Biogas or Biomethane plays an important role in the conversion to a sustainable energy system. However, its benefits are to be spread further afield. It will be necessary to introduce efficient new production methods to complements today’s anaerobic digestion technology.
Thermal gasification of forestry byproducts is one such method. Transforming branches tree tops and other forest residues into clean biomethane.
If there’s one thing we have an abundance in this world, is forest. Large scale biomethane production can be produce from forest residues. But how exactly does one transform residues from the forestry industry into gas?
Let’s take a closer look at this advanced process in which the laws of physics and chemistry interact in an unique way.
Converting forestry byproducts into biomethane is a two-stage process.
The first, is a thermal gasification stage in which the raw material is converted into a blend of various gasses known as syngas. Second is methanation where the syngas is purified and refined into methane gas at the same high quality as natural gas.
Let’s begin by looking at the gasification process.
Wood pallets or branches and tree tops that have been chopped into chips are delivered to the facility where they are fed into a gasifier. Here, they are subjected to very high temperature. The heat comes from a separate combustion chamber and is transferred to the gasifier with the help of a recirculating heat bearing bed material, in this case, sand.
Since the environment inside the gasifier is oxygen free, complete combustion does not take place. Instead, the woody materials are transformed into a gas mixture that later can be refined. Combustion also creates flue gasses. These are taken care of in a post-combustion chamber which completely incinerates the carbon monoxide and reduces the nitrogen oxide content.
After this, the flue gasses are cleaned once again with particles being separated via a flue gas filter. Here, and at several other stages the flue gas or syngas is cooled down, making it possible to recover heat which is used in other parts of the plant or delivered into the district heating system to warm up homes, schools and workplaces.
When they syngas leaves the gasifier, it consists primarily of carbon monoxide, carbon dioxide, hydrogen, water vapor and methane as well as sulfur compounds and heavier hydrocarbons that is say tars such as naphthalene which must be removed.
This cleaning process takes place in several steps.
First, the gas is cooled after which particles are separated in a special gas filter and then the gas is cleaned with biodiesel in a device known as RME-Scrubber. The tars that are removed accompany the used biodiesel into the combustion chamber.
In the next stage, the gas is further filtered using activated carbon. The tars that are extracted here, are also sent to incineration. Before methanation can begin, the gas pressure is raised from just above atmospheric pressure to 16 bar. In addition, the gas has to be cleaned of all traces of chlorine and sulfur compounds since these may damage the catalysts used in the methanation process.
Cleaning takes place both in an Absorption Vessel and in Amine Washer. Now, the gas is ready for the first stage of methanation. With the help of a device known as a shift reactor and a catalytic process, the gaseous chemical compound is altered so the ration between its hydrogen and carbon monoxide content is adjusted to three to one.
As a result, additional carbon dioxide can be removed. This takes place in another Amine Washer. The extracted carbon dioxide is returned to the process and is used for various purposes such as unearthing fuel silos during filling. In the next stage of methanation, the gas is refine still further and yet another catalytic process but in several stages converts it into pure methane gas that is biomethane.
Before the biomethane can be fed into the gas grid, it is dried to prevent the formation of condensation in the gas. The pressure is now raised to 35 bar and the biomethane is ready to be delivered to transmission grids for gas.
That’s what the journey from forest to biomethane looks like. Forest residues become clean energy and vision becomes reality.