Thermochemical pre-treatment technology for residues from breweries and other biomass to enhance anaerobic digestion

Final Report Summary - THERCHEM (Thermochemical pre-treatment technology for residues from breweries and other biomass to enhance anaerobic digestion)

Executive Summary:
TherChem represents sustainable energy and waste management based on a combined thermo-chemical and biotechnological process. The first step in the TherChem process is a thermochemical pre-treatment of chosen feedstock with high amount of ligno-cellulosic compounds, a chemical structure which cannot degraded by bacteria. Hereby, parameter settings such as acid concentration, temperature, solid/liquid ratio and exposure time were varied for six different ligno-cellulosic containing substrates (e.g. brewers´ spent grains, wheat straw). Finally for each an optimal setup was identified resulting in high amount of solubilized sugar monomers and low concentration of unwanted thermochemical by-products. Biomethane potential tests of pre-treated substrates showed in almost all cases both, higher degradation rates as well as higher specific methane yields compared to the control.
In a second step pre-conditioned material was anaerobically treated in a single and a two phase process system. During microbial acidification solubilized carbon compounds were converted to energy rich liquid intermediates, mainly short chain volatile fatty acids and alcohols. Process parameters for this step were optimised and significant improved yields of intermediates were obtained in the majority of the applied substrates. Beside production of intermediates a hydrogen sulphide rich gas was formed. This gas was directed to a bio-trickling filter system in order to produce sulphuric acid, which was reused in the pre-treatment step. Various sulphur oxidizing bacteria were selected and different setups of parameters (e.g. flow rate, oxygen rate, retention time, immobilisation carrier material) tested. 100 % conversion rate and lowest possible pH (between 0.7 and 1.5) was achieved.
Finally, thermo-chemically pre-treated and microbial acidified material was converted to biogas. Improved results compared to the reference system could only be achieved when previous acidification step was operated properly. Otherwise inhibition and low biogas yield due to presence of sulphate and low pH occurred.
All sub-steps were carried out in both, in lab-scale as well as in demonstration scale. The design, scale up and the continuous operation of the complete system was one of the main challenges in this project. As all sub-processes were optimized at lab-scale a mobile demonstration plant was designed and build inside a 20 feet container including four sub reactor units (pre-treatment, acidification and methanogenesis; sulphuric acid recovery unit). The demo plant was continuously operated for the period of 6 months accepting brewer´s spent grains in two different modes. Although initial difficulties in operation occurred, finally very promising results in terms of efficiency and improved methane yield and faster degradation rates, respectively could be achieved.

Project Context and Objectives:
Aims and introduction
The EU climate and energy strategy for 2020 is summarised in the 20-20-20 targets. By the year 2020, greenhouse gas emissions should be reduced by at least 20% in comparison to the level of 1990. The use of renewable resources has to be increased to 20% of total energy production. The energy consumption should be decreased by 20% of projected 2020 level.

The brewing industry has a high potential to decrease the ecological footprint. This can be achieved by proper conversion of generated organic residues into renewable energy, e.g. in the form of biogas. In long-term, this energy cycle will decrease costs and energy demands. The integration of the TherChem process into an industrial process will make a big contribution to environmental protection.
Brewers’ spent grains (BSG) have a biogas potential of about 120 NL/kg fresh matter (60%v methane). However, due to the biochemical composition (e.g. hard degradable ligno-cellulosic compounds) of the brewers’ spent grains the whole potential of this feedstock cannot be exploited. By pre-treating the substrate thermo-chemically at high temperatures and under the influence of an acidic catalyst the complex chemical structure of the substrate is broken down. Consequently the pre-treated substrate is more easily accessible to microorganisms. But due to chemical reactions thermal by-products are formed that might act inhibitory to the anaerobic digestion process. High amounts of sulphuric acid in the substrate may also lead to pH drop and in this way to inhibition of the microbial consortium.

The TherChem-process comprises the following three sub-processes:
1. Pre-treatment based on a thermo-chemical process
2. Anaerobic digestion process
3. Desulphurisation process

The challenge of this project was to design pre-treatment parameters suitable for a stable anaerobic fermentation which results in high biogas/methane yield. The applied catalyst should be reused for pre-treatment of fresh substrate to provide an economic and ecological cycle. As the gained biogas is utilised on site, the electrical and/or thermal energy is consumed directly in the industrial plant. Such concepts are implemented effectively in the food and beverage industry (slaughter house, dairy factory), but this approach is completely new to the widespread brewing industry. Therefore a (mobile) pilot plant was built to prove that results from lab scale can also be achieved in a larger scale and to demonstrate the interaction between the different sub-processes.

Substrates
The innovative TherChem process was not only applied on the anaerobic digestion of brewers’ spent grains but also on five further ligno-cellulose rich substrates, which were agreed upon by the Advisory board. Therefore the following substrates were assessed: wheat brewer’s spent grains (WBSG), bagasse (BAG), wheat straw (WS), maize straw (MS) and organic materials from landscape maintenance (also called cut grass and green waste CGGW). Since BSG is an industrial product from beverage technology, its composition may vary a lot. The chosen substrates were characterised and TS, VVS, lignin-content, cellulose-content, hemicellulose, total ammonium content, total Kjedahl nitrogen and chemical oxygen demand were analysed.
Pre-treatment
Substrates with high (hemi-)cellulose and high lignin contents are harder biodegradable and result in low biomethane yields compared to the theoretical potential based on total organic carbon present. High values of total ammonium content and total Kjedahl nitrogen arise from the protein contents of the substrates. Those substrates with high protein contents (like BSG, WBSG and CGGW) might lead to higher concentrations of thermal by-products and especially Maillard reaction product during pre-treatment, because a nitrogen source is necessary for the Maillard reaction.
Thermo-chemical pre-treatment is influenced by different parameters which were varied in the lab scale experiments: temperature, TS-ratio, concentration of sulphuric acid and retention time of treatment. The analysed responses are the contents of sugars (sum of cellobiose, maltose, lactose, glucose, xylose, fructose and arabinose) and thermal by-products (sum of furfural and 5 hydroxymethylfufural), given in the unit [mg/g TS].

