Converting biomass to energy — A South African perspective

Martina Meincken explains the importance of biomass in providing energy in South Africa.

With fossil fuels becoming ever scarcer and more expensive, renewable energy options are gaining interest worldwide. In developing countries, such as South Africa where the electricity grid is not yet completely developed, it is more economically viable to expand the grid with renewable energies than with fossil fuels. There is currently a strong drive to develop the solar, wind and bioenergy sector in South Africa. The move from first-generation biofuels (e.g. maize and sugarcane), which were accompanied by problems of food security, to second-generation biofuels, such as lingocellulosic (woody) biomass, is currently being attempted all over the world, although it poses its own problems.

The term lingocellulosis refers to plant biomass that is composed of cellulose, hemicellulose and lignin. It is usually grouped into four categories – agricultural residues (including maize and sugarcane leaves and stalks), dedicated energy crops, wood residues (including sawmill and paper mill discards) and municipal paper waste.

In South Africa, for example, wood is a valuable commodity. There is not enough produced to be used for industrial purposes and for energy conversion. Residues are used largely at the production site, e.g. to fire boilers to dry the wood and are therefore not available for further processing.

Alternative fuel resources, such as agricultural residues, will have to be found if biomass is to be used for energy production. One possibility is to use the lignin residues of the pulp and paper industry, but at the moment this requires (expensive) pre-treatment.

Something that has to be remembered is that in South Africa  – in common with other developing countries – a large proportion of the population are already using biