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NEA’s BlueStroke adjusts flows to process needs

Controlling The Efficiency Of Recip Compressors


NEUMAN and ESSER (NEA) has delivered three reciprocating compressors for German automaker Audi’s project to produce 1102 tpy (1000 T/yr) of synthetic renewable methane (also called e-gas).

Highly efficient storage media is required for wind power and photovoltaics. Renewable energy sources like wind and solar fluctuate with weather and seasonal variance, which can only be compensated using such storage. Another problem that occurs in the field of renewable energy is that the power grid is not prepared for the necessary, larger quantities of electricity distribution and transportation volumes.

In Germany, for instance, wind energy produced in the North cannot be transferred to the South as the old supply grid is overstrained. Thus, wind power plants have to be switched off in case of surplus production. To avoid this, the power-to-gas concept stores and transports regenerative energy as hydrogen or methane. By this means, renewable resources can be made optimally useable and fossil sources can be replaced reliably.

Converting energy

The world’s first industrial plant for generating synthetic methane from carbon dioxide (CO2) and renewable electricity has been built at Werlte, Germany. It draws electricity primarily when an excess is available.

“This power-to-gas technology opens up new possibilities for sustainable mobility and tomorrow’s energy industry,” said Reiner Mangold, head of sustainable product development at Audi. “The e-gas project marks a transition toward alternative forms of energy for automobiles.”

The company has launched this endeavor without any investment grants or subsidies.

The power-to-gas plant works in two process steps: electrolysis and methanation. In the first step, the plant uses surplus green electricity to break water down into oxygen and hydrogen in three electrolyzers. The hydrogen could one day power fuel-cell vehicles. For the time being, however, in the absence of an area-wide infrastructure, a second process step is carried out directly: methanation. The hydrogen is reacted with CO2 to produce synthetic methane. It is virtually identical to fossil natural gas and will be distributed via an existing infrastructure, the natural gas network.

Carbon dioxide required for the e-gas production is a waste product from a nearby biogas plant operated by the energy provider EWE AG. The biogas plant is fed with organic waste as opposed to energy crops, avoiding any competition with food production. In this way, the power-to-gas plant produces 141 Mcfd (4000 m3/d) of methane.

Audi built the e-gas plant in Werlte on an EWE site in cooperation with the equipment manufacturer Etogas GmbH (formerly SolarFuel GmbH).

Roles for NEA recips

Three NEA units are part of it all. The vertical e-gas reciprocating compressor 2TZS25GT is new in NEA’s range and supplies 12.3 Mcf/hr (350 m³/hr). The two-crank and two-stage machine is a pressure-resistant supplement for low volumes. The second NEA recip is a V-type biogas compressor supplying a quantity of 24.7 Mcf/hr (700 m³/hr). The third and largest compressor is also a V-type recip with 37 Mcf/hr (1050 m³/hr) that can compress both e-gas and biogas. The NEA compressors are, on the one hand, involved in feeding the processed biogas and on the other hand when passing the renewable methane on.

EWE has cooperated with NEA for several years. Initially, NEA supplied the energy company large boxer compressors for natural gas storage. As the biogas sector developed, EWE sought compressors for feeding in the high pressure area more and more frequently.

There are 7600 biogas plants in Germany with an overall output of more than 4 million hp (3 GW). They supply more than 5 million households and replace three nuclear power stations. Most of the plants generate electricity with the biogas and supply the surrounding houses with heat. However, this is not the only means to use biogas.

Due to the large quantity of agricultural waste and liquid manure, the Netherlands, Switzerland and Sweden have the most experience with biogas, which is processed as biomethane in these countries. The Netherlands and Switzerland feed it into the natural gas grid. It is used for motor vehicles in Sweden. In Germany, biogas and the feeding of biomethane into the natural gas grid will be an important mainstay for energy politics in the future.

As the biogas has low pressure following its preparation, it must be compressed accordingly using a reciprocating compressor. Thirty-five NEA compressors run at German biogas plants of various sizes and feed the purified biomethane into the existing natural gas grid. Depending on the grid operator, the NEA compressors must increase the gas pressure between 58 and 116 psi (4 and 8 bar) to up to 1015 psi (70 bar).

The efficient use of energy flows is the top priority in the production sequence of the power-to-gas plant from Audi. The waste heat given off during methanation is used as process energy in the adjacent biogas plant, significantly increasing overall efficiency. In return, this plant supplies the highly concentrated CO2 required as a basic building block for the e-gas. This CO2 thus serves as a raw material and is not emitted to the atmosphere.

In June 2013, Audi opened the power-to-gas facility. Major German energy utilities have since taken up the idea of power-to-gas cogeneration and are following Audi with initial projects of their own.

In parallel with the e-gas plant in Werlte, Audi also operates a research facility in Hobbs, New Mexico, for the production of e-ethanol and e-diesel in collaboration with Joule. At this facility, microorganisms use water (brackish, salt or wastewater) sunlight and carbon dioxide to produce high-purity fuels.

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