Biomethane potential tests (BMP-tests)
Biomethane potential tests were performed under mesophilic condition (at 35 °C) to compare the biomethane potential of pre-treated substrates and not treated substrates. The biodegradability of the substrates and the degradation rate under anaerobic conditions were evaluated.
Continuous fermentation
To assess the biodegradability of these substrates in long-term conditions, (semi-)continuous fermentations were carried out in four simultaneously running reactor set-ups:
• A single stage fed with substrate which has not been pre-treated (reference system).
• A two stage system fed with substrate which has not been pre-treated.
• A single stage fed with pre-treated substrate.
• A two stage system fed with pre-treated substrate.
During operation differences among the applied substrates were observed. Therefore parameters like pH, content of chemical oxygen demand, gas production and gas composition were measured at a daily or weekly basis.
Through an intensive process monitoring campaign a parameter setup for optimal performance (e.g. high loading rate) was found.
In some cases the biogas process was split to a two phase process: the first stage, an acidification step and the second stage, a methanogenic stage. During acidification the substrate is hydrolysed and converted to intermediates mainly consisting of volatile fatty acids or alcohols. This process favours acidic condition (pH 4 – 6).
It was aimed to combine acidification and sulphate reduction. During sulphate reduction sulphuric acid (from the pre-treatment step) is converted to (gaseous) hydrogen sulphide which leaves the system together with the hydrolysis gas (which mainly consists of carbon dioxide).
After the acidification step, the fermentation broth is fed into the methanogenic reactor (pH 7 – 8.5) in which the volatile fatty acids were converted to biogas. As the sulphur already left the system during the acidification, microbial inhibition as well as damage to equipment due to the corrosive effect of hydrogen sulphide was prevented.
Acidic catalyst recovery
As the hydrogen sulphide leaves the system together with the (worthless) hydrolysis gas, there is the option to recover the sulphur. The microbiological hydrogen sulphide (H2S) removal and subsequent conversion into sulphuric acid was assessed. A number of different sulphur oxidising bacteria have been reported in literature to be able of this conversion. Based on their metabolic pathways, elemental sulphur and sulphuric acid are the main end products. By adaption of oxygen-levels during the oxidation it is possible to target towards the production of sulphuric acid.
In order to optimise the microbiological desulphurisation of H2S-rich biogas with respect to fast and efficient sulphuric acid production, a microorganism screening was conducted to identify the most potent microbial strains to oxidise sulphur components to sulphuric acid.
Subsequently, the principle of converting H2S to sulphuric acid was to be proved at lab-scale. A continuous desulphurisation unit for microbial H2S conversion was constructed. Potent sulphur oxidising bacteria were immobilised in this column to produce sulphuric acid in a concentration sufficient to be used for substrate pre-treatment.

Demonstration plant
In order to prove the concept of the TherChem-project, a mobile pilot plant was built. The pilot-plant includes a four reactor system (pre-treatment, acidification and methanogenesis; sulphuric acid recovery unit). As the plant was suited inside a 20 ft.-container. The most important reactor was the custom built thermo-chemical pre-treatment reactor, which had to be designed specifically for this project. After the built demonstration runs were to be conducted to prove if the performance of the sub-process is comparable or even better after scale-up. Furthermore, as this was nearly impossible in lab-scale the sub-processes were connected together for the first time as one fully automated process plant.

Project Results:
Introduction
The EU climate and energy strategy for 2020 is summarised in the 20-20-20 targets. By the year 2020, greenhouse gas emissions should be reduced by at least 20% in comparison to the level of 1990. The use of renewable resources has to be increased to 20% of total energy production. The energy consumption should be decreased by 20% of projected 2020 level.
Many breweries mainly cover their energy demands with fossil fuels. Within the sector of food and beverage industry, the brewing industry has a high potential to decrease the ecological footprint. This can be achieved by proper conversion of generated organic residues into renewable energy, e.g. in the form of biogas. Production of biogas from the own organic residues and the following exploitation of the produced biogas can provide energy for the industrial process. The production of biogas has many advantages: Among them, the supply of renewable energy in the form of electricity and heat as well as the reduction of disposal costs, are the ones with the highest impact. Furthermore, a high value organic fertiliser is provided. In long-term, this energy cycle will decrease the costs for energy demands. The integration of the TherChem process into an industrial process is a contribution to environmental protection.
Brewers’ spent grains (BSG) have a biogas potential of about 120 NL/kg fresh matter (60%v methane). However, due to biochemical composition (e.g. hard degradable ligno-cellulosic compounds) of brewers’ spent grains, the whole potential of this feedstock cannot be exploited. In order to improve the exploitation (realistically up to 20%), a pre-treatment step is necessary. Among many techniques available, in our project we introduced a multi-stage fermentation process.

