PROCESS FOR INJECTING BIOMETHANE INTO A NATURAL GAS NETWORK

The present invention relates to a process for injecting biomethane into an L type natural gas network and to the corresponding plant thereof.

Biogas predominantly contains methane (CH₄) and carbon dioxide (CO₂), but also water, nitrogen, hydrogen sulfide, oxygen, and other organic compounds.

It is essential to develop various upgradings of the biogas in order to respond to the problems caused by global warming, on both a global and regional level, and also in order to increase the energy independence of the territories that produce it.

Biogas may, after slight treatment, be upgraded in the vicinity of the production site in order to provide heat, electricity or a mixture of both (cogeneration); the high content of carbon dioxide reduces its heating value, increases the compression and transport costs and limits the economic advantage of upgrading it to this local use.

A more thorough purification of the biogas enables a broader use thereof.

In particular, a more thorough purification of the biogas makes it possible to obtain a biogas that is purified to the specifications of natural gas; this highly purified biogas is referred to as “biomethane”. Biomethane thus supplements natural gas resources with a renewable portion produced at the heart of territories. It can be used for exactly the same uses.

The injection of produced biomethane is booming. However, in France for example two types of natural gas networks exist: the H type (high calorific value) network and the L type (low calorific value) network. The biogas purification units produce a biomethane containing 2.5 mol % CO₂in CH₄mainly, with therefore a gross calorific value and a Wobbe index that are too high for being injected into the L type networks.

Hence, one problem that is faced is that of providing an improved process for injecting biomethane into the natural gas network.

One solution of the present invention is a process for injecting biomethane into a network having a gross calorific value of value X between X1 and X2, comprising the injection of nitrogen into the biomethane network before the injection of the biomethane into the network having a gross calorific value of value X so as to lower the calorific value of the biomethane network to a value between X1 and X2, with the nitrogen resulting from the retentate of at least one membrane stage.

One specific solution of the invention is a process for injecting biomethane into an L type natural gas network, comprising the injection of nitrogen into the biomethane network before the injection of the biomethane into the natural gas network so as to lower the gross calorific value of the biomethane network to a value between 9.5 and 10.5 kWh/Nm³, with the nitrogen resulting from the retentate of at least one membrane.

Depending on the case, the process according to the invention may have one or more of the following features:

• • the membrane stage is supplied with air resulting from an internal network of the process or resulting from an air compressor; and the amount of nitrogen injected into the biomethane network is controlled via a control valve located on the feed of the membrane stage or via the adjustment of the production capacity of the air compressor. In the case where a control valve is used, the set point of the flow rate of nitrogen to be injected is calculated knowing the content of methane in the natural gas, and also its flow rate, these two parameters making it possible to deduce the GCV of the gas when CH₄is the only fuel present. An “internal network” is preferably understood to mean air used for the operation of instruments used in the process such as the valves; reference may also be made to “instrument air”;
  • the gas stream passing through the membrane stage is air that is dried so that the nitrogen, retentate of the membrane stage, mixed with the biomethane meets the specifications of the network having a gross calorific value of value X; and that is de-oiled at a pressure greater than or equal to the pressure of the biomethane network, in general between 5 and 15 bar. In general, the air is dried so as to have a dew point below −5° C. at the maximum pressure of the injection network;
  • the purity of the nitrogen injected into the biomethane network is controlled via an analysis of the concentration of oxygen in the nitrogen retentate, or by a measurement of the pressure of the retentate. For this, use is preferably made of a control loop, the actuator of which is a control valve installed on the retentate of the membrane, with the control valve making it possible to adjust the operating pressure of the membrane. An oxygen analyzer located on the nitrogen retentate makes possible to control the purity and constitutes the measurement of the control loop. The oxygen purity may also be deduced by the measurement of the pressure of the retentate. The measurement of the control loop is then constituted by a pressure sensor;
  • the membrane from which the nitrogen-enriched retentate is derived also produces an oxygen-enriched stream;
  • the oxygen-enriched stream is injected into a digester that produces biogas or upstream of activated carbon filters of a biogas purification unit. A “digester” is understood to mean an anaerobic production of biogas. This injection of oxygen-enriched stream facilitates the desulfurization of the biogas actually within the digesters, or when the oxygen-rich stream is injected into a biogas purification unit it facilitates the abatement of H₂S by the activated carbon.

Another subject of the present invention is a plant for injecting biomethane into a network having a gross calorific value of value X, comprising:

• • a biomethane production unit;
  • a biomethane network;
  • a network having a gross calorific value of value X;
  • a nitrogen-selective membrane enabling the production of a nitrogen-enriched retentate from an air stream;
  • a system for producing air at a pressure greater than or equal to the pressure of the biomethane network;
  • a first injection means for injecting the retentate of the membrane into the biomethane network;
  • a second injection means for injecting the biomethane resulting from the biomethane network into the network having a gross calorific value of value X,
    with the second injection means downstream of the first injection means according to the flow direction of the biomethane in the biomethane network.

One specific plant according to the invention is a plant for injecting biomethane into an L type natural gas network, comprising:

• • a biomethane production unit;
  • a biomethane network;
  • an L type natural gas network;
  • a nitrogen-selective membrane enabling the production of a nitrogen-enriched retentate from an air stream;
  • a system for producing air at a pressure greater than or equal to the pressure of the biomethane network;
  • a first injection means for injecting the retentate of the membrane into the biomethane network;
  • a second injection means for injecting the biomethane resulting from the biomethane network into the L type natural gas network,
    with the second injection means downstream of the first injection means according to the flow direction of the biomethane in the biomethane network.

Depending on the case, the plant according to the invention may have one or more of the features below:

• • said plant comprises an oxygen concentration analyzer located on the retentate of the membrane upstream of the first injection point, a pressure sensor located on the retentate of the membrane upstream of the first injection means, and a control valve located on the retentate of the membrane downstream of the analyzer and upstream of the first injection means;
  • said plant comprises a control valve on the feed stream of the membrane;
  • the system for producing compressed air successively comprises, in the flow direction of the air, an air inlet, an air compressor, a compressed gas cooling system, a condensate separator, an activated carbon filter that makes it possible to remove the residual oil particles, a particle filter that makes it possible to remove the activated carbon particles, a dryer and a compressed air storage tank.

Tables 1 and 2 below illustrate the need for injection of nitrogen in order to comply with the biomethane injection specification from the point of view of the GCV and the Wobbe index in L gas networks: