Biogas – more cost effective upgrading using biocatalyst

by Thomas Hernø

One major barrier for increased use of biogas in the natural gas grid is the cost of upgrading the biogas. Upgrading is basically removing Carbon Dioxide from biogas so only Methane remains in the gas. However the cost of energy required in the commercially available technologies add a substantial cost to the upgraded gas. By using an biocatalyst the efficiency of the upgrading process can be increased and the cost of energy reduced. A partnership has been formed to demonstrate the technology on a full scale basis. The project also demonstrates how the biogas upgrading can be integrated in the biogas system at a waste water treatment plant producing the biogas and injection of the upgraded gas in the natural gas grid. When the ENZUP upgrading facility commences in full operations end 2015, this is expected to be the largest demonstration of a biocatalyst-based approach to remove Carbon Dioxide from an industrial gas stream anywhere in the world.

1. BACKGROUND 

HMN Gashandel A/S (Denmark) and five partners have initiated a 3-year full scale demonstration project to show that ENZymatic UPgrading of biogas (ENZUP) is a more energy- and cost-effective method for upgrading biogas to natural gas grid quality than the commercial technologies in operation today. The project addresses one of major barriers for increased use of biogas in the natural gas grid: the cost of upgrading the biogas. The biogas for the ENZUP plant is produced from waste water treatment, but could be produced from agricultural or industrial residues as well.

Upgrading of biogas from waste water treatment, agricultural and industrial residues support the Danish national energy policy on using biogas produced on residuals and waste instead of fresh crops. The upgraded biogas will meet natural gas specifications and will be injected in the natural gas distribution grid, replacing fossil natural gas and reducing Green House Gas emissions to the atmosphere. Lower cost for upgrading biogas will accelerate the use of renewable gasses in the natural gas grid and supports Danish gas utilities’ business strategies on increasing the amount of renewable gasses in the natural gas grid.

The upgraded biogas will be injected into HMN’s distribution natural gas network. The ENZUP upgrading plant is scheduled to operate for two years as a demonstration unit and is planned to continue operation on commercial basis afterwards. Another demonstration project “Power-to-Gas via Biological Catalysis – Phase 2” (BioCat), will be located beside the ENZUP biogas upgrading plant.

2. METHODS 

The biogas will be upgraded in a commercial-scale demonstration plant, producing up to 2.6 million normal cubic meters per year of grid quality natural gas. The upgrading process is developed by Akermin Inc. (USA) in collaboration with Novozymes A/S (Denmark). Akermin’s technology transforms enzymes into proprietary industrial biocatalyst that can be readily incorporated into conventional chemical absorption processes for Carbon Dioxide removal.

By addressing the key issue of enzyme stability in harsh industrial environments, Akermin’s approach to Carbon Dioxide removal significantly reduces operating costs. Carbonic Anhydrases (CAs) belong to a family of enzymes that are particularly useful in biogas upgrading as they convert Carbon Dioxide and water to bicarbonate.

Akermin uses developmental versions of recombinant, highly active and thermo stable CAs available from major enzyme suppliers. The enzymes are manufactured using conventional fermentation processes.

Akermin’s approach enables the use of non-volatile, carbonate-forming solvents, which, without an enzyme, absorb Carbon Dioxide at a rate that is far too slow to have any practical application, but that otherwise have outstanding physical properties. By addressing this key limitation, Akermin creates an environmentally friendly solution for removing Carbon Dioxide from biogas.

In the ENZUP biogas upgrading facility Akermin’s proprietary biocatalyst what one might call “nature’s perfect catalyst” – the enzyme Carbonic Anhydrase – to accelerate Carbon Dioxide absorption into a non-volatile, environmentally-friendly solvent. The enzyme is immobilized in a polymer that helps to protect the enzyme from high pH, temperature and shear forces. This approach not only increases enzyme stability but improves operational performance.

In the upgrading process Carbon Dioxide is separated from Bio Methane in an absorber column where the former is absorbed by the re-circulating solvent. The un-reactive Bio Methane leaves the absorber while the solvent containing Carbon Dioxide is sent to the stripper. Following the release of Carbon Dioxide, the solvent is returned back to the absorber. The upgrading process works at a near-atmospheric pressure and at lower temperatures compared to traditional Amine upgrading plant.

The expected results of the demonstration of the biocatalyst upgrading process are a 27% reduction of power and heat costs compared to a traditional Amine upgrading plant. The total cost advantage including investment costs for the ENZUP demonstration plant is estimated to 6%. Total cost advantage for future upgrading plant is estimated to 14% and is expected that higher cost advantages can be achieved as a result of scale up and heat integration. The process does not have any methane slip and there is no emission of the solvent used in process.