Objectives
The main objectives of the project comprised the following sub processes :

1) Pre-treatment based on a thermo-chemical process
2) Anaerobic digestion process
3) Desulphurisation process

These three units were the main process steps within the multi-stage fermentation process of the TherChem-project. The aim of the combination of these steps was to improve the performance in terms of methane yield and feedstock utilisation.

Aims of pre-treatment
By pre-treating the substrate thermo-chemically at high temperatures and under the influence of an acidic catalyst the complex chemical structure of the substrate is broken down. Consequently, the pre-treated substrate was more easily accessible to microorganisms of the anaerobic digestion process. But due to chemical reactions thermal by-products occur that might act as inhibitors in the anaerobic digestion process. Under thermal and acidic conditions different chemical reactions took place. Furfural and 5 hydroxymethylfurfural, summarised as thermal by-products, and Maillard reaction products (not investigated) were produced. These substances are considered to have inhibitory effects on microorganisms.
To find the suitable process parameters for pre-treatment meant to find the balance between sufficient break down of the complex cellulose structure and the production of undesired thermal by-products and Maillard reaction products. The challenge was to design the pre-treatment parameters suitable for a stable anaerobic fermentation with a high biogas/methane yield.

Aims of anaerobic digestion
The aim of the anaerobic digestion was to optimise the anaerobic digestion process for the fermentation of brewer spent grains. Optimisation was to be done by assessing the digestibility of the substrate before and after being thermo-chemically pre-treated, as well as finding the maximal loading rate. The use of a two stage system, in comparison to single stage system, was also investigated. The performance of the acidification stage was extra interesting, as it was planned to be a combined acidification and sulphate reduction.

Furthermore, different substrates were assessed to broaden the field of application of the TherChem-process. These substrates were selected by the advisory board.

Aims of desulphurisation
The aim was to recover the catalyst, sulphuric acid, which could be reused for pre-treatment of fresh substrate. This catalyst loop provides an ecological process.
The research on the sulphur cycle dealt with the microbiological H2S removal and subsequent conversion into sulphuric acid which was recycled for lignocellulose pre-treatment prior to anaerobic digestion. A number of different sulphur oxidising bacteria are reported in literature. According to their metabolic pathways elemental sulphur and sulphuric acid are their main end products. By adaption of O2-levels during the oxidation a targeted production of sulphuric acid is possible.
The aim was to optimise microbiological desulphurisation of H2S rich biogas with respect to fast and efficient sulphuric acid production. One important task was a microorganism screening to identify the most potent microbial strains to oxidise sulphur components to sulphuric acid. Another task was to prove the principle of H2S conversion to sulphuric acid in lab scale. A continuous desulphurisation unit for microbial H2S conversion was constructed. Potent sulphur oxidising bacteria were immobilised in this column with the aim to produce sulphuric acid in sufficient concentration for substrate pre-treatment.

Aims of pilot plant
Similar concepts are implemented effectively in the food and beverage industry (slaughter house, dairy factory), but this approach is completely new concerning the widespread brewing industry. For this reason a mobile demonstration plant was to be constructed inside a 20 ft.-container which can be used for on-site demonstrations at a brewery. Furthermore, the effect of upscaling of the single sub-processes can be assessed. At the same time it was the first time that the three sub-process (pre-treatment, anaerobic digestion and catalyst recovery) were joined together in a fully automated process.

WP2 – Thermochemical Pre-treatment
Substrates
The innovative TherChem process was not only applied on the anaerobic digestion of brewers’ spent grains but also on five further lignocellulose rich substrates, which were chosen by the Advisory Board: brewers’ spent grains (from barley grain BSG), wheat brewers’ spent grains (WBSG), bagasse (BAG), wheat straw (WS), maize straw (MS), organic materials from landscape maintenance (also called cut grass and green waste CGGW). The latter substrate is a mixture of different types of green waste: cut grass, reed, Polyporus rhizophilus, Convolvulus arvensis, Elymus, Nepeta cataria. Since BSG is an industrial product from beverage technology, its composition may vary a lot.
The chosen substrates were characterised and TS, VVS, lignin-content, cellulose-content, hemicellulose, total ammonium content, total Kjedahl nitrogen and chemical oxygen demand were analysed. In most cases, the results fit together well with values found in literature. But the cellulose and hemicellulose contents of WBSG and the hemicellulose content of BSG were found to be only half of the values found in literature. Substrates with high hemicellulose and high lignin contents are harder biodegradable and might result in lower concentrations of released sugars via pre-treatment and thus lower biomethane yields. High values of total ammonium content and total Kjedahl nitrogen arise from the proteins of the substrate. Those substrates with high protein contents (like BSG, WBSG and CGGW) might lead to higher concentrations of thermal by-products and especially Maillard reaction product during pre-treatment, because a nitrogen source is necessary for the Maillard reaction.
Volatile fatty acids, free sugars and thermal by-products of not treated (NT) and thermo-chemically pre-treated (TC) substrates were analysed. The expectation was fulfilled that the content of free sugars is higher in pre-treated substrates than in not treated substrates, except for the substrate maize straw. The amount of free sugars in TC-MS is slightly lower than in NT-MS. The substrate CGGW is very inhomogeneous since it consists of a mixture of different plants and part of plants. This is the reason why in respective results, the standard deviation might be high although no mistakes in performance of experiments and analyses have been made. Results show that the amount of free substances is a little bit higher in pre-treated than in not treated CGGW. Since the standard deviations are high and overlapping, this difference is not significant. However, the occurrence of thermal by-products shows that the pre-treatment has had an effect on the substrate, setting free sugars and starting the Maillard Reaction. Especially the substrates wheat brewers’ spent grains, bagasse and wheat straw show that pre-treatment can significantly increase the amount of free sugars per g TS of the substrate, since the measured sugar amounts are a few times higher in pre-treated than in not treated wheat brewers’ spent grains, bagasse and wheat straw.

