the temperature of the process), a heating resistor, a moisture adsorption column, and an electrolyser consisting of a stack of 11 spaced graphite discs. Figure 2 shows an aspect of the prototype unit.

The production of synthesis gas via electrochemical means is based on the following reactions:

(i)

Cathode reactions:

2H₂O + 2e⁻ → 2H₂ + 2OH⁻

(1)

(ii)

Anode reactions:

4OH⁻ → O₂ + H₂O + 4e⁻

(2)

2C + O₂ → 2CO

(3)

2CO + O₂ → 2CO₂

(4)

(iii)

Global reaction:

2H₂O +C + W_e → 2H₂ + CO₂

(5)

Current experimental work focuses on carrying out optimisation tests to determine the ideal conditions for the production of the syngas to be used, in the future, as a feedstock for methane production. For this specific purpose, the syngas produced aims to have as little oxygen content as possible (to minimise the risk of catalyst deactivation during methanation) and a CO₂:H₂ ratio that is ideally close to the one associated with the conversion of CO₂ into methane (1:4) [22].

Two different bio-oils obtained from different biomass types were tested for two different concentrations (2.5 and 5% (w/w)) in order to allow for comparisons with each other and the respective performance with and without the use of any liquified biomass (bio-oil).

Table 1. Elemental analysis of fresh biomass and obtained bio-oil extracts.

Biomass Types		C (%)	H (%)	N (%)	S (%)	O (%)
Energreen	Fresh	46.0	5.3	1.2	0.1	47.2
	Bio-oil	65.3	9.7	1.2	0.3	23.5
Acacia	Fresh	47.4	5.8	<0.5	<0.1	46.8
	Bio-oil	68.2	10.7	<0.2	<2.0	21.1

shows the elemental analysis of both the fresh biomass and the extracted bio-oils. The extraction procedure is described in detail elsewhere [23]. Both vegetable biomass sources were collected from Portuguese forests: “Energreen” is from a mixture of Eucalyptus globulus, Pinus pinaster, and Quercus suber, and “Acacia” is from Acacia melanoxylon only [24].

In all tests, the electrolyte used was a 1 M solution of NaOH, as indicated schematically in Figure 3.

The 1 kW prototype plant has a stack with 12 cells (graphite bipolar electrodes), with the distance between electrodes being 5 mm. The electrodes have a diameter of 117 mm (100 cm² area) and a 5 mm thickness with two 10 mm holes to allow for circulation. The electrolyte used is NaOH (0.4 M). The dimensions of each electrode can be found in Figure 4.

The electrolyte used is NaOH (1 M). A pump is used to circulate the electrolyte from the reservoir to the stack. The electrolyte solution together with the produced gaseous phase leaves the stack from the top and is guided through the tubes into the initial tank where the liquid phase is recirculated while the gaseous phase is separated and leaves the tank through a pressure valve. Since the produced gas phase leaving the tank still carries some steam, it needs to be passed through a cooling serpentine to condense the bulk of the water that is then collected in the following drainable reservoir. The remaining gas exits from a second hole on the top and is passed through a cylindrical tank filled with a molecular sieve (2.0–5.0 mm) to remove any remaining moisture. Following the removal of the majority of the humidity, the gases enter a group of sensors to analyse and determine their pressure, temperature, and remaining humidity as well as their composition. The control and adjustable operational parameters are the following: the voltage/current and the temperature and pressure, and the current of each stack is separately monitored.

To optimise syngas production and its composition, three different groups of tests were executed, one without the addition of liquified biomass, one using a liquified sample of Acacia biomass, and another using a sample of non-specified composition designated as Energreen, obtained from the liquefaction of a mixture of cork, eucalyptus, and pine wood [23]. Each sample was diluted in the electrolyte solution at 2.5 and 5% (w/w), and each one was tested within an interval of pressure (4 and 5 bar gauge) and temperature (100 and 110 °C).

Each test was executed for 3 h with the temperature, pressure, flow of gas produced, and current applied, among other data, being monitored every 5 min. To simplify the analysis and presentation, the average of the last 3 measurements in a stationary state was collected and presented as the shortened results for each test. Through these methods, it was possible to evaluate how the operational conditions affect the flow rate of the gas produced and its composition (which constitute the most important set of results). These tests were performed bearing in mind the intended use of the syngas produced for subsequent methane production, with the following aims:

(i)

Have an oxygen content as low as possible, to minimise the risk of catalyst deactivation during methanation;

(ii)

A CO₂:H₂ ratio ideally close to the one associated with the conversion of CO₂ into methane (1:4) via the following reaction:

CO₂ + 4H₂ → CH₄ + H₂O

(6)

(iii)

A good relation between the flow of gas produced and the energy consumed to produce it.

3. Results

As a way to resume the results of each individual test,

Table 2. Global test results.