3. BIOGAS SOURCE AND ENZUP LOCATION 

The ENZUP demonstration facility will be located at the BIOFOS Waste Water Treatment Plant Avedoere, near Copenhagen, Denmark. The plant is owned by municipalities and handles waste water from ten municipalities with . mill. residents and Industries.

The plant has a strategy to become Carbon Dioxide neutral by 2025. The means are:

• Electricity savings by installing energy efficient waste water pumps etc.
• Waste water process changes, eg. change of aeration system.
• Utilisation of waste heat from sludge incineration and optimization of the waste heat recovery system by adding a heat storage tank and construction of heat exchangers.
• Increased energy production by hosting ENZUP and BioCat projects.

Sludge from the water treatment is treated in large digesters (4×6,000 cubic metres) in an environmentally-friendly manner to produce biogas. Today the all energy in biogas, approximately 3.5 mill. cubic meters per year, is used in a 1 MW gas engine, producing electricity for the grid and heat for the waste water treatment plant.

The existing biogas system includes a biogas storage tank (1,500 cubic metres capacity), an activated carbon filter to remove Siloxanes and a flare in case the biogas engine is out of operation (see Figure 1). 

4. FUTURE BIOGAS SYSTEM AND ENERGY PRODUCTION 

BIOFOS Waste Water Treatment Plant Avedoere considered and evaluated several options for use of the biogas to help achieve the target of becoming Carbon Dioxide neutral. BIOFOS ended up signing an agreement with HMN Gashandel A/S selling all biogas production in a 10 year period for upgrading biogas to natural grid quality gas in the ENZUP plant. The value of the upgraded biogas injected in natural gas grid is approximately 0.77 EUR per normal cubic metre (Grant 0.50 EUR decreasing to 0.30 EUR in 2024, gas selling price 0.26 EUR and value of biogas certificates 0.01 EUR).

Another demonstration project “Power-to-Gas via Biological catalysis – Phase 2” (BioCat), lead by Electrochea. dk, will be located beside the ENZUP biogas upgrading plant. The BioCat plant uses an advanced version of an alkaline electrolyser. The electrolyser is connected to the local power grid and draws electricity in times of low electricity prices. In cell stacks, the electricity splits water into its molecular components Hydrogen and Oxygen. Both gases are immediately separated and leave the electrolyser in separate pipes. The Hydrogen is then delivered to the methanation reactor, while the oxygen is supplied to the activated sludge basins for wastewater treatment.

In BioCat’s biological methanation system, the hydrogen from the electrolyser is combined with Carbon Dioxide, and this gas mixture is then introduced to a liquid phase methanation reactor. Over the course of the experimental testing phase in the BioCat Project, two sources of Carbon Dioxide will be used: 1) raw biogas with a composition of approximately 60% Methane and 40% Carbon Dioxide, and 2) a pure stream of Carbon Dioxide supplied by the ENZUP plant.

The capacity of the ENZUP plant will be approximately 300 normal cubic meters of Bio Methane per hour and the BioCat plant has a capacity of approximately 50 normal cubic meters of Bio Methane per hour (see Figure 2).

In the future biogas system biogas is produced as today. The biogas pressure is increased slightly before it enters the activated carbon filter. In the filter air is injected so both the Siloxanes and Hydrogen Sulphur contents are reduced. The Hydrogen Sulphur content is reduced from 200-300 ppm to a few ppm. The biogas quality and amount is measured at the BIOFOS selling point. The billing is related to energy content, so the exact biogas composition must be known. Most of the biogas will be upgraded to Bio Methane in the ENZUP plant. A smaller amount will be used in the BioCat plant. The Bio Methane is then sent to the grid injection facilities described in the next section.

During normal operation or during short stops for maintenance and repairs the biogas storage tank can be used as a buffer. The capacity of the biogas storage tank equals 3-5 hours of the biogas production. The biogas engine will be back-up in case the ENZUP plant is out of operation, so it is very unlikely that biogas is flared and value of the gas is lost. Waste heat from the ENZUP plant will be used in the BIOFOS WWTP. The rejected Carbon Dioxide from the ENZUP upgrading plant will be used by the BioCat plant.