Thermo-chemical pre-treatment
Under thermal and acidic conditions different chemical reactions take place. Furfural and 5 hydroxymethylfurfural, summarised as thermal by-products, and maillard reaction products (not investigated) are produced. These substances are considered to have inhibitory effects on microorganisms. In the enol forming reaction monosaccharides are turned into furan- and pyran-components. Glucose (aldose) is turned into 1,2 endiol and fructose (ketose) produces 1,2 endiol. Eventually 5 hydroxymethylfurfural (HMF) is built out of hexoses and furfural (FF) is produced from pentoses. The production of thermal by-products is visible due to colour change of the sample solution. The different intermediates from the earlier stages react either with free amino acids to low molecular weight melanoidins or with proteins to high molecular weight Maillard reaction products. These furans may damage cell walls and cell membranes, inhibit activities of glycolytic enzymes and proteins, and inhibit RNA synthesis in microorganisms, reduce enzymatic and biological activities and break down DNA. Furans are considered among the most potent microbiological inhibitors. On the other hand, conversion of furfural to furfuryl alcohol by yeast, bacteria and archaea has been previously reported. Biotransformation of furfural and HMF means usually conversion into furfuryl and HMF alcohol. Biotransformation of furfural and HMF to less microbial inhibitory furfuryl alcohol and HMF alcohol by Clostridium species has also been reported.
Thermo-chemical pre-treatment is influenced by different parameters which were varied in the lab scale experiments: temperature, TS-ratio, concentration of sulphuric acid and retention time of treatment. Substrate characterisation of TherChem substrates clearly showed that the substrates differ a lot. Especially the total solids content makes a big difference. It was important to reach the same TS-ratio with all substrates in the sampling vessels for pre-treatment. The thermo-chemical pre-treatment was performed in a high pressure microwave autoclave. After the pre-treatment step, the released substances were separated and analysed via HPLC-method. The analysed responses are the contents of sugars (sum of cellobiose, maltose, lactose, glucose, xylose, fructose and arabinose) and thermal by-products (sum of furfural and 5 hydroxymethylfufural). For reasonable comparison of the chosen substrates, results are given related to the TS-content of the respective substrate in the unit [mg/g TS]. The pre-treatment should fit easily into a brewery and thus steam from the same source should be used for brewing and pre-treatment processes.
Most of the experiments were planned via design of experiments (DoE) and analysed via response surface methodology (RSM). Mathematical models for the concentrations of sugars and thermal by-products were produced with statistical software. The results were statistically analysed and showed significance in most cases.
Generally the amount of released sugars increases with temperature, TS-ratio, sulphuric acid concentration and retention time. This result is valid for all tested substrates. At medium settings of the parameters, chemical reactions start which lead to a conversion of sugars into the thermal by-products furfural and 5 hydroxymethylfurfural. At the beginning, the increase of the concentrations of sugars and thermal by-products correlate. Low parameter settings lead to no or low thermal by products concentrations. Low thermal by-product concentrations are desired since the pre-treated substrates should be further used as substrate for anaerobic digestion processes. When increasing the severity of the pre-treatment, the amount of sugars decreases but the amount of thermal by-products further increases. This proves the occurrence of the chemical reaction “conversion of sugars into thermal by-products under acidic conditions”. At even higher parameter settings, the concentrations of thermal by-products should decrease due to the Maillard reaction and thus Maillard reaction products (not measured) will occur. A high conversion rate of sugars into thermal by-products will lead into a reduced amount of total sugars.

Higher temperature and longer retention times caused higher yield of sugars in pre-treated BSG. But there is a peak of sugars before their concentration drops. This decrease of sugars can be explained with the production of thermal by products out of the released sugars. The retention time of 30 minutes results in a higher amount of thermal by products. From these results it is already clear that the temperature should not be too high to avoid the severe production of thermal by-products. Considering that the pre-treatment should fit easily into a brewery, a pre-treatment temperature below 160 °C fits together with the steam temperature of 120 to 140 °C.
Pre-treatment of WBSG for 60 minutes and pre-treatment of maize straw and bagasse for 15 minutes showed a high conversion of sugars into thermal by-products. The highest concentrations of sugars and thermal by-products do not occur at the same parameter settings for these substrates. The highest amounts of thermal by-products occur at increased severity of the parameter settings. This phenomenon could not be seen in the data of WBSG (at a retention time of 15 and 30 minutes), wheat straw and CGGW. The reason is that the applied parameters weren’t severe enough. The parameter settings should be further increased to research this phenomenon.
Comparison of the results of pre-treatment from maize straw and wheat straw shows that the production of thermal by-products occurs only at higher temperature in wheat straw. Also, the produced amounts are twice as much in maize straw compared to wheat straw. To reach a comparable sugar concentration, a lower temperature is enough for maize straw, but for wheat straw higher temperature is necessary. The reason why pre-treatment of wheat straw resulted in less thermal by-products can be found in the substrate characterisation. Since wheat straw has higher lignin content it is more difficult to crack. Thus the sugar release and also the chemical reactions do not start until more severe parameter settings are chosen.
Thermo-chemical pre-treatment of cut grass and green waste resulted in the lowest amount of released sugars in comparison to all other TherChem substrates. Also the lowest concentrations of thermal by-products were achieved.
The occurrence of thermal by-products is an interesting research topic. The parameter set has to be shifted to higher values in the experiments. The parameter set which resulted in the highest thermal by-products concentration should be shifted to represent the centre point. To be able to confirm the conversion from sugars into thermal by-products and further into Maillard Reaction products, Maillard Reaction Products should be measured.
The results show that all parameters (temperature, TS-ratio, sulphuric acid concentration and retention time) are significant and none of them can be neglected. Every substrate requires a different parameter set. It is not possible to recommend one generally valid pre-treatment method. It is recommended to use one charge of substrate for one experiment and to hold the storage time as short as possible. It is also important to characterise the substrate to have background knowledge of the chemical composition to understand the chemical reactions to which the substrate is exposed. The statistical results show, that all tested parameters do have significant influences on the outcome of the pre-treatment. The significance and the severity of the influences differ for sugars and thermal by-products as well as for all substrates. None of the tested parameters is negligible. The aim of the pre-treatment has to be chosen to decide for the suitable parameters.