Carbon Source	Test Conditions	O2 (%)	CO (%)	CO2 (%)	H2 (%)	F (l/h)	W/f (Wh/L)	CO2:H2 Ratio
No biomass added	4 bar gauge; 100 °C	4.1	1.8	25.4	68.7	46.84	10.58	2.71
	4 bar gauge; 110 °C	3.5	1.8	26.2	68.5	44.95	11.39	2.61
	5 bar gauge; 100 °C	8.8	2.1	29.6	59.5	57.19	8.62	2.01
	5 bar gauge; 110 °C	6.7	2.1	31.8	59.4	61.79	7.92	1.87
2.5% Acacia biomass	4 bar gauge 100 °C	3.9	2.0	34.5	59.6	67.53	7.05	1.72
	4 bar gauge 110 °C	2.1	2.0	42.6	53.3	62.92	6.71	1.25
	5 bar gauge 100 °C	3.8	2.0	45.4	48.9	59.67	7.33	1.08
	5 bar gauge 110 °C	3.4	2.0	53.4	41.1	62.64	7.17	0.77
5% Acacia biomass	4 bar gauge 100 °C	3.3	2.0	54.9	39.7	63.21	7.96	0.72
	4 bar gauge 110 °C	3.1	2.0	55.0	39.9	71.35	8.00	0.72
	5 bar gauge 100 °C	4.0	2.0	53.8	40.1	63.91	8.85	0.75
	5 bar gauge 110 °C	3.2	2.0	44.5	50.3	60.40	8.89	1.13
2.5% Energreen biomass	4 bar gauge 100 °C	6.2	2.0	40.6	51.2	56.83	10.74	1.26
	4 bar gauge 110 °C	3.2	2.0	46.0	48.7	65.88	6.25	1.06
	5 bar gauge 100 °C	4.8	2.0	39.1	54.1	72.28	6.36	1.38
	5 bar gauge 110 °C	3.0	2.0	46.7	48.3	63.32	6.72	1.03
5% Energreen biomass	4 bar gauge 100 °C	4.1	2.0	40.6	53.3	67.68	7.34	1.31
	4 bar gauge 110 °C	3.4	2.0	35.0	59.6	67.16	7.64	1.71
	5 bar gauge 100 °C	4.2	2.0	36.5	57.4	62.35	8.33	1.57
	5 bar gauge 110 °C	2.9	2.0	38.3	56.8	66.36	8.01	1.48

displays an average of the last three measurements, in a stationary state, for each key parameter, as well as a final CO₂:H₂ ratio to allow for a better comparison.

The obtained results are shown graphically in Figure 5 and Figure 6, as follows: (a) Figure 5: performance results obtained without the use of biomass; (b) Figure 6: performance results obtained with the use of biomass of different origins and added in different contents.

4. Discussion

By analysing the results obtained, it can be concluded that the additions of liquified biomass boosted CO₂ production leading to a significant increase in its content within the gas outlet. As a consequence, it also lowered the CO₂:H₂ ratio. Additionally, the higher CO₂ production also requires more O₂ consumption, generating syngas with a lower O₂ concentration.

Focusing on the production of syngas specifically for future use in methanation, the addiction of biomass, specifically Acacia, shows favoured results regarding the production of a syngas mixture while lowering its O₂ content, and this can be due to the existence of less oxygen species in Acacia than in Energreen.

On the other hand, the CO₂:H₂ ratios are far from the ideal value when compared to the ideal 0.25, as expected. However, this can be easily resolved either by a later addition of H₂ or, alternatively, by the partial removal of CO₂ (with this still being an easier approach compared to removing the O₂).

Energreen, on the other hand, produced a more optimal CO₂:H₂ ratio (at the cost of lower O₂ consumption) and, most importantly, has shown the best relation between energy consumed and gas production, probably due to its composition having more oxygen species, fewer carbon species, and slightly fewer hydrogen species than Acacia

Regarding process conditions, both biomasses showcased different behaviour towards pressure, temperature, and their concentration. In the case of Acacia, the preferable conditions are 4 bar gauge at 110 °C, as these conditions result in lower O₂ and energy consumption. Between the two tested concentrations, arguably the 2.5% mixture offered the best performance by consistently exhibiting the lowest energy expenditure while producing a more optimal CO₂:H₂ ratio. This could be due to the fact that 2.5% mixtures result in the reaction medium having lower viscosity than 5% mixtures. As the chemical reactions involved are, mainly, surface processes, the influence on mass transfer could explain the observed results. In regards to Energreen, on the other hand, it is not as clear which of the two concentrations offers the best performance, with the 2.5% mixture producing syngas at a lower energy cost and being close to 5% in terms of O₂ content but producing a less ideal CO₂:H₂ ratio. The same can be said for pressure and temperature. If the priority is a reduction in O₂ content, the observed ideal conditions were 5 bar gauge at 110 °C. Although, if reducing energetic costs is deemed more important, than the best conditions were 4 bar gauge at 110 °C.

Although the use of this co-electrolysis process has not been investigated much, it is important to stress that similar tendencies were found by Liu and Li [25], regarding the reduction in overall energy costs by using added biomass, which enables high-value products at both electrodes. Also, Xiang et al. [26] found that CO₂ fixation can be enhanced by the addition of bio-oils to co-electrolysis processes.

5. Conclusions

This study clearly showed some advantages of adding liquefied biomass to the reaction media of the co-electrolysis process, regarding the subsequent production of methane and/or methanol. In fact, this biomass addition, as a whole, enhanced the reaction process towards the production of syngas. The additions of liquified biomass resulted in a significant increase in the CO₂ content of the gas outlet, resulting in a lower CO₂:H₂ ratio. Additionally, higher CO₂ production also requires more O₂ consumption, generating syngas with less O₂. Focusing on the production of syngas to be used in methanation, Acacia shows favourable results, compared to Energreen, in lowering the O₂ content. On the other hand, the CO₂:H₂ ratios are still far from the ideal 0.25, as expected. However, this can be easily resolved either by the later addition of H₂ or, alternatively, by partial removal of the CO₂ (with this still being an easier approach compared to removing O₂). Therefore, this study clearly recommends the use of liquefied biomass from Acacia in a mass content of 2.5%, a pressure of 4 bar gauge, and a temperature of 110 °C. Nevertheless, further investigations, namely regarding energy usage, are in order.