The ENZUP facility compromises a building of 150 square metres with process equipment, room for a natural gas boiler for process heat supply and a control room with electric boards and control systems. Outside the building the absorber and stripper columns are located. The absorber column is 25 meters tall and the stripper 15 meters tall, each 0.7 meter in diameter. A small cooling tower is also located besides the building.

5. NATURAL GAS GRID INJECTION 

The upgraded biogas will be injected in the natural gas distribution grid. After upgrading the gas is called Bio Methane due to the content of approximately 98% Methane. The Bio Methane is compressed to 5,5 bar and dried. The gas is then sent to a receiving station, where the gas quality is checked to make sure the Bio Methane meets natural gas grid quality requirements. The checking includes measurement content of Methane, Oxygen, Carbon Dioxide and Hydrogen Sulphur content in a gas chromatograph and calculation of Wobbe index. Temperature, dew point and gas amount are also measured. The grant (0.5 EUR per normal cubic meter) for biogas upgrading is calculated here.

The gas is odorized and transferred with a pressure of around 5,5 bar in a 1,8 km gas pipe to a Meter and Regulator station. In this station the gas is injected in the 4 bar grid and distributed to natural gas costumers. If case the Bio Methane does not meet grid quality requirements a valve in the receiving station is closed and the rejected gas is sent back to the gas storage. A small amount of Bio Methane is continuously checked and as soon as this gas meets requirement the valve is opened and all Bio Methane is send to the natural gas grid.

The heating value of the Bio Methane is around 10 kWh per normal cubic metre, approximately 10% lower than fossil natural gas. One option is to increase heating value by adding propane, which is adding a substantial cost to the gas. In this project the distribution company keeps track of the heating value in the natural gas distribution grid, so the heating value is known for each customer and no propane is needed.

6. PROJECT PARTNERSHIP, FINANCING AND POLICY 

The project has six partners, each of them contributing and benefitting within their fields of competences and development strategies.

HMN Gashandel A/S will lead the ENZUP project and oversee the construction and operation of the biogas upgrading facility. HMN Gashandel A/S is the commercial part of HMN Naturgas I/S, Denmark’s biggest operator of natural gas distribution.

Other partners in the project are:

• Akermin Inc. (USA) is responsible for providing the upgrading unit process design, production and supply of the industrial biocatalyst and technical support to the project.
• Novozymes A/S, Denmark, is responsible for development and production of enzyme used in the production of Akermin’s industrial biocatalyst.
• Ammongas A/S (Denmark) will design and supply the upgrading system. Ammongas will also provide technical support during commissioning and operation.
• BIOFOS Spildevandscenter Avedoere A/S (Denmark) produces the biogas and the ENZUP plant will be located on the premises of the waste water treatment plant.
• Dansk Gasteknisk Center A/S (Denmark) will be responsible for coordinating site testing and developing the subsequent technical and economic analyses to establish commercial viability.

The ENZUP budget is EUR 5.5 million. Roughly 50 % is funded through a grant from the Energy Technology Development and Demonstration Program (EUDP). The programme supports the development and demonstration of new energy technologies.

7. ROADMAP 

The 3-year project stared in January 2014. In 2014 and first quarter of 2015 the upgrading plant was designed and engineered. Construction and erection starts in second quarter of 2015. Commissioning and preliminary operation of biogas upgrading plant is expected to start in fourth quarter of 2015, followed by full scale operation end 2015.

Until April 2017 the plant will be in full scale operation testing different operation modes and operation strategies. The results will be reported and published. When the demonstration project ends the plant is expected to continue operation on commercial basis if the enzymatic upgrading is proven successful.

8. CONCLUSIONS 

When the ENZUP plant begins full operation end of 2015, it is expected to be the most comprehensive demonstration of a biocatalyst-based approach to remove Carbon Dioxide from an industrial gas stream anywhere in the world. When the BioCat plant begins operation beginning of 2016 the BIOFOS Waste Water Treatment Plant Avedoere will change into a full-scale demonstration center for new technologies for biogas upgrading and power to gas.

The biogas-based upgraded gas will replace fossil natural gas and reduce Green House Gas emissions to the atmosphere. Lower cost for upgrading biogas will accelerate the use of renewable biogas in the natural gas grid.

REFERENCES 

“Demonstration of a Novel Biogas Upgrading Technology”: www.enzup.com

“Power-to-Gas via Biological catalysis – Phase 2”: www.biocat-project.com

BIOFOS: www.biofos.dk

AUTHOR 

Mr. Thomas Hernø 

HMN Gashandel A/S

Søborg | Denmark

Phone: +45 51 61 92 62

Email: toh@naturgas.dk