Biomethane potential tests (BMP tests)
Biomethane potential tests were performed to show the methane potential of substrates by evaluating the biodegradability of a substrate and the degradation rate under anaerobic conditions at 35 °C. Biomethane potential tests were performed according to VDI 4630 guidelines.
The BMP tests were performed as triplicates. Untreated substrates were fermented as reference and compared with the gas yields of the respective thermo-chemically pre-treated substrates. As blank, inoculum without any feeding was fermented.
The best results for pre-treated BSG were achieved with a pre-treatment temperature of 140 °C. Lower retention time showed better results than longer retention time. The gas yield was lower after fermentation of pre-treated BSG at 160 °C for 60 minutes and 180 °C for 15 minutes than from untreated BSG. This is caused by the increased concentration of Maillard reaction products. Additionally methane formation rate was reduced. This leads to the presumption that inhibition of microorganisms takes place.
The pre-treatment of the substrates wheat brewers’ spent grains, bagasse, maize straw, wheat straw and cut grass and green waste had a positive influence on the rate and yield of methane production. For the substrates maize straw, wheat straw and bagasse the biodegradability could be increased due to pre-treatment. This resulted in faster biomethane production.
For wheat straw additionally a higher biomethane yield was determined. The pre-treatment of maize straw resulted in faster fermentation but the total methane yield was comparable to fermentation of not treated substrate. The fermentation of not treated and pre-treated CGGW didn’t show significant difference in the rate of biomethane production but the total methane yield could be increased due to pre-treatment.
The degradation rate (at day 7 and day 21) could be increased by up to 54% when comparing pre-treated with not treated substrates. On the one hand pre-treatment of bagasse and maize straw did not result in higher gas yields after 77 days of fermentation but on the other hand for wheat straw and CGGW higher gas yields of 14 and 20% were obtained, respectively. The higher gas yields might result from the higher lignocellulose content or the crystallinity of the cellulose.
Generally, the degradation rate of methane production can be increased by thermo-chemical pre-treatment of lignocellulose rich substrates. However, the total methane yield is not increased in every case when pre-treatment was applied. The comparison of pre-treated and not treated bagasse and maize straw showed almost no difference. But pre-treatment of wheat straw and cut grass and green waste resulted in increased total methane yield.
The increased degradation rate is advantageous because it means that the hydraulic retention time and thus the reactor size could be decreased if thermo-chemical pre-treatment of lignocellulose rich substrates is performed.

