By Dave Elliott
‘Turning certain rubbish materials and farm and food waste into various types of biogas – ‘green gas’ – holds the potential to cut costs, radically reduce pollution, and decrease our reliance on imports. Crucially, using more green gas could make a real impact on the decarbonisation of heat without the need to overhaul our national gas pipeline and heat delivery infrastructure and without significant technical barriers’. So say Labour MPs Lisa Nandy and Caroline Flint in the Green Gas book published by the Parliamentary Labour Party Energy and Climate Change Committee.
In her separate chapter, Flint notes that there are 172 on-farm AD plants producing enough heat for 240,000 homes and a further 50 biomethane plants generate enough heat for 155,000 homes. But she says ‘dwarfing this are some 447 AD plants generating electricity from biogas. 556MW each year, electricity for 750,000 homes. The problem is that taking biogas and using it to power turbines to generate electricity is considerably less efficient than using the gas to heat homes. The subsidies available from Government to help grow low carbon energy have distorted the investment, leading to far more electricity generation than green gas.’
Clearly Flint wants a change. As do the other contributors to the booklet, like Labour MP and well known sustainable energy supporter Dr Alan Whitehead. He notes that the currently favoured decarbonisation option, heating using heat pumps powered by non-fossil electricity, simply won’t work on a large scale, given the scale and variability of heat demand: ‘the amount of additional capacity in electricity generation that would need to be envisaged would be simply enormous’ and ‘the variation in demand for fuel for heating vastly exceeds that of the variation for electrical power for other uses’. Whitehead mentioned an estimate that the equivalent of 30 new nuclear plants would have to be built to meet peak heat demand. That estimate was by Mike Foster, from the Energy Utilities Alliance, who added that another alternative would be a vast increase in renewables or imported low-carbon electricity. Foster also noted that ‘the UK’s current distribution networks could not cope with such extra demands on them’ – we would need to upgrade the local power grid extensively. And also fit expensive new end-use devices like heat pumps – ripping out existing gas boilers. All of which seems to suggest that it would make more sense to replace fossil gas with green gas using the existing gas main and existing heating devices.
However, there is a potential problem – there may not be enough green gas. Certainly AD biomass is relatively limited at present, although it could be expanded. In his chapter, 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’.
In his chapter, Dale Vince from Ecotricity added an extra AD dimension: they were looking to use grass to make green gas. He claimed that ‘grass can yield twice as much gas per tonne of feedstock than food waste’ and he saw this approach as having low environmental impact. No artificial fertilisers would be needed and ‘grass can be grown on marginal farmland’. He estimated that their proposed 10MW ‘gas mills’ would each use 5,000 acres of land and produce enough energy for 5,000 homes – an acre each! 500 of them could, he said, meet the expected shortfall in green heat, which he put at 42.5TWh p.a., based on the DECC target of getting 12% of heat from renewables by 2020; so far we had only managed 4.9%.
There is clearly some way to go before anything like that is possible but, in addition to Ecotricity’s plan, the new H21 project proposed for Leeds goes some way towards it. It gets regular mentions and a chapter in Labour’s booklet. What’s proposed is a switch over to hydrogen, made by steam reformation of mains gas, injected in the Leeds gas mains, with CCS taking care of the CO2 produced.
There will be losses associated with this multi-stage process, so that 47% more gas will be needed to get the same heat output as would be obtained if the gas was used directly for heating, as a UKERC post noted. It also noted that the net emissions saved, even with CCS, would only be 59% compared with the conventional gas route:
So it’s good to see that ITM’s Power to Gas (P2G) green gas electrolysis technology is also being looked at in the H21 programme, alongside fossil gas. That wouldn’t need CCS – it could use surplus zero carbon renewable electricity. But although that electricity would in effect be free (it would be dumped otherwise), the conversion process is still inefficient and the Leeds team seems to see P2G as marginal for now, compared to steam reformation of fossil gas. They say ‘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. Renewable-based electrolysis could be used, but for the foreseeable future the required quantities do not look realistic.’
There are other options for the H21 system. Some AD biogas might also be used as a feedstock for hydrogen production via steam reformation, rather than fossil gas. But if we have green gas, then why go for conversion to hydrogen? Why not just just inject AD-derived biomethane into the gas mains? Or perhaps go for a blended mixture.
One of the chapters 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 MacLean 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’. Maybe, although the Leeds team sees it as hopefully being replicable in cities elsewhere.
Cities do need to convert to greener energy use but some say that, in addition to the direct use of renewables like PV solar for electricity, district heating networks can and should play a major role. That would involve installing expensive pipework and would still, at least initially, rely on fossil gas, though it could be gradually replaced with green gas. Heat networks clearly have a place, especially in high density urban environments. But switching to hydrogen, delivered via the existing gas mains, might be easier in some locations. I will be looking at green energy options for cities in my next two posts.
The Labour booklet is downloadable free from: https://alansenergyblog.files.wordpress.com/2016/07/final-the-green-gas-book_96pp_v5.pdf