Thursday, 31 May 2012

How green is my charcoal barbecue?

The barbecue season has started. In my book I looked at cooking on the hob and in the oven but I didn't think of barbecues. You might think that using charcoal on your barbecue is carbon neutral because charcoal is made from wood which is renewable fuel. This is not entirely true because of greenhouse gas emissions (a) during manufacture of the charcoal and (b) in transport.  So how does cooking on the barbie compare with, say, under a grill in the kitchen?


My friend Neil is a barbecue expert and he says he uses about 1.2 kg charcoal to cook for 2-4 people. This is close to the minimum viable size.

Burning 1.2kg charcoal produces around 3.1 kg CO2. That is assuming the charcoal is 70% carbon, which is a reasonable quality [1], and that it is burnt cleanly. 3.1 kg CO2 is the equivalent in greenhouse gas emissions of about 6 kWh of electricity or 17 kWh of natural gas. To put that into perspective, the grill mode on my oven says it generates 3.2 kW heat so burning the charcoal for one barbecue produces the same CO2 emissions as about 5 hours of gas grill.

However, that doesn't matter because all the carbon in your charcoal came from trees and you did use FSC charcoal (didn't you?) so the trees are being replaced. Phew.

However, to make charcoal you have to heat the wood to burn off impurities and break down the wood structure and this process generates all sorts of gas products, one of them being methane which is a very potent greenhouse gas.  Methane is produced because the wood is being heated with insufficient oxygen to burn it. Charcoal production is a delicate balance between enough oxygen to burn and generate the heat needed to break down the wood but not enough to burn it completely. Partial combustion products like methane are inevitable and traditional charcoal production in a closed kiln ultimately vent it into to the air. There is another method called the retort method in which heat is applied to the charcoal oven from outside. Then the gas emissions, including the methane which is a useful fuel, are collected and burnt to help heat the oven; the whole process is relatively clean and efficient. However, the equipment for this is more expensive. There are good descriptions of the kiln method and the retort method here. 

A very large proportion of the charcoal we use is imported from places like Brazil and East Africa where it is made almost exclusively using the closed kiln method. We produce about 5000 tonnes of charcoal in the UK (2010)  [2] but we use more like 60,000 tonnes [3] so 92% is imported. Producing 1 kg of charcoal in a typical kiln in Kenya produces methane equivalent to 200-400g CO2 [4]. Also, bringing the charcoal back to the UK by ship is 11000 km by container ship which adds another 150 g CO2e [5]. The lorry journeys from the producer to the port, and in the UK to wholesaler and then retail warehouse and then retail outlet can easily add another 1000km which is another 90g even in an efficient container lorry. If you make a special trip to the shop to buy the supermarket, a 3 mile round trip would be another 750g of CO2 but if you buy a 6 kg bag this will do you for 5 barbecues. All told then we have at least 545g worth of CO2 equivalents per kg of charcoal or 625 g per barbecue session, as shown in the table below.

GHG emissions (in CO2 equivalents) from typical barbecue charcoal,  imported from Kenya


g CO2e per kg charcoalg CO2e per 1.2 kg barbecue session
Charcoal production200-400 *240-480
Ship Kenya to UK (11000 km)150180
Lorry here there and everywhere (500-1000 km)45-9055-110
5km (3m) by car to buy 6kg charcoal from supplier, typical new car 2009 [5].125150
Total545-790625-920
* This figure is based on the 100 year GWP for methane -  the 20 year figure would be 10 times higher because methane impact is intense but relatively short term.

If you were burning natural gas instead, 625g worth would be 3.4 kWh or an hour of my gas grill. That is probably twice as long as it would take to grill a piece of chicken and maybe 4 times as long as to cook a steak if you like it rare. This means your barbecue, even with FSC produced charcoal, probably generates at least twice the emissions that you would have generated with fossil fuels under a gas grill.  For large parties, you won't need so much charcoal per person so the emissions overall could be just about the same as using the gas grill.

If you want a barbecue with less carbon emissions, use UK produced charcoal, as local as you can find, and don't make a special car trip to buy it: combine with some other shopping, use public transport or cycle. UK producers are helping to manage our woodland sustainably too, using traditional techniques such as coppicing. However, even in the UK many charcoal producers use the closed kiln method for charcoal production. The websites I have found all show production using kilns.  Bioregional (who supply charcoal to B&Q) have a network of local producers and they say 'We are aware of the methane emissions from traditional kilns and have worked over the years with our producers to encourage the use of retorts whenever possible, as well as investing time and money into research for even cleaner technologies.' If your charcoal was made in a kiln you still have at least 240g CO2e or so emissions per barbecue, not counting any for transport. That is equivalent to 25 minutes of my gas grill.

Which brings us to the carbon emissions in the meat. That will be another 540g for chicken or 1800g for beef per 100g portion [6] - but you would have eaten that anyway.

[1] FAO Simple technologies for charcoal making chapter 10 Using charcoal efficiently
[2] Forestry Commission, 2010, UK Wood Production and Trade
[3] Guardian, 2008, Tread lightly: Buy British barbecue charcoal
[4] Bailis, R., 2009, Modeling climate change mitigation from alternative methods of charcoal production in Kenya, Biomass and Energy (33) 1491-1502
[5] Emission factors for a container ship from  DEFRA via my book
[6] from Manchester Business School, 2006, Environmental Impacts of Food Production and Consumption, final report to DEFRA via  my book

3 comments:

  1. For a Charcoal barbecue it is considered green only if it is less consuming and more output is given

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  2. Hi there. Could you please share with me how you calculated the ghg emissions. Thanks.
    Arthur

    ReplyDelete
    Replies
    1. The emissions from making the barbecue come from reference 4. The transport emissions come from my book which got them from DEFRA's GHG emissions factors. They are now here http://www.ukconversionfactorscarbonsmart.co.uk/

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