WP3 Anaerobic digestion
To assess the biodegradability of the previously mentioned substrates, (semi-)continuous fermentations were carried out in four simultaneously running reactor set-ups:
• A single stage fed with substrate which has not been pre-treated (reference system).
• A two stage system fed with substrate which has not been pre-treated.
• A single stage fed with pre-treated substrate.
• A two stage system fed with pre-treated substrate.
The fermentations were monitored by regularly analysing parameters like pH, VFA-concentration, COD-content, gas production and gas composition. The aim was to optimise the fermentations and maximise the loading rate. Based on these results the different systems were compared, leading to the conclusion whether the substrate is or is not suitable as a substrate for the TherChem-process.
In some cases the biogas process was divided into a two stage process. The first stage was the acidification step and the second stage the methanogenic stage. During the acidification the substrate is hydrolysed and converted to volatile fatty acids. This process favours acidic condition (pH 4 – 6). Within the TherChem-process the aim is to combine this acidification with the sulphate reduction, converting the sulphuric acid to (gaseous) hydrogen sulphide which leaves the system together with the hydrolysis gas (which mainly consists of carbon dioxide).
After the acidification step, the fermentation broth is fed into the methanogenic reactor (pH 7 – 8.5) in which the volatile fatty acids are converted to the actual biogas (mainly consisting of energy-rich methane and the unwanted carbon dioxide). As the sulphur already left the system during the acidification, damage to equipment due to the corrosive effect of hydrogen sulphide is being prevented.
Results
Digestion of brewers’ spent grains was successful. However, as the gas production measurements turned out to be susceptible to failure, the performance of the reactor was measured by the COD- degradation. Based on these results it turned out that using a two stage fermentation system increases the COD-degradation by over 10%. Regarding the systems fed with pre-treated BSG, the hydrogen sulphide content in the biogas was significantly less when a two stage system was used in comparison to a single stage system.
The comparison of the two-stage fermentations showed that there is only minor advantage of the applying a pre-treatment. However, it is thought that the system performance of the two stage system fed with pre-treated BSG can be increased significantly by further optimisation the interaction between pre-treatment and the acidification.
WBSG: using a thermochemical pre-treatment made both the single and two stage processes more prone to process instability, causing high fatty acid concentrations and incomplete degradation. The use of the chosen pre-treatment did also not result in the expected increase in COD-degradation (though in the two-stage process this was due to losses in the acidification stage) or an increased biogas quality.
Maize straw: It has to be noted, that none of the systems was entirely stable without the addition of extra buffer capacity. However, after the addition of sodium carbonate, the use of a thermo-chemical pre-treatment seemed to be advantageous. Based on the formation of volatile fatty acids, the performance of the acidification reactors was poor. However, a two stage system with pre-treatment is still recommended as the specific methane yield was increased by 11 %.
Wheat straw: Based on the similarities between maize straw and wheat straw, it was decided only two reactor systems would be run. As the advantages of a two-stage system is clear for straws. It was decided to only run single stage fermenters as the advantage of a pre-treatment can be best compared and because the response of single-stage reactors is faster. All in all it turned out that the pre-treatment of wheat straw enhanced the specific methane yield by 29 % and therefore a two-stage system with pre-treatment is favourable.
Bagasse: The advantage of using of a thermochemical pre-treatment was not proved during the continuous fermentations. A normal two stage system seemed to be the best choice as the formation of floating layers was less, without a significant loss in specific methane yield.
Grass cuttings and green waste: With the fermentation of grass cuttings and green waste, basically two problems occurred. First of all the reactor design was poorly chosen. The width to height ratio was false, causing problems with the mixing, leading to the fibre structure of the substrate forming large substrate balls. Secondly it was hard to accomplish a good acidification, causing a loss in methane yield. Therefore the most ideal system was a single stage system with pre-treatment.

WP4 Acidic catalyst recovery - Desulphurisation
Screening of microorganisms
Four different bacterial strains that have been reported in literature to grow chemolitho-autrotrophically were selected to screen their sulphuric acid formation capacity. Selection criteria were temperature and pH optima. Sodium thiosulphate was chosen as model substrate mimicking hydrogen sulphide to keep the experimental setup simple. The substrate is metabolised to sulphuric acid and elemental sulphur, depending on the oxygen concentration of these acidophilic bacteria resulting in a decrease of the pH.
Examined species included Acidithiobacillus thiooxidans, A.albertensis A.caldus and Acidiphilium acidophilum.
The first two strains listed were already adapted to minimal thiosulphate medium. In contrast, A.caldus and A.acidophilum needed to be pre-cultured on media containing trace elements or glucose at optimal temperatures of 45 °C and 30 °C, respectively, prior to incubation in thiosulphate medium. The adaption of especially these two strains to growth conditions of the screening experiment (thiosulphate medium at 30 °C) was not successful. Both strains showed little biomass formation in preliminary experiments. Consequently, A.caldus was not included in further screening experiments. However, Acidiphilium acidophilum was reported to show enhanced biomass formation under mixotrophic growth conditions on glucose and thiosulphate by Pronk et al. (1990). As a consequence, this strain was included in further experiments and was monitored on its sulphate formation during chemolithoautrophic as well as mixotrophic growth.
Two acidophilic sulphur oxidising bacteria with high potential for sulphuric acid formation until pH of 1.6 – 1.8 were identified: Acidithiobacillus thiooxidans and A. albertensis. The cultivation on minimal medium with thiosulphate as sole energy source at 30 °C and 35 °C was successful. Complete substrate conversion with resulting sulphate concentrations of in the range of 0.07 – 0.11% was achieved with both strains.

Operation of a desulphurisation unit
A continuous desulphurisation column at lab scale was constructed and three different carrier materials (porous glass, cylindrical carriers made of stainless steel or polypropylene) were characterised according to void space, water holding capacity, structural stability and suitability for the process.
The carrier materials chosen were as follows:
a) Porous glass cut to approximately 1 cm x 1 cm
b) Cylindrical stainless steel carrier with 1 cm height
c) Cylindrical polyethylene carrier with 1 cm height
Polyethylene cylindrical carrier was chosen for further experiments due to its physico-chemical stability and high void space volume. Although porous glass has a higher void volume and significant water holding capacity when compared to both cylindrical carrier forms it was not considered applicable for further experiment. The column will block due to both, bacterial growth and the formation of elemental sulphur, when filled with this material. The stainless steel carrier was also rejected as the acidic conditions caused the surface of the stainless steel to oxidise, resulting in the formation of insoluble iron-sulphide during an experimental run. In the following, a summary of the evaluated carrier properties is listed:
Acidithiobacillus thiooxidans and A. albertensis were pre-cultured separately for inoculation of the column. In order to immobilise the microorganisms on the carrier material both pre-cultures were simultaneously added to the column. During the start-up phase, a steady increase of pH was observed. The same effect of increasing pH over time was observed when the experiment was repeated at lab scale.
Consequently, lab scale experiments were performed with a sulphur oxidising biocoenosis with high acid formation capacity withdrawn from an industrial desulphurisation plant. With this sample the pH dropped below 1 within one week. However, when fresh thiosulphate medium was inoculated with biomass from this sample, which had been exposed to a pH below 1, no substrate utilisation was observed. For the continuous operation of the desulphurisation column medium was replaced as soon as the pH dropped below 1.2. Bacteria were able to efficiently metabolise H2S to sulphuric acid (indicated by decreasing pH) within one week of operation. From then on immobilised bacteria were able to oxidise high concentrations of H2S to sulphuric acid within less than 24 h at a removal rate of 100%. When the H2S supply was stopped for a few days the sulphuric acid concentration in the medium continued to increase. This indicates that sulphuric acid accumulates within the column and is eluted bit by bit once there is no substrate left for conversion.
In other experiments the removal efficiency of H2S shock loading was examined.

