Biogas purification from anaerobic digestion in a wastewater treatment plant for biofuel production

Abstract

The main objective of this investigation consists in the biogas purification coming from the anaerobic digestion of sludges in a wastewater treatment plant, in order to be used later as biofuel for vehicles. This article consists of the optimization of the biogas desulphurization. In our case, this process was achieved in a chemical way. Besides the scrubbing towers, the pilot plant used included filters of activated carbon at the end of the line. The H₂S inflow concentrations were quite high. The effluent biogas from the scrubbing towers presented an H₂S concentration less than 1 ppm and zero or undetectable values were obtained for up to 58 analyzed trace elements.

Introduction

As a consequence of new wastewater plants and the large amount of wastewater treated, there has been an enormous increase in the production of sewage sludge over the last few decades. Nowadays in Spain the average production of stabilized and dehydrated sludge is approximately 40–60 grams/inhabitant/day (15–20 tons/inhabitant/year) [1], [2].

With reference to energy, the European Environment Agency (EEA) identified and prioritized five environmental and sectorial areas which the European Union has included in its Sixth Environment Action Programme as well as in its Sustainable Development Strategy. Regarding climate change, there is a movement to reduce greenhouse gases as well as to enhance the rational use of fossil fuels (energy efficiency and renewable and sustainable energy sources) [3].

In the majority of large wastewater treatment plants (i.e. those with the greatest potential for energy use) sludge is generally stabilized by means of anaerobic digestion [4]. This process produces biogas, which is composed of various gases such as methane (60–70%), CO₂ (30–40%), nitrogen (<1%), and H₂S (10–2000 ppm). However, as can be observed, such biogas is mostly composed of methane.

In order to obtain energy from biogas in a more productive and cost-efficient way, the gas must be enriched and its pollutants eliminated. This means that all gases except for methane (CH₄) must be removed. The removal of hydrogen sulphide (H₂S) is particularly crucial because it can cause corrosion, which can seriously damage energy co-generation equipment or other installations. Water must also be eliminated because of the accumulation potential of condensate in the pipe line. Finally, CO₂ must be removed if the biogas is to be upgraded to standard natural gas or car fuel since CO₂ reduces the energy content of the biogas [5].

As previously mentioned, once enriched, the biogas obtained from sludge is mostly composed of methane (CH₄), a colourless, odourless, inflammable gas that is lighter than air (a key factor in the greenhouse effect). Its calorific value is 37781.6 kJ/Nm³ and its energy generation capacity is 5 kWh/Nm³.

The major reasons for gas upgrading include the need to fulfil the requirements of gas appliances (engines, boilers, fuel cells, vehicles, etc.); to increase the heating value of the biogas; and/or to standardise the biogas quality. So it is important to point out that the required quality depends strongly on the application [2].

Biogas is generally enriched by any of the following methods to remove carbon dioxide [2], [6]: Pressure swing adsorption (PSA) with activated carbon or molecular sieves, physical absorption, chemical absorption, biological absorption, use of organic solvents, cryogenic separation, and membrane purification gains interest.

Sewage sludge contains a significant concentration of sulphur (0.3–2.3 wt%) that hampers sludge (co-)combustion as the final disposal route due to the formation and emission of gaseous SO₂ and associated acid rain problems. Dewil et al. [7] have recently studied the distribution and transformations of sulphur compounds occurring during the successive steps in the sewage sludge treatment. It was seen that sulphates are the predominant compounds in secondary sludge. During thickening, the oxygen level in the sludge decreases due to microbial activity, and the sulphates are gradually transformed into sulphides. The process continues when thickened sludge is stored in the sludge storage tanks. After anaerobic digestion, the ORP of the sludge has decreased to such an extent that all inorganic sulphur is transformed into sulphides.

