Biogas and renewable natural gas
Natural gas is almost entirely methane (CH4). It includes small amounts of heavier gases like ethane, propane, butane and pentane. It is a non-renewable fossil fuel that was generated deep underground from decomposing organic matter millions of years ago.
Biogas is a mixture of gases produced from anaerobic decomposition (in which microorganisms break down material in the absence of oxygen) or thermochemical conversion of biomass (in which material is broken down by using heat in the absence of oxygen). It is mostly methane, which is the primary component of natural gas. Biogas also includes CO2 and small amounts of water vapor and other gases.
Biogas can be burned as a fuel, or it can be treated to remove CO2 and other gases. Treated biogas is called renewable natural gas (RNG). It is almost entirely methane, just like natural gas. RNG is interchangeable with natural gas. It can be:
- Injected into the natural gas pipeline system
- Delivered to industrial and residential utility customers
- Burned in natural gas vehicles
Benefits of biogas and renewable natural gas (RNG)
Biogas and RNG are produced from the decomposition of living biomass, unlike natural gas, which was produced by decomposition of biomass millions of years ago.
The carbon in biogas and RNG comes from the decomposition of plants that were recently living. Living plants pull CO2 out of the atmosphere through photosynthesis and use that CO2 to build biomass. The carbon in plants is therefore part of the active carbon cycle. It comes from the atmosphere, and the plants pull it in. During decomposition, it is converted into CO2 and methane. That methane may end up in biogas, which can then be purified to make RNG. When biogas or RNG are burned, they are converted to CO2, which then returns to the atmosphere.
In this process, the amount of CO2 in the atmosphere doesn’t change. It simply cycles through plants, decomposition, biogas, RNG, combustion and back to the atmosphere. This process produces clean energy.
Conversely, when we burn natural gas, which is really fossil biogas, we are releasing CO2 into the atmosphere that was stored deep in the Earth for millions of years, and we are releasing it in a matter of decades.
The urgent need to capture biogas
Landfills, manure lagoons, sewage treatment facilities and other anaerobic environments (those without oxygen) produce biogas whether we capture it or not. When we capture biogas from these sources and use it as clean renewable energy, we are reducing the amount of natural gas that is burned and the amount of new CO2 that enters the atmosphere.
We need to capture as much biogas as possible to reduce the amount of methane going into the atmosphere. In some cases, it might not be economically feasible to convert biogas to energy. In these instances, it is still important to burn or flare the biogas so that it goes into the atmosphere as CO2 instead of methane, which is a much more potent greenhouse gas.
Pyrolysis and gasification
Pyrolysis and gasification are thermal decomposition of biomass.
Pyrolysis involves heating biomass in the absence (or near absence) of oxygen, at or above 932°F (500°C). Biochar is produced during pyrolysis, and it is often the desired product. Pyrolysis also produces hydrogen, bio-oil, heat and synthetic natural gas (syngas).
- Hydrogen can be used as transportation fuel.
- Bio-oil is a renewable fuel that can be used to create renewable jet fuel or diesel.
Heat can be used on-site to offset fossil fuel use. Much of the heat produced is used to drive the pyrolysis process. The extra heat can be used for building or process heating.
The process can be adjusted to maximize production of a particular product. For instance, you can maximize the amount of biochar that you produce, or you can maximize the amount of hydrogen. This is a source of green hydrogen.
Gasification occurs at higher temperatures. The products of gasification are mostly syngas, hydrogen and carbon monoxide. This is also a source of green hydrogen, which can be burned on demand to generate renewable electricity. This balances wind and solar fluctuations on the grid.
Typical solar farms are tight rows of solar panels that may cover several acres. Sometimes solar farms are installed on former farmland. If the topsoil is removed, it destroys the agricultural value of the land. This makes it much more difficult to return the land to agricultural use after the solar panels are retired.
In some cases, the ground is covered with gravel to inhibit weeds and reduce landscaping expenses. Farmers struggling to make ends meet may be tempted to convert some of their land to solar farms for a steady stream of revenue. However, there are other options that are better in the long run.
We need more clean power, but we also need as much agricultural land as possible.
Agrivoltaics allows farmers to generate clean power and continue to farm their land. Agrivoltaics, as the word implies, is simply the combination of agriculture and photovoltaics on the same land.
There are several ways to combine solar panels and agriculture:
- Mix solar arrays with pasture or crops.
- Space rows of solar panels further apart to allow sun to reach the ground.
- Mount the solar panels high enough to drive a combine or tractor underneath.
- Mount the panels in such a way that they can be rotated to let sun through when the crops need it most. Shade-loving crops can be grown under the panels, or sheep may be allowed to graze.
Solar panels are more efficient when they are cooler. Evapotranspiration (evaporation from the land plus transpiration from plants) from the crops or pasture keeps them cooler than they would be if they were installed over gravel or on a rooftop. The shade from the solar panels reduces heat and evaporation, which can save on irrigation expenses.
Farmers benefit from having two revenue streams from the same land. It helps them preserve their farm rather than converting it into a solar farm.