Electrification with renewables isn’t the
only energy decarbonisation option- gas can also be greened and there are a range
of options for how to do this, with conversion to hydrogen currently being talked
up: www.ukerc.ac.uk/network/network-news/guest-blog-decarbonising-heat-by-replacing-natural-gas-with-hydrogen.html
The H21 plan for Leeds looks to a switch to hydrogen gas for use
in the gas network, rather than fossil gas. All domestic gas boilers and
cookers would be converted to run on clean-burning hydrogen under the proposal
to make Leeds a ‘hydrogen city’. At a cost estimated by Northern Gas Networks of £2
bn, Leeds would be converted by 2025-30
and this could then be replicated in other major UK cities.
Steam-fed fossil-gas
methane ‘reformer’ (SMR) plants around the city would convert methane from the
national gas grid into hydrogen with the resultant carbon dioxide captured and
piped to offshore undersea geological storage wells:
www.northerngasnetworks.co.uk/wp-content/uploads/2016/07/H21-Executive-Summary-Interactive-PDF-July-2016-V2.pdf
www.northerngasnetworks.co.uk/wp-content/uploads/2016/07/H21-Report-Interactive-PDF-July-2016.pdf
It’s a bold plan. Gas
conversion SMR technology is well established but CCS is untested on any scale,
and cannot deliver 100% carbon sequestration. Moreover, though hydrogen burns
without creating CO2, domestic appliances would have to be modified to burn it
rather than methane, as they were in the 1970s, when the UK switched from
hydrogen-rich ‘town gas’ to methane-rich North Sea natural gas. But that would
be less disruptive than installing electric heat pumps in houses for heating
and upgrading local power distribution grids to cope with the large extra
demand- houses already have gas grid links with modern plastic pipes able to
handle hydrogen.
Steam reformation is straight forward and the main current way of
making hydrogen: CH4+ H2O > 3H2+ CO2. However, some say why
not use biogas produced from anaerobic digestion of domestic food and farm waste
for at least some of the feedstock? Then the CO2 produced will be more or less balanced by biomass is growth- so
no CCS is needed: www.sciencedirect.com/science/article/pii/S0961953414003663 Or how about synthetic
green gas made by electrolysis using surplus electricity from wind farms- the
so called Power to Gas (P2G) idea being developed in Germany? That’s based on
the the Sabatier reaction: CO2 +
4H2 > CH4
+ 2H2O.
That will actually be looked at in the H21 programme, but, for
now, the
H21 team sees P2G as marginal: ‘Renewable based electrolysis could be
used, but for the foreseeable future the required quantities do not look
realistic’.
So for the moment it will be based on using
fossil gas. The H21 team says ‘natural gas (predominantly methane),
the lowest carbon dioxide emitter per unit of energy of any fossil fuel,
produces about 180 gm/kWh CO2 equivalent whereas hydrogen emits zero (at the
point of use). The change over from natural gas to hydrogen has the potential
to provide a very deep carbon emission reduction. The true carbon footprint of
hydrogen depends on its source. For example, grid power electrolysis has very
high emissions whereas hydrogen made from stripping the carbon atom from
natural gas has about 50 gm/kWh CO2 equivalent including indirect emissions, a
large reduction over the existing unabated natural gas fuel’.
Labour has been pushing green gas generally and a parliamentary
group has produced a new report, the Green
Gas Book, with a series of essays by MPs and experts exploring the various biomethane, hydrogen, bioSNG and biopropane options. It includes a chapter on the H21 plan and
many mentions of it. The MPs see it, and green gas generally, as better than
ripping out existing gas boilers! https://alansenergyblog.files.wordpress.com/2016/07/final-the-green-gas-book_96pp_v5.pdf
There are nevertheless
some reservations about replacing natural gas with hydrogen. Safety issues are often raised, and certainly
hydrogen gas, like all flammable gases, needs careful handling. But being lighter than air hydrogen tends
vent out if there are leaks rather than filling up buildings. And, as already noted, the coal-derived Town
Gas used before the UK converted to North Sea gas had a high hydrogen content. However,
the chapter in Labour’s booklet by Dr. Keith MacLean from the Energy Research Partnership, notes that ‘Current regulations only allow for 0.1% by volume of hydrogen to be blended into gas supplies. Since the level is much
higher in other countries, like Germany where it is over 10%, there appears to
be no insurmountable technical or safety reasons for this low limit. Upper ends
of estimates of what could be added before adjustments would be required to
appliances are about 20% by volume. However, although hydrogen has a high
energy density by weight, it has a very low density by volume – about one third
of natural gas. Therefore, even 20% by volume would only be equivalent to 6% by
energy’.
So
he says that ‘considering the supply side
and network developments needed to enable hydrogen use in any quantity, it may
make better technical and economic sense to convert to 100% hydrogen in a
limited number of locations, rather than to convert many more areas for a
blended solution, especially if this remains limited to such low levels’.
A
similar view emerged in a UKERC blog: the hydrogen option was an outlier, with
the H21 approach only at best reducing CO2 by 59%: http://www.ukerc.ac.uk/network/network-news/guest-blog-heat-decarbonisation-calls-for-proven-technology.html
Maybe,
but the Leeds H21 team see it as hopefully being replicable in cities elsewhere.
Perhaps by then other feedstocks than fossil methane could be used e.g. biogas
or P2G syn gas. That would avoid the need for CCS. But it would still be a complex
multi-stage process with significant conversion losses, requiring more gas
input to get the same heat output as would be available if the green gas was
used directly for heating. Certainly direct
use of AD-derived biogas might be an easier option in some locations, and even
though the volumes available are limited at present, they could be expanded. Ecotricity,
Good Energy and Green Energy are already offering
green biogas options to consumers via admixtures in the gas supply, and this
route could expand as new biogas plans are developed- Ecotricity is planning to
use grass as a feedstock in new gas mills!
P2G and other syngas routes could
also open up. In his chapter in the Green Gas Book Tony Glover, from the Energy Networks
Association, notes that ‘National Grid’s
2015 Future Energy Scenarios report highlights the potential
for a 10-fold increase in the number of green gas connections to the grid over
the next decade, indicating a possible 416 connections by 2025 and 700
connections by 2035. This equates to approximately 40 TWh/year of green gas
from AD injected to the grid by 2035, around 5% of the total UK gas demand and
around 10% of the UK domestic gas demand. More recent industry estimates, which
also include other renewable gases such as Bio-substitute natural gas (Bio SNG)
and Power to Gas (P2G), suggest that the full potential of renewable gas may be
over twice this level. Additionally, as UK gas demand continues to decrease,
this proportion could become much higher’.
For
a detailed analysis of all the green gas options and a review of some
pioneering examples, see ‘Renewable Gas’, Jo Abbess, Palgrave, so far the
definitive text, although this new more technical one looks good too: www.wiley-vch.de/publish/en/books/forthcomingTitles/CG00/1-118-54181-2/?sID=qq5uj8c4nsi1ombe73jpijkli0.
