Smokeless fuel

Smoke rising in Lochcarron, Scotland, is stopped by an overlying layer of warmer air (2006).

Coal was widely used for domestic cooking and heating during the Victorian period and up to the early 1950s in most towns in Britain. However, poor coal fuels and badly designed fireplaces created a good deal of local pollution from smoke and tars produced from partial combustion of the coal and disappeared up the chimney. As a result, the smoke and noxious gases were often trapped locally when a weather inversion occurred. Such trapped gases and smoke caused fogs and worse, smogs which discoloured clothing and was a serious health hazard. Such pea soupers were generally more prevalent in the larger towns such as Manchester, Birmingham and London. There were also toxic gases such as sulphur dioxide and nitrogen oxides present in the same noxious mixture, both of which irritated the lungs of those exposed to the smog. The former gas produced by sulphur impurities in the coal, was especially iniquitous since it oxidises further in the air to produce sulphuric acid, a highly corrosive and strong acid.

London

Nelson's Column during the Great Smog of 1952

In 1952 The Great Smog of London killed directly a great number of people varying between 4 and 12 thousand people by exacerbating existing lung diseases such as bronchitis and breathing disorders like asthma. It also killed indirectly by causing many rail, road and pedestrian accidents created by the impenetrable gloom of the smog.[2] Clothes were stained by the soot and generally discoloured by the sulphurous gases. Because of this tragedy the 1956 Clean Air Act was passed to attempt to solve this problem.[3] One of the results of this act was the development of smokeless fuels, which were designed specifically to aid the environment by reducing the amount of smoke produced, as well as removing some impurities such as sulphur in the coal. Such manufactured fuels also burnt at a higher temperature and so made for a better and more efficient fuel for open fires as well as stoves.

Ogatan, charcoal briquettes made from sawdust

Hexamine fuel tablets

Smokeless fuels can have several benefits over other solid fuels such as bituminous coal or anthracite. They are usually more expensive than coal, and often consist of briquettes of standard size (ca 3 inches long and wide by 1 inch thick) aiding transport, handling and fire-making. Coal however, may often be supplied in a large variety of sizes from fines to large lumps. Slack such as stone and shale or even coal dust can choke a fire and usually produces a great deal of smoke. The variable size of the coal results in inconsistent burning, the smaller particles burning faster than the larger. The advantages of a standard sized briquette of smokeless coal include regular burning for longer (less refueling needed) and burning at a higher temperature, so making an open fire more efficient. They can also improve air quality by reducing smoke formation[4] They are generally of more consistent quality since are mass made by compression moulding of powdered coal and a binder such as starch. They can be used in many different appliances such as enclosed stoves as well as open fires.[5]

Smokeless coal is more efficient than a conventional open coal fire because the high working temperature is radiated into the room as infra-red radiation, as can be judged by the bright red colouration from a mature fire. The hot gases produced are still lost up the chimney, so reducing efficiency just as in an open coal fire. The gases mainly consist of carbon dioxide and a little water vapour, which is lost to the environment while carbon dioxide is essential for plant life in photosynthesis. With no or little smoke, chimneys remain cleaner for longer and require brushing less frequently since fewer volatiles or smoke are produced, and so cannot be condensed in the cooler chimney. The main combustion reaction is

C(s) + O2(g) → CO2(g)

In a restricted supply of air or oxygen, then toxic carbon monoxide can be formed:

2C(s) + O2(g) → 2CO(g)

Charcoal either in unprocessed sticks or as shaped briquettes is widely used for barbecues owing to its relatively low production of smoke and the intense heat generated needed for fast cooking of meat and fish. What little smoke is made can give the food a special flavour. Charcoal and raw wood are also commonly used for manufacture of various smoked products such as smoked salmon. Charcoal is widely used in African countries for domestic cooking purposes.

Hexamine fuel tablets are entirely for the convenience of hikers and campers, and the fuel consist of white tablets made from trioxane, so it is a completely synthetic fuel and made from formaldehyde.

Smokeless fuels generally have a high calorific value, with that of anthracite being greater than dry wood for example, and many smokeless briquettes are made from this type of coal. Thus anthracite has a calorific value of 32.50 MJ/kg compared with that of dry wood of about 21 MJ/kg. Lignite or brown coal is even worse with a heat of combustion of only 15.00 MJ/kg owing to the presence of non-combustible impurities. Bituminous coal has a value lower than anthracite, but neither lignite nor bituminous coal are smokeless owing to volatile content.

Smokeless fuels also have some downsides, typically they can be harder to ignite owing to the lack of volatiles present in coal for example. There are fewer flames, due to reduced volatiles but a generally higher ash content. Fines and dust can be produced by mechanical movement but the amount is minimal compared with that produced by bituminous coal. It is sometimes supplied wet, even in sealed plastic bags, but is easily dried since the moisture is superficial. There is an increased cost to the user or consumer owing to the costs of mass manufacture although the loss of volatile chemicals such as coal tar can offset those costs to the manufacturer. Those extra costs make the fuel more expensive than coal, but the effect is small at about a 30% premium over coal at 2020 prices. Since solid fuels are bulky and heavy, they need manual transport to the fireplace and storage in a convenient spot near to the house.

• Briquette
• Clean Air Act Amendments of 1990
• Clean Air Act 1956