Conclusion
The conducted microorganism screening identified two potent sulphur oxidising bacteria for sulphuric acid production with thiosulphate as model substrate for H2S. Acidithiobacillus thiooxidans and Acidithiobacillus albertensis showed fast and almost complete thiosulphate conversion. The minimum pH values reached corresponded to the respective acid concentrations and were within the optimum range for the acid pre-treatment of lignocelluloses.
Lab scale experiments proved the suitability and stable operation of the designed desulphurisation column for H2S conversion into sulphuric acid. The inoculation material was withdrawn from an industrial desulphurisation plant and could withstand both, H2S shock-loading and longer periods of operation without any H2S feeding. Nevertheless, sulphuric acid did accumulate in the column and was gradually eluted from the column in times without H2S supply. This guarantees the sufficient production of sulphuric acid for substrate pre-treatment even under unbalanced gas supply.
The constructed column for microbial desulphurisation using sulphur-oxidising Acidophiles such as A.thiooxidans and A.albertensis is capeable of both, stable H2S removal at 100% removal efficiency and subsequent conversion to sulphuric acid in sufficient concentration for lignocellulose pre-treatment prior to anaerobic digestion.

WP5 Demonstration of TherChem Process: mobile Demo plant
The TherChem process integrates three different steps. During the first step the input material is thermo-chemically pre-treated using diluted sulphuric acid and heat. In the second step pre-treated material is anaerobically digested and acidified, respectively. The main aim is the conversion of solubilised carbon compounds to energy rich intermediates, mainly short chain volatile fatty acids and alcohols. These intermediates can be easily converted to biogas in the third step.
Sulphuric acid used in the pre-treatment is recycled during the second step. Beside production of intermediates a hydrogen sulphide rich gas is formed. This gas is directed into a bio-trickling filter system. Here hydrogen sulphide is biologically oxidised by the immobilised microbial consortium is able to produce sulphuric acid. Part of this sulphuric acid can be re-used in the first step, the pre-treatment.
In the course of the project experiments were carried out in both, in lab-scale as well as in demonstration scale. The design, scale up and the continuous operation of the complete system at demonstration level was one of the main challenges in the project.
The demonstration plant was built inside a 20 ft.-container and includes a four reactor system (pre-treatment, acidification and methanogenesis; sulphuric acid recovery unit) and is fully automatically operated. It allows gas measurement in both, quality and quantity as well temperature, filling level and pressure.
Pre-treatment reactor
A custom reactor was designed and constructed to perform the thermo-chemical pre-treatment. This reactor consists of two vessels which are mounted on top of each other. The upper vessel is the actual treatment reactor and has a working volume of 35 L. To monitor the pre-treatment parameters this vessel is equipped with both a thermometer and a barometer. Heating is possible either by indirect heating by leading steam into a heating jacket or by directly injecting the steam into the substrate-mixture. The steam that is needed for the heating is being generated by two steam generators (P = 7.6 kW; Pmax = 5.1 bar).
The lower vessel serves as 200 L storage-vessel to store the pre-treated substrate to enable the transition between batch (pre-treatment) and continuous (anaerobic digestion) operation. The storage vessel is large enough to enable 3 days of full operation without the need to perform a pre-treatment.
Anaerobic digestion
The biogas process consists out of two reactors, one for acidification and one for methanogenesis. The combined acidification and sulphate reduction was conducted in a stirred stainless steel reactor with a volume of 278 L, however during operation this working volume will be maintained at a level around 240 L. This is possible thanks to the fact that the reactor is mounted on three weighing sensors. The reactor has an online temperature measurement, enabling the temperature control via the heating jacket
The methanogenic reactor was made out of stainless steel and has a working volume of 2m3. The temperature inside the reactor is regulated with the help from a temperature sensor and an electrical heating coil, which is wrapped inside the reactor. To ensure good stirring the reactor is equipped with a centred stirrer. The reactor is also equipped with multiple ports. Besides an inlet port, liquid outlet port and multiple sampling ports, a gas port with small an internal small desulphurisation column was installed.
To transfer the feed into the acidification reactor and to further on into the methanogenesis, two progressive cavity pumps were installed which are especially suitable to pump viscous liquids containing small particles.
Gas measurements are done both quantitatively and quantitatively. The amount of produced gas is measured by a drum type gas counter, while the gas composition is determined by a gas analyser.
The entire process is controlled via a PLC-unit which was build in-house at the BOKU University. This unit communicates with a PC which has the FactoryTalk-software suite installed.
Results
The first demonstration run of the pilot plant proved that the anaerobic digestion of thermochemical pre-treated brewers’ spent grains at pilot scale is possible. The composition of the produced biogas contained about 50%v methane. The performance of the acidification reactor was more important, at the beginning the process was somewhat instable as the buffer capacity seemed to be insufficient resulting in a gradually decreasing pH. Without interference the pH would drop below 3.0 inhibiting all microbial activity. Different measures were tried out, whereas recirculation seemed to be the best option.
During the second run of the demonstration plant, pH of the acidification step was kept stable for a long period of time. In terms of fatty acid production good results were achieved.
In the methanogenic stage nearly all substrate is being degraded, resulting in a gas production of around 520 nml g-1-TS with a methane content of 54%v. This corresponds to 75% of the measured COD-degradation, hinting that the methanogenic stage is still building biomass.

