Gas (methane) is
a useful fuel. It is easy to store and can be transmitted long distances with
low energy losses. And it’s cleaner to burn than coal. Moreover, relatively
cheap and flexible gas-fired power plants are useful for providing grid
balancing for variable renewables. However, natural gas won’t last forever,
even with a shale gas input, and in any case, as a fossil fuel, we ought to be
reducing its use to avoid carbon emissions. So what’s the alternative?
The most obvious
non-fossil ‘renewable gas’ is that produced directly from biomass i.e. biogas,
biomethane generated via anaerobic digestion of biomass and wastes, and used as
a fuel. Biomass can also be gasified at high temperatures or converted into
liquid biofuels via medium temperature pyrolysis. You can also make synthetic
methane using other renewable sources e.g. by using wind-derived electricity to
electrolyse water to make hydrogen and then convert that to methane using
carbon dioxide from the air or from power plant exhausts. This ‘wind-to-gas’
idea is being developed in Germany for grid injection and also for vehicle fuel
production. There is also some interest in this idea in the UK: http://www.gridgas.co.uk/partners.html
However these two
‘green’ sources of non-fossil methane will take time to expand. The
multiple-stage wind-to-gas option is going to be expensive and some see biomass
as being relatively limited given land-use constraints. Wider use of food and agricultural
wastes may change that (though we ought to reduce these wastes at source), and
if it is (effectively free) surplus wind power that is being used to make
syngas, then the costs are lower.
Even so it will take time.
So what can be
done in the interim? Well there is
a range of low-carbon fossil gas conversion options which may been seen as
transitional technologies, making use of fossil fuels, but laying the basis for
a low carbon energy system. There are a number of industrial processes,
including waste management processes, which can be used to make low carbon
synfuels and hydrogen gas from, initially, fossil fuels or some wastes, but which
might also later use 100% biomass. The gasification or pyrolysis of fossil
fuels can produce a range of fuels which, and if the process is combined with
carbon capture, can be low carbon overall, and in time the fossil feedstock can
be replaced with biomass, making it potentially carbon negative. One even more advanced idea is to use
green hydrogen (e.g. from wind powered electrolysis) to upgrade biogas to
higher value synfuel, thus reducing the biomass land area/kWh needed.
What we are
seeing with ideas like this is clever ‘green chemistry’, playing with carbon
and hydrogen to reduce carbon dioxide emissions, initially using fossil energy
and fossil feed stocks, but increasingly using renewable sources and
feedstocks, including biomass and even carbon from the air, to make low carbon
renewable fuel as an alternative to fossil gas. Biomethane production via AD is of course quite widespread
and there is some hydrogen production via gasification. Germany may be leading,
but the UK government also seems keen to press ahead. In its report on energy
networks last year it said ‘‘innovative alternatives to natural gas, such as
biomethane, low carbon hydrogen and gas from anaerobic digestion plants are
potential ways of maintaining the existing gas grid while reducing our reliance
on fossil fuel gas.’
There are of
course limits. As noted above,
land-use is a key issue. Biomass is land (and water) hungry. Its large-scale
production for energy use may undermine natural carbon sinks and local
biodiversity and possibly lead to conflicts with food production. Especially if
we are talking about large plantations producing biofuels for vehicle use. Producing renewable gas would however
only be a subset of biomass use and many of the techniques being developed for
its production and use are less environmentally aggressive than those for
mass-burn power production e.g. using forest-derived wood pellets to fire large
converted coal plants. For example, AD biogas production can be done at a range
of scales and with a range of farm and other wastes. Some environmentalists worry that the interim continued use
of fossil fuels, albeit at least partly decarbonised via CCS or other carbon
cycling approaches, will delay the development of fully renewable systems, but
the pragmatic view is that these interim approaches will lay the basis for a
fully sustainable biomass based system later on. The debate goes on, although
usually focused on specific projects. For example, while some environmentalists
are often concerned about emissions from some waste combustion plants,
gasification and pyrolysis are usually seen as a less problematic. It may all
come down to need to proper, and convincing, regulation.
As can be seen,
it’s a complex field, still under development, but the potential does seem to
be there for non-fossil gas to replace at least some fossil gas long term and
for clever green chemistry and biology to allow for the production of interim
cleaner synfuels.
For an overview
of all the options and a review of some pioneering examples, in the food
industry waste management field especially, see ‘Renewable gas’, Jo Abbess, Palgrave. She doesn't
cover AD in so much detail. For that see a Routledge book, ‘Bioenergy Production by Anaerobic Digestion’ Edited
by Nicholas E. Korres, Padraig O'Kiely, John A.H. Benzie, Jonathan S. West, and ‘Anaerobic Digestion
- Making Biogas - Making Energy’, The Earthscan Expert Guide, by Tim
Pullen. Stewart Boyle’s ‘The
Sleeping Giant Awakes’ is also worth looking at.
