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Charcoal & the Environment - Pros & Cons

Papers by Rudiger Meicherczyk and Walther Hennig, reproduced from "GATE TOPICS" 1192

Critics of appropriate technology occasionally point out to its supporters that the slogan "Small is Beautiful" does not always apply. Charcoal production is one example; with the simple earth and pit kiln technology, the emissions of toxic substances are substantial. Should one therefore advocate large charcoal production plants? Does it, in any case, make more sense to burn fuelwood for domestic cooking and heating? Even within GTZ, questions linked to charcoal production cause some controversy. We print two authors views on the subject.

 

Advantages of Small-Scale Charcoal Production

by Rudiger Meicherczyk, GTZ

More than two thousand million people throughout the world depend on wood for cooking and heating. In the early 1980s the biomass consumed accounted for 47% of all the wood cut down in the world - a total of 1.3 billion cubic metres. Since the industrialized countries' share of this total is a mere 12%, it is clear that the Third World, which consumes the remaining 88%, is going to depend on wood as a fuel for a long time to come. And by doing so it is playing a major part in the destruction of forests in tropical and subtropical regions.

Most charcoal is produced from waste resulting from timber felling and forest clearance for other forms of utilization. The calorific value of charcoal is approximately double that of wood. Also, with its low bulk per calorific unit it has distinct advantages as regards transport and storage, and can be used for a wider range of applications. This compensates for the conversion losses in charcoal production.

In 1979, world charcoal production was 16million metric tons, produced from about 140 - 160 million metric tons of fuelwood, most of which originated from the sources mentioned above. Some of the charcoal used for metallurgical processes is produced from wood grown on plantations.

Table 1: Charcoal Production Methods

 

Yield%

Carbonization duration

Investment Costs

Capital intensiveness

Labour

Earth pit kilns

10-15

1-5 weeks

low

low

high

Brick & steel kilns

15-30

1-12 days

1,013 US$ -6,750 US$

medium high

medium

Large scale plants/ retorts

30-32

20-30 hours continuously

6,750 US$-6,756,750 US$

high

low

 

There are three basic methods of producing charcoal: earth and pit kilns, brick and steel kilns and large-scale plants or retorts. These three methods differ essentially as regards the investment costs involved, the duration of carbonization, yield and labour -intensiveness (see Table 1).

The availability of raw materials and the distance to consumers are crucial in deciding where and how charcoal should be produced. The available production methods enable both small and large quantities to be produced by their carbonization process and can be operated on a centralized or decentralized basis.

Traditionally, charcoal is usually produced where the raw material is available and where a demand for it exists not too far away. In the past, environmental aspects were ignored (and for economic and/or political reasons were only considered in relation to large plants).

Charcoal production generates toxic substances in solid, liquid and gaseous form - tars, phenols, carbon dioxide and nitrous oxides. The amounts vary depending on the type of wood and the production method, but the pollutants and residues can be intercepted and used as raw materials for other products.

In particular this applies to large plants: as the amount of charcoal produced increases, so does the quantity of pollutants, so that collecting and processing these substances can be economically viable. However, the pollutant emission levels of large plants are so high that although some of these by-products are utilized, the majority are not. As a result the pollution they cause is more serious than with small plants.

Small plants not a "clean" alternative

But small plants do not represent a "clean" alternative either, because they produce the same pollutants. Although the quantities per plant are smaller and are distributed over a larger area, the total quantity of pollutants is more than that produced by large plants. We are forced to consider how production in small plants can be made more environmentally-friendly.

With earth and pit kilns this is practically impossible. Operators of such plants should therefore be made aware of improved technologies such as brick or steel kilns; and the design of these processes should be such that the toxic substances they produce can be intercepted and the amount of unavoidable pollution minimized. It has been shown with experimental plants that this can be done. However, the investment costs are higher.

A further advantage of small-scale plants is that their output can be adjusted more readily to seasonal fluctuations in charcoal demand for cooking and heating. They can also be located close to consumer centres. Since they are highly labour-intensive and are usually situated in economically neglected regions, their importance in providing employment should not be underestimated. In addition, small plants can be operated as individual kilns or in batteries.

For large plants, on the other hand, a number of preconditions have to be met. Due to the high capital investment involved they have to operate continuously and at the highest possible capacity. This presupposes an availability of sufficient raw material and steady demand. Moreover, they require only a small - but highly qualified workforce. Local and regional markets seldom permit continuous operation and large-scale plants are usually built for a specific purpose (ea. the iron and steel industry) or to supply international markets.

Large charcoal production facilities offer considerable scope for environmentally compatible operation and reducing pollution. However, in the Third World, there is little environmental legislation or monitoring and toxic emissions can attain levels which jeopardize health. For these reasons preference should be given to small plants despite the environmental problems associated with them. Large plants should only be built when they have no negative impact on the labour market and when the availability of raw material, demand and quality criteria make production in small kilns unrealistic. However, an environmental impact assessment should be performed and its results verified during construction and operation of the plant.

Earth and pit kilns should be rejected, for environmental reasons and because of their low yield. Brick and steel kilns should be further improved - reducing toxic emissions and increasing the yield - and their operators should be trained. Charcoal production is still a major cause of pollution. Intensive research and prompt application of research results in practice are needed to reduce il.

