Biogas Desulphurisation: History
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Subjects: Biotechnology & Applied Microbiology
Contributor:
Emky Valdebenito-Rolack

The agriculture and livestock industry generate waste used in anaerobic digestion to produce biogas containing methane (CH4), useful in the generation of electricity and heat. However, although biogas is mainly composed of CH4 (~65%) and CO2 (~34%), among the 1% of other compounds present is hydrogen sulphide (H2S) which deteriorates engines and power generation fuel cells that use biogas, generating a foul smell and contaminating the environment. As a solution to this, anoxic biofiltration, specifically with biotrickling filters (BTFs), stands out in terms of the elimination of H2S as it is cost-effective, efficient, and more environmentally friendly than chemical solutions. BTFs are a devices in which a microbial biofilm (mainly bacteria) formed on a packing material, degrades the pollutants (in this case hydrogen sulphide).

  • anoxic biofiltration
  • hydrogen sulphide
  • desulphurisation
  • biotrickling filtration
  • biofilter performance

1. Introduction

Anaerobic digestion (AD) is a biological process where solid organic matter originated in the agriculture and livestock industry, among others, is used to obtain biogas containing methane (CH4), which in quantities of ~65% is burned to produce bioenergy as heat and electricity in cogeneration engines . The world’s biggest waste sources used in AD come from rice farming, corn farming, and wheat. Among them, the major energy crops for biogas production are maize silage and grass, followed by lignocellulosic residues or other plant biomass in general (rich in lignin content) from sources such as banana plants (leftover banana trees, flowers, leaves), palm oil residues after harvest, sugar and palm oil refinement processes (that produce large quantities of fibrous lignocellulosic biomass), forestry residues, coffee pulp, and field residues such as corn stover [1]. AD is conducted by breaking up the organic matter (anaerobically) generating biogas, mainly composed of methane (CH4) and carbon dioxide (CO2), with a lower concentration of hydrogen sulphide (H2S), ammonium (NH3), and other trace compounds .

AD is carried out by several groups of bacteria and methanogenic archaea in four stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis, CH4 is produced in the last stage . In the intermediate stages of AD, H2S is produced from the degradation of proteins containing sulphur-rich amino acids and from the sulphate reduction (SO42−). This process is carried out by sulphate-reducing bacteria (SRB) [2].

At low concentrations (0.01–0.3 ppmv) H2S produces foul smells and the effects of H2S on human health at different concentrations are summarized in Table 1. Thus, the maximum exposure concentration permitted in the industry is 10 ppmv, with peaks of 50 ppmv accepted for a duration of 10 min [3][4].

Table 1. H2S concentrations and subsequent effects on human health [3].

H2S Concentration (ppmv) Effect on Human Health
0.01–0.3 Olfactory threshold. Rotten egg smell.
20 Strong odour. Eye irritation may occur.
20–50 Eye and lung irritation.
100 Eye and lung irritation, olfactory paralysis, apparent disappearance of odour.
>150 Severe eye and lung irritation. Sensation of olfactory loss.
>250 Pulmonary edema and risk of death.
>500 Highly dangerous, risk of death. Evacuation is mandatory.
>1000 Loss of consciousness, apnoea, immediate collapse. Death.

In electricity or heat generation by biogas combustion, the H2S contained in biogas increases the sulphur dioxide (SO2) emissions, producing acid rain, corrodes metals and degrades oil in electrical power cogeneration engines (> 500 ppmv) and, in the application of biogas as raw material for high-efficiency fuel cells, especially solid-oxide fuel cells (SOFC), forms a nickel sulphide precipitate (NiS) creating a metal sulphide layer on the anode surface, reducing the cell electrochemical activity (> 5-20 ppmv) [5][6][7][8]

The aforementioned problems and the international regulations referring to H2S concentrations prior to biomethane injection into natural gas grids (among others) have motivated the development of many physical-chemical and biological methods for eliminating H2S in biogas [9]. Among these solutions, the biofilters stand out because they are cost-effective, efficient, and more environmentally friendly than physical-chemical methods, as it requires significantly lower chemical use and energy consumption, which translates into a low carbon footprint and lower costs [10][11].

2. Biofilters

Biofilters for gas treatment are devices in which a residual gas current containing pollutants is passed through a packing material containing bacteria immobilized as biofilms (BF) metabolizing the pollutants (in this case H2S in biogas) [11]. BFs are organized microorganism communities attached to surfaces, embedded in an exopolysaccharide (EPS) matrix, whose physiology and gene expression is different than a planktonic community. Among these differences are: (a) BF microorganisms are more resistant to environmental factors; (b) they do not limit their growth by substrate, as they can interact with the environment and among themselves since they are not fixed structures; and (c) they produce EPSs [12][13][14].

There are two groups of biofilters: (1) open biofilters: usually a box with packing material containing the biofilms inside which is open to the atmosphere, in which parameters such as temperature, humidity, and the purity of the microbial cultures are difficult to control as they are weather dependent (due to rain and temperature fluctuation, among others); and (2) closed biofilters: in which the biofilm-containing packing material is inside a closed container (usually made of steel or plastic), making it easier to control parameters such as culture media flow, nutrients, gas flow, temperature, and humidity [15]. Biotrickling filters (BTF) are a type of closed biofilter combined with a bioreactor containing a microorganisms culture or a nutrient solution container [16]. Anoxic BTFs are one of the most widely used biological technologies to perform biogas desulfurisation, because of their lower investment, operation, and maintenance costs, as well as for their greater energy efficiency removing foul smells and in industrial-scale applications for biogas treatment [17][18]. Furthermore, this type of biotechnology presents a low pressure drop and an equitable distribution of microorganisms through the biofilter bed of the BTF [19]. Due to these factors, this article focused on BTFs.

