How to transform waste into green energy

The project to purify biogas from the Can Mata controlled landfill in Barcelona will make this Ferrovial Services facility the first in Spain to recover landfill gas for conversion into biomethane and injection into the natural gas transmission grid. Waste deposited in the landfill will result in the production of 40 million cubic metres of biogas per annum and a reduction in emissions of 17,449 tonnes of CO2 equivalent.

Alex Grande, Head of Projects, Environment Area, Ferrovial Services

The Can Mata facility, located in the municipality of Els Hostalets de Pierola (Barcelona), is one of the largest controlled landfills in Spain. It occupies an area of 68 hectares and has been operated for 20 years by Ferrovial Services, during which time over 20 million tonnes of waste have been treated. The waste stored at Can Mata produces almost 40 million cubic metres of biogas per annum. This biogas is currently recovered as fuel to produce electricity at a ceramics plant and for heat generation in the leachate treatment processes.

How to turn waste into natural gas that can be injected into the grid

Thanks to the driving force of the circular economy, different initiatives have been embarked on in Europe in recent years for the upgrading and transformation of biogas from waste treatment facilities into biomethane of sufficient quality for injection into natural gas transmission grids. With this objective, Ferrovial Services undertook this project with Waga Energy for the installation of a biogas purification unit capable of treating up to 2,200 m³/h (2,000 m³/h nominal) at the Can Mata landfill. The biomethane obtained will be injected into the NEDGIA (NATURGY Group) natural gas distribution network. For this purpose, a gas pipeline will be built to connect the biogas purification unit with the gas pipeline in the municipality of Capellades, where the biomethane will be consumed.

Process to convert biogas into biomethane

Landfill gas is a mix of different gases generated spontaneously through the action of microorganisms in the landfill cells where the waste is deposited. It is mainly made up of methane (CH4), carbon dioxide (CO2), nitrogen (N2) and oxygen (02). This composition is variable, even during the same day, depending on weather conditions. To obtain biomethane suitable for grid injection, the methane molecules must be separated from the other compounds, which is not an easy process. Because nitrogen, oxygen and methane are molecules of similar size, separation by filtration is very difficult. Moreover, landfill biogas contains other compounds (VOCs, ammonia, siloxanes, etc.) that also need to be removed to meet grid specifications. For this reason, Ferrovial Services opted for WAGABOX® technology, which purifies the landfill biogas to convert it into methane by means of an innovative, two-stage process. The first stage consists of membrane separation to remove the CO2 from the biogas, while the second stage consists of cryogenic distillation to separate the methane from the N2 and O2 in order to obtain biomethane.

Membrane separation is the leading technology implemented for the purification of digester biogas, which is similar to landfill biogas in terms of CO₂ content. This technology is widely acknowledged for its technical efficiency and cost effectiveness. The system is very flexible and can be adapted to varying biogas compositions and flow rates, whilst guaranteeing very high availability. Cryogenic distillation consists of cooling the biogas to a temperature of 112 K (-116 °C) so that methane is obtained in the liquid phase while nitrogen and oxygen remain in the gas phase.

In order to size the purification unit to be installed in Can Mata, an estimate of biogas production was made using a calculation model that takes into account, as main parameters, the quantity of waste deposited in the landfill and its composition. To calculate biogas production, a mixed theoretical/practical method was used to determine the kinetic model, using a mathematical algorithm (derived from the algorithm proposed by Tabasaran, 1976) based on a typical model that allows a standard specific productivity curve to be constructed for every tonne of waste.

The biogas produced at Can Mata will be delivered to the Nedgia injection module, where the pressure will be increased to 17 bar. In this module, it will be verified that the biogas meets the required parameters and it will undergo odorization treatment through the addition of THT (because after the upgrading process, the biomethane is completely free of VOCs that would provide it with odour). From here, the biomethane will be sent through an embedded pipeline to the injection point of the Nedgia transmission grid, around 4 km from the Can Mata Landfill.

Published in: Nº78 FuturENVIRO March – April 2021