Even though the purpose of this article is to present the results of the first phase of our research project (i.e. the chemical desulphurization of biogas), the second phase, which is presently underway, will treat the biogas in a new pilot plant where it will undergo chemical scrubbing with amines. In this second phase, we plan to extract all biogas components that do not contribute to its use as biofuel (mainly CO₂ and H₂O). The quantity of raw biogas to be treated will be 5 Nm³/h. The previous removal of H₂S improves the efficiency of the chemical absorption process and reduces the costs of the absorbent.

As shall be shown, the upgraded biogas obtained in the pilot plant in the first processing phase has an H₂S concentration of less than 1 ppm. However, as an added precaution, we have installed an activated carbon filter at the end of the line, which assures the maximum retention of hydrogen sulphide particles

Once the possible traces of hydrogen sulphide are eliminated in the first phase, the second phase focuses on the removal of carbon dioxide (CO₂). After desulphurization, the carbon dioxide volume in the resulting biogas is 38.5%.

Chemical gas scrubbing technologies are based on the contact of a chemical scrubbing agent with the gas stream to be treated. The two main technologies used for this purpose are packed towers and atomized mist systems [8], [9].

This research study uses a system of countercurrent packed scrubbers. A possible configuration of scrubbing towers consists of a vertical reservoir with a gas stream that passes up through the filler, and a liquid scrubbing solution that flows downwards through the filler [10]. Scrubbing towers are very effective in the removal of hydrogen sulphide, but do not work as well when it comes to removing organic compounds that are not soluble in water. This system is generally the most cost-efficient chemical technology for scrubbing large quantities of biogas, and it is also the simplest to operate. The scrubbing solution usually consists of a hydroxide, generally sodium hydroxide and sodium hypochlorite although alternatively, other chemical solutions can be used.

For both economic and operational reasons, the choice of the right chemical agent is crucial [11]. One possibility is to use sodium hydroxide as the sole scrubbing agent. Previous research has shown that high concentrations of hydrogen sulphide can be removed in this way at a very low cost. However, this method can only eliminate from 90% to 95% of the H₂S concentration, and furthermore, it only eliminates H₂S, and no other gas compounds. This means that a second treatment phase is necessary in most gas scrubbing installations.

In sludge, ammonia is produced during the degradation of nitrogenous matter, mainly proteins and urea. Ammonium (NH₄⁺) and free ammonia (NH₃) are the two most predominant forms of inorganic nitrogen present [2]. When there is a high ammonium concentration, it is frequently more economical to remove it in a scrubbing tower with a low pH value (normally 3) by using a sulphuric acid solution. The oxidizing scrubbers, particularly scrubbing towers that use sodium hypochlorite, can eliminate ammonium very efficiently, but at a very high cost. In addition, high ammonium concentrations can reduce the efficacy of the hypochlorite when it comes to removing other compounds. This may occur because the ammonium reaction takes place very rapidly, and at the same time also consumes considerable amounts of chlorine, which produces localized areas of low oxidation–reduction potential in the tower.

For certain treatments such as the treatment of gaseous effluents in wastewater plants, the most commonly used scrubbing tower configuration consists of the use of sodium hypochlorite with or without the addition of sodium hydroxide for pH control.

Finally, given the wide use of chemical desulphurization in gas scrubbing towers, we selected this method for the pre-treatment of biogas from anaerobic digestion in wastewater plants before treating it with an organic solvent, amines, in a second chemical absorption phase. Although the organic solvent removal units are more expensive than those using water as a solvent, and suffer from the need to periodically partly discharge, dispose and replace its solvents, low-pressure operation is possible and reductions of CO₂ to 0.5–1 vol% in biogas are possible [2].

Since our biogas is characterized by high inflow concentrations of H₂S, the main objective of this first research phase was to verify whether chemical desulphurization was a sufficiently effective technique to remove it. We also wished to find out the extent to which the activated carbon phase would be necessary. In addition, it was our intention to optimize the chemicals used as well as discover the most effective pH range.