Clean coal- does such a thing exist?

 

Ideally, to protect the climate, we should avoid all processes that produce carbon dioxide gas (CO2). But some of the CO2 produced by burning fossil fuel can be captured (chemically) from power plant exhausts and stored (e.g. in empty undersea oil and gas wells), at a cost. This is one of the ideas behind the label 'clean coal'.  But it’s not that clean. And it's also complicated.

Carbon Capture and Storage (CCS), as it’s called, is not very efficient and may not be a long-lasting solution (the CO2 may eventually escape), but it does allow you to cut the net emissions from continued fossil fuel use by about 60-70% (not 80-90% as some claim, since the various CCS processes use energy and supplying that adds more emissions). Coal CCS is usually harder and more expensive than gas CCS. That’s partly because, in most modern plants, to get high efficiency, the coal has to be gasified as a first stage.

Gasifying coal at high temperatures was how we made 'town gas' before North Sea gas became available. In some modern coal fired power plants designs, some of the hydrogen gas in the output from the gasification process can be extracted (chemically) and used as a clean-burn fuel (then there is no CO2 production). The rest can be burnt as normal e.g. in a gas turbine, but, if coupled with CCS to deal with the CO2 that is produced by gasification and subsequent full combustion, then you have a low carbon option.  In some new so called pre- combustion plants, CO2 extraction is carried out at the gasification stage, while in others oxygen is injected prior to combustion to increase the proportion of CO2 in the resultant flue gases, to make its capture more efficient.

As can be seen, whether pre or post combustion, carbon capture is complex stuff, usually requiring a radical redesign of plant, and extra stages in plant operations, adding perhaps 50% to the cost of power generation. It also requires costly new pipe infrastructure to convey the captured CO2 gas to a storage well, usually with intermediate pumping stations to sustain gas pressure and then injection equipment at the well head.

Most environmentalists don't see CCS as anything more than an expensive interim option and the risk is that it will be used as an excuse to downplay the proper answer to halting emissions - a switch to renewables. But this simple anti-CCS line can be complicated by some other possible options which might make the development of CCS more useful long term, such as Biomass Energy with Carbon Capture and Storage  (BECCS).  

In the case of biomass combustion, the CO2 produced is nearly balanced by the CO2  absorbed when the biomass is growing (not quite since there are harvesting, transport and processing energy requirements). Even so its nearly carbon NEUTRAL over time. Adding CCS would make it carbon NEGATIVE- actually taking CO2 out of the atmosphere. Be warned though, it all depends on the type of biomass used.  Biomass stores CO2, so you don't want to interrupt that too much or for too long, or have CO2 in the air for long times before it is reabsorbed by new plant growth. So using trees is a very bad idea- most take a long time to grow fully and then can store CO2 for a long time (before they die, rot or catch fire). A key carbon sink. Fast growing energy crops (e.g. using short rotation coppicing) are much better as a fuel source, since replanting can start reabsorbing CO2 fast. Using farm (crop/animal) and household (human/food) bio-waste to make biogas is even better still. The supply is continually available and using wastes to make a bio-fuel is much better than leaving them to rot, potentially releasing methane into the atmosphere in an uncontrolled way, a much worse greenhouse gas than CO2. And BECCS, biomass use with CCS, could take it one step further.

However there’s also another approach: Carbon Capture and Utilisation (CCU). This uses some of the captured CO2 (from fossil fuel or biomass combustion) to make new synthetic fuel, synfuel or synthetic natural gas (SNG). To do that you need a source of hydrogen. You can get it by processing biomass (or of course fossil fuels) at high temperatures OR via the electrolysis of water- and, if using electricity from wind turbines or PV solar cells, it is carbon neutral, and the same (almost) if it’s from suitable biomass feed stock.  You then have a choice. There is a non-CCU route. The hydrogen gas can just be used as a fuel itself, and, if used to run a fuel cell or a gas turbine, you can get carbon neutral electricity back- there is no CO2 produced when hydrogen is used as a fuel.  So if the electricity initially used was from surplus wind or PV solar output, and you store the hydrogen until power is needed (e.g when wind/PV production is low), you have a way of dealing with the variable output from wind and solar plants, with no CO2 emissions.

Alternatively, as in the modern Power to Gas (P2G) systems being developed in Germany, the hydrogen gas can be converted to methane (CH4) using captured CO2. So that is CCU. It’s much easier to use CO2 from power plant exhausts than to try to capture it from the air (there's not much in the air despite all our efforts!)  But burning the resultant methane (to make electricity, or for heating, or in a car) produces CO2 again, although, overall Power to Gas conversion (even using CO2 from power plants) can, like CCS, be near carbon neutral, depending on the original source. Though if the synthetic methane gas is used for electricity production, so that you can capture the resultant CO2 (again), you can make electricity production carbon NEGATIVE  (especially if the CO2 came from biomass combustion), although each time you repeat this process you get diminishing returns - it's overall conversion efficiency is low. And of course it can’t be done (easily!) with car exhausts or with domestic gas-fired central heating boilers. Using fossil fuel to make hydrogen feed stock for CCU synfuel production would of course not be carbon neutral, since some CO2 would be produced, but if this was captured and the synfuel was used to make electricity with CCS, then it could be.  

As can be seen there may be some clever new biomass (or even fossil fuel) to synfuel options, with low or even negative carbon implications. Does this mean we can still use coal? If so, where would it come from? Most of what the UK now uses is imported (about half of it from Russia) although we do still have some open cast strip mines. But that's very environmentally unappealing. Some coal (or more likely) gas CCS may happen eventually and coal gasification for hydrogen production is ready now (at a cost) and might just be condonable in extremis! It’s certainly better environmentally than just burning coal in conventional plants. But if we must burn coal, then burning it in Combined Heat and Power (CHP) plants is arguably even better: the overall energy conversion efficiency can be 80% or more, since you can make use to the otherwise wasted heat, assuming there is a big heat load reasonably nearby.  CHP plants can be very flexible- the ratio of heat to power output can be varied to help balance varying grid supplies and demand. Though that may make it hard to use CCS with them. However that may not matter too much, since the net CO2 emissions per useful kWh of heat and power output will be low, compared with conventional power only fossil fired plants. It would be even lower if they are gas fired, as most of the new CHP plants are, and even lower if they are biomass fired, depending on the biomass source. 

So there are some options for cleaner coal use, but they all have limitations and costs and none avoid all emissions. To do that you need renewables like wind and solar. Developing CCS, CCU and CHP initially for fossil fuel might be seen as a way to pioneer low carbon or even negative carbon technologies for subsequent use with biomass, but the use of biomass can be problematic, depending on the source. Wind and solar (and possibly wave and tidal power), are arguably less problematic, and if you want green gas, then the Power-to-Gas idea, using surplus wind and/or solar derived electricity, is a way forward-  for heating, transport, or, with storage, for (later) power generation to balance the grid.

For more see ‘Renewable Gas’, Jo Abbess, Palgrave.

More on CHP in my next post in this Renew Extra series.