3. BTFs

The basic setup of a BTF is usually a cylinder-shaped container filled with a packing material (Figure 1B) that is basically the biofilter part of the device, consisting of a sprinkler at the top (Figure 1, “shower”), from which the prokaryote microorganisms culture originated from a bioreactor or fermenter (Figure 1A) is applied. This culture drains through the packing material, contributing to the growth of prokaryote microorganism BF (usually bacteria or archaea). In the case of treating contaminated biogas, a counter current is usually applied from the lower part of the BTF (Figure 1, red arrow), ascending through the packing material containing the BF, where the pollutants are metabolized resulting in a reduced or eliminated contaminant concentration biogas through the upper part of the device (Figure 1, green arrow). This type of solution has been tested at an industrial scale from very low concentrations to values close to 15,000 ppmv [20][21].

Figure 1. Basic set up of a biotrickling filter (BTF), accompanied by the bioreactor supplying bacterial culture or nutrient solution. Red arrow: H2S contaminated biogas inlet; green arrow: BTF desulfurized biogas outlet. A) Bioreactor or nutrient liquid container, B) working volume, filled with packing material containing microbial biofilms and C) port to take samples of packing material for microbiological analysis.

4. Markers for the Comparison of the Performances of Anoxic BTFs in Biogas Desulphurisation

Traditional and microbiological markers are useful to compare the performance of BTFs (and biofilters) in terms of their removal of H2S from biogas, and the need to simultaneously report and relate them is of vital importance for selecting more efficient microorganisms and choosing the most apropiate packing materials for bacterial biofilms  [22][23][24][25][26]. In this article, the recommended traditional and microbiological markers to the assesment of the BTFs performances in biogas desulphurisation are summarized in the Table 2, representing a framework of minimum parameters to be considered to compare the performance of anoxic BTFs for biogas desulphurisation and thus evaluate research in this field in a complete and uniform way. These parameters can be applied to BTFs for any type of gas treatment, not only for desulphurisation.

Table 2. Suggested set of traditional and microbiological parameters that every research on BTFs should include.

Parameter Description
Traditional Markers of Efficiency in BTFs for Biogas Desulphurisation
ECcrit (g m−3 h−1) Elimination capacity at 90–100% of RE.
LRcrit (g m−3 h−1) Loading rate of pollutant at 90–100% of RE, theoretically its value is equal to ECcrit.
EBRT for ECcrit (min) Empty bed retention time at the ECcrit, sometimes is equivalent to EBRT for ECmax.
ECmax Maximum elimination capacity (EC). Corresponds to the inflexion point or the first value of the asymptotic part of the EC versus LR curve.
LRmax (g m−3 h−1) Loading rate (LR) at the ECmax.
RE for ECmax Removing efficiency at ECmax.
EBRT for ECmax Empty bed retention time at the ECmax, sometimes equivalent to the EBRT for ECcrit.
Microbiological markers
Inoculum description Detail of the sample, culture, or strain for the inoculum.
Quantification of biomass Bacterial cell count by packing material surface, through microscopic cell counting in Neubauer chamber or Baclight stain. CFU by packing material surface, through the drop plate method.
Initial microbial ecology (after start-up) DGGE or TEFAP for the biofilm in the packing material of the BTF and the liquid culture in the bioreactor.
Final microbial ecology (at ECcrit and ECmax times) DGGE or TEFAP for the biofilm in the packing material of the BTF and the liquid culture in the bioreactor.
Microbial ecology under perturbation DGGE or TEFAP for the biofilm in the packing material of the BTF and the liquid culture in the bioreactor.

Abbreviations: ECcrit: critical elimination capacity, ECmax: maximal elimination capacity, LRcrit: critical loading rate, LRmax: maximal loading rate, EBRT: empty bed retention time, RE: remotion efficiency, DGGE: denaturant gradient gel electrophoresis, TEFAP: tag-encoded FLX-amplicon pyrosequencing. CFU: colony forming units.

On other hand, in silico experiments utilizing mathematical modelling have gained attention in recent years, simulating the behaviour of real BTFs for biogas desulphurisation, to enable the better understanding of their processes and allowing the selecting of appropriate operational parameters for real solutions [27]. In this regard, a recent work of López et al. (2021) evaluated the effect of different control strategies in an aerobic BTFs for biogas desulfurisation under different H2S LR conditions [28]. Although this research is focused on aerobic BTFs, similar modelling results were described by Almenglo et al. in 2015 for anoxic BTFs [29]. This type of mathematical model development, including the traditional performance parameters and microbiological markers suggested in the Table 2, are a powerful tool to evaluate operational strategies.

For more detailed information, please read "Markers for the comparison of the performances of anoxic biotriclikng filters in biogas desulphurisation: a critical review" in the link below.

This entry is adapted from the peer-reviewed paper 10.3390/pr9030567

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