Etamax – Biogas from lignocellulose-poor waste and algae residues

In order to reduce dependence on dwindling oil resources and at the same time reduce carbon dioxide emissions, the use of renewable energies is a sustainable alternative. The use of plant biomass to produce bioenergy – electricity, heat or fuels – plays a prominent role here.

However, the potential of waste biomass for the production of biogas and its use as a fuel has not yet been fully exploited. Organic waste materials with a high water content and low lignin and lignocellulose content, e.g. waste from the food industry, wholesale market waste or algae residues, are best suited for fermentation.

Coordinated by Fraunhofer IGB, a project consortium has therefore set itself the goal of completely converting easily fermentable wet biomass with a low lignocellulose content into biogas using an adapted high-load fermentation process with maximum energy production and at the same time closing all material cycles. In the EtaMax project, the consortium focuses in particular on low-cost biowaste and residual algae biomass that does not compete with food.

The project focused on the regional production and use of renewable methane from biogas. The purified biomethane was used as vehicle fuel for CNG (compressed natural gas) vehicles. Liquid, nutrient-rich fermentation residues produced during fermentation were used to grow microalgae, as they contain sufficient amounts of the inorganic nutrients necessary for algae growth.

Modular high load digestion

Optimally suited for fermentation are waste materials with a very high water content and low lignin and lignocellulose content from the food industry, kitchen waste or wholesale market waste, which today are usually taken to composting plants, whereby the energy they contain is lost as heat. In a high-load fermentation process, which was developed years ago at Fraunhofer IGB and has been technically implemented several times for sewage sludge, the solids of these biowaste fractions are almost completely converted into biogas in just a few days.

In order for a fermentation plant to convert the different substrates to biogas as effectively as possible, the process technology is specifically adapted to the respective substrates by using a flexible multi-substrate high-load fermentation plant. As a second stage, the fermentation water is cleaned in a central reactor with maximum efficiency and also converted to biogas.

Additional biomass through algae

Additional wet, low-lignocellulose biomass for the multi-substrate high-load fermentation is contributed in the form of residual algae biomass. The generation of energy with algal biomass is already possible today thanks to a photobioreactor platform developed at Fraunhofer IGB. In the reactors, algae grow to high cell densities only with sunlight as energy source and carbon dioxide as carbon source, as well as inorganic nitrogen and phosphate.

EtaMax uses the carbon dioxide produced during fermentation as a co-product and during the combustion of biogas as a carbon dioxide source for algae cultivation. In addition, the filtrate water from the fermentation process, which contains nitrogen and phosphorus as nutrients, is used to grow algae. Here it is important to find robust algae that grow quickly with this flue gas and also in Central Europe with seasonal fluctuations in light and temperature conditions.

Hydrothermal gasification of fermentation residues

Catalyst-supported hydrothermal gasification at high pressure and high temperature is being investigated for the small amounts of fermentation residues that are produced in each fermentation process and cannot be further degraded anaerobically. The same products are produced here as in fermentation: carbon dioxide and methane.

Purification of biogas to biomethane

In a large-scale plant, 300,000 cubic metres of methane gas per year could be produced from the municipal biowaste of the city of Stuttgart. This gas will be purified as a vehicle fuel using membrane technology and used for a small fleet of natural gas-powered refuse collection vehicles.

In a pilot-scale fermentation plant (2 x 30 l reactors), the process parameters for the transfer to the pilot scale (2 x 3.5 m³) were determined at Fraunhofer IGB within the first year of the project. The two-stage pilot plant produced 850 l of biogas from wholesale market waste in terms of organic dry matter (oTR), which corresponds to an average of 190 l of biogas per day at a volume load of 7 g oTR/ld at the present reactor volume. During the investigations, unsorted market waste from the Stuttgart Wholesale Market was provided at regular intervals, shredded and fed into the fermentation process. The feeding of such unsorted waste poses a challenge to the microorganisms. Our investigations have shown that the fluctuations in the composition of the substrate can only be absorbed by intelligent process control: By feeding substrate portions of different fermentation process stages via several pre-fermentation tanks, we can guarantee continuous biogas production with only slight fluctuations in gas yield despite a large range of substrate variations.

With the investigations in the pilot plant, we were also able to successfully determine the limit values that are important for a transfer to a larger scale, such as minimum residence time and maximum room load as well as parameters for the optimised supply of different substrate compositions.

The common factor for all sub-components used is the minimum energy input for the realization of the respective tasks. In 2012, the findings determined on a pilot plant scale were transferred to a demonstration plant on the site of the EnBW combined heat and power plant in Stuttgart-Gaisburg. The biogas produced is purified in a membrane plant and then fed into vehicles.

Conversion of wholesale market waste to biogas

For the first time, superimposed fruit and vegetable waste from a wholesale market (Stuttgart Wholesale Market) was converted very efficiently into biogas in a two-stage process in two gas lift reactors with 3.2 m3 each. For this purpose, the high-load fermentation process, which was developed at Fraunhofer IGB and has been technically implemented for sewage sludge several times since 1994, was extended and adapted for this substrate.

With a hydraulic residence time of 17 days per stage, the plant could be operated in a permanently stable manner even with changing fruit and vegetable waste. Degradation rates of up to 95 percent were achieved, with most of the degradation occurring in stage 1. The biogas yield here was between 840 - 920 standard litres of biogas per kilogram of added organic dry substance, the methane content was 55 - 60 percent.

Biogas production and algae cultivation as efficient material cycles

For the energetic use of algae ingredients, a two-stage fully automated process for the production of lipid-rich algal biomass in flat plate airlift reactors (FPA) was developed at Fraunhofer IGB in the field and transferred to pilot scale.

The material cycles for nitrogen and phosphate between biogas production and algae cultivation were closed by using the liquid fermentation residue from the biogas reactors as a medium component for the production of algae biomass. An algae mixed culture specially adapted to this liquid fermentation residue was successfully cultivated with liquid fermentation residues from the fermentation of fruit and vegetable waste in 180-litre FPA reactors for four months. The ammonium concentration of approx. 800 mg / L contained in the liquid fermentation residue was completely consumed. The biomass concentration in the FPA reactor ranged from 2.5 g / L to 5.5 g / L between harvest times. The volumetric biomass productivity fluctuated between 0.1 and 0.35 g L-1 d-1 depending on weather conditions.

The fermentation of fruit and vegetable waste in changing composition could be operated for the first time in long-term operation under continuous conditions with a residence time of 17 days stable with a high degree of degradation and high biogas yield.

The material cycles were closed by the utilisation of the biogas (not shown) and the utilisation of the fermentation residues. The use of liquid fermentation residue as a medium component for algae cultivation is a step towards reducing the production costs for algae biomass. However, these results also show a way to reduce the nitrogen and phosphate load of liquid fermentation residues and to produce phototrophic biomass which can be used both energetically and materially. This goes hand in hand with a reduction in costs both in algae production and in the treatment of biogas effluents.