Potential Impact:
Each SME involved in the project has had to play a specific role. It was expected that the outcome of the RTD-outcomes of this project would increase the competitiveness of each participating company. During the course of the project SME-partner Atres has had several inquiries on implementation of the TherChem process into breweries. By the end of the year 2014, no TherChem system has been established. It is expected that in the following years some of the inquiries, especially in Germany but also in other countries, will lead to the realisation of a TherChem-installation.
For the participating manufacturing SMEs, the TherChem project offered a very good opportunity to bring a new technology to the market. At the beginning it was planned to focus only on the brewery sector, but in the future the general biogas market should be penetrated too. The penetration will realistically be limited to a few countries, but Germany, the leading country in both the field of biogas technology and brewing industry, shows large potential. The development of the TherChem process is aiming for a modular build¬-up, meaning (parts of) the process-module can easily be implemented to existing plants. Alternatively, this technology can be offered to biogas plant construction companies in order to extend their portfolio with an entire TherChem-based biogas plant.

Within the project consortium each SME-partner has a different engineering focus:
- Thermo-chemical pre-treatment (Aigner)
- Multi-stage anaerobic digestion (Atres)
- Desulphurization (S&H)

Although this technology can be applied to a broader market, the initial goal is the implementation of the TherChem process into existing breweries. This leads to several benefits for the brewery as it increases the self-sufficiency and lowers the energy cost (which make up for about 10-20% of total production costs) and improves the corporate image.
Up to now, anaerobic digestion systems are not yet established in breweries, with exception of water treatment installations.
ATRES, a well-known company with respect to anaerobic digestion of brewers’ spent grains (BSG), was frequently present at specific trade fairs for brewer-technologies. Estimations for the demand of biogas plants in brewery-sector showed a realistic potential of 60 to 75 sites until 2020. Further support for this estimation comes from the research site; the amount of research projects related to biogas from BSGs has increased significantly in recent years.
In addition, the participation of the German Brewers’ Associations in the projects´ Advisory Board underlines the importance of economically efficient exploitation of brewers´ spent grains.
However, in a lot of European countries the situation at the biogas market changed in the last years due to diminishing framework conditions. But still, there are countries such as Italy, Czech Republic and Germany in the lead where the overall situation for biogas technology is still promising in comparison to the others, although framework conditions there are also declining.
To broaden the application of this technology other residues, mainly from the agricultural sector, were tested. For instance, thermo-chemical pre-treatment of wheat and maize straw showed significant better performance compared to untreated material. Because of that additional economic growth in terms of implementation can be further stimulated. Due to the increasing amount of biogas plants in Europe, participating manufacturers see an uprising potential of 500 to 1 000 plants in Europe (outlook 2010).
The overall economic situation in Europe is at the moment (2015) in a crisis (small to no economic growth). This fact of course has a big effect to the biogas market too. For that reason the potential is expected to be lower, in a range of approximately 400 to 600 plants.

For the involved breweries, economic benefits can be generated by reduction of energy costs, the freedom from fluctuating energy prices or the marketing advantages of using green energy. The demonstration of WÄDI as the first self-sufficient brewery was expected to be a huge boost to the company. But due to the recent economic development in Switzerland investment activities are little hampered, but Wädi is still interested to implement such a system in the near future.

The realisation of the TherChem-project strongly improved the cooperation between the SMEs and RTDs. The development of the TherChem-process brought several SMEs closer and a long term commitment for cooperation in the field of biogas technology was strengthened.
The development of the transnational approach is led by the high numbers and excellent reputation of breweries in the participating countries. Additionally, most biogas production and almost all biogas enterprises are located in Germany and Austria. The cooperation between SME and RTD was deepened through this project.
Besides the focus to Brewers´ spent grains, further feed stocks were investigated as well. Biogas production from organic raw material and residues is one important backbone of the energy production in Europe, and more feed stocks (based on organic residuesor waste) will be considered in the future. The consortium of the TherChem project is confident that, with the gained experience and knowledge, it will be able to meet the upcoming demand of this technology.
Dissemination of the project was one of the major tasks. The established project website was seen by a lot of visitors (http://therchem.eu) and the consortium received positive feedback. Among a lot of project relevant information, an animation video explains the TherChem process in a simplified and fun way. Furthermore, project folders in German and English were distributed at all visited fairs and conferences.
A big number of international conferences, fairs and workshops were visited by our consortium-members within the course of the project. The TherChem-process was actively promoted to interested visitors (industry, R&D field) in more than 30 oral presentations and poster sessions as well as in a huge number of personal chats and talks. Furthermore, all SME-partners have already had opportunities to promote the TherChem project to significant number of interested clients.

List of Websites:
website: http://therchem.eu
contact: Markus Ortner
email: markus.ortner@bioenergy2020.eu
tel: +43227266280-536

Final Report Summary - THERCHEM (Thermochemical pre-treatment technology for residues from breweries and other biomass to enhance anaerobic digestion)

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