 

Charcoal Production - An Industrial Task

by Walther Hennig of GTZ

Unless integrated into a sustained forest management plan (to prevent deforestation), charcoal production is a problem in many respects. As regards energy efficiency, the widespread use of charcoal instead of firewood in households, ea. for heating or cooking, makes little sense.

The use of charcoal can only really be justified where a higher energy density and its specific physical and chemical properties are required, for example in metalworking trades and the metallurgical and chemical industries.

This article does not advocate charcoal production. However, if charcoal is produced, then it should be under efficient industrial conditions, which are monitored and thus also controllable, rather than by the more simple (and widespread) kiln method.

The process

Wood is carbonized by a process known as "dry distillation", ie. thermal decomposition. The raw material is wood, either air-seasoned or dry. A typical analysis of the raw material is presented in Table 2.

The carbonization process can be divided roughly into two ranges. In the range between ambient temperature and somewhat above 200°C, the reaction is endothermic (absorbes heat). The principal effect, at about 150-177ºC, is expulsion of mechanically bound water from the wood. In the range above 200ºC, which can extend to about 800ºC depending on the desired charcoal quality (high temperature = high carbon content), the carbonization process is exothermic (gives out heat) ( 270 kcal/kg). In the exothermic range large quantities of gas and crude wood vinegar are expelled.

Arguments against carbonization in kilns

Taking energy-related, ecological and economic factors into account, charcoal production in kilns, as opposed to modem industrial facilities, involves major disadvantages and hazards.

Energy efficiency:

When producing charcoal in a kiln, the energy required in the endothermic, low-temperature carbonization range is generated by burning wood. The wood is wasted. Also, the energy produced in the exothermic range of the process is simply given off unused - again a waste.

In an industrial plant the thermal energy contained in the hot gas is recovered and used to preheat the wood.

In a kiln, for instance, between 15% and 20% of the air-seasoned wood is burned as kiln fuel and about 8% of the energy consumed is exhausted with the wood gas. The respective energy balances are shown in Table 3.

In practice, the percentage of the energy obtained from charcoal which is actually used is even lower. Kilns are not usually situated at central locations. Industrial and domestic users need (or prefer) charcoal in lumps. In the course of production, handling and transport charcoal dust is produced, which is discarded as "waste".

At a steelworks in Brazil it was established that only 50% to 70% of the lumpy charcoal delivered to the works could actually be used directly. (Not only are the high charcoal dust dumps at Brazilian foundries a typical sight, the dust emanating from them is also a permanent nuisance.)

In industrial charcoal plants, the fines can be agglomerated and then used to make a variety of products, ea. barbecue briquettes.

Environmental considerations:

With kiln technology, wood gas is given off during the carbonization process. Apart from the loss of energy, this exhaust gas also constitutes an environmental hazard. In addition to the principal gases, (carbon dioxide and carbon monoxide), wood gas contains approximately 13% methane (CH4).

Table 2: Typical analysis of wood as raw material

Wood

air seasoned

15-20% moisture content

dry

2-8% moisture content

specific weight

0.47 - 0.78 kg/dm³

av. chemical composition(% by weight)

C = 50.6%, H2 = 6.2%, O2 = 42%

Charcoal kilns also produce the substance known as crude wood vinegar (or condensate because in industrial plants it condenses during cooling of the exhaust gases). As Table 4 shows, the quantities of wood gas and crude wood vinegar are substantial.

The low yield of charcoal (and therefore of energy) from kilns as compared to industrial plants also means a waste of natural resources.

Wood vinegar has been shown to contain about 10,000 ingredients (ea. phenols, cresols, benzol, toluol, anthracene, pyridine), many of which are toxic and/or carcinogenic. They should not be allowed to escape freely into the environment.

Table 3: Energy balances of kiln and industrial charcoal production plant

Input

Kiln

Industrial

Wood (air-seasoned) (av 3,500 kcal(kg))

 

100%

100%

Yield (weight basis)

20-25%

35%

Yield (energy basis)

4045%

0%

Charcoal = 82% carbon, 4% hydrogen, 14% oxygen

Note:

Yield (wt basis) = wt charcoal/wt wood × 100%

Yield (energy basis) = energy in charcoal/energy in wood x 100%

Economy:

In addition to the higher efficiency of industrial plants with regard to the charcoal yield, the raw wood vinegar fraction obtained is an important economic factor. Apart from the acetic acid, many other valuable substances are obtained by further processing. They are used in pharmacies, industry or the home. The value of these substances obtained from the crude wood vinegar is approximately the same as that of the charcoal itself.

When charcoal is produced in kilns neither the wood gas nor the crude wood vinegar produced are recovered and utilized. Thus, both energy and valuable substances contained in the wood vinegar are wasted - a major economic disadvantage. The method also appears highly undesirable due to the toxic emissions involved.

Table 4: Material losses to the environment in kiln carbonization

Wood Gas

Basis: 1000 kg wood (dry)

 

= 190 kg (=125 Nm³)

 

CO2 49%, CO 34%, CH4 13%

 

C2H4 2%

 

Calorific value: 2200 kcal/Nm³

Crude wood vinegar

including

= 460 kg

tar (sol)

7-8%

acetic acid

11 %

wood spirit

3 %

dissolved tar

7 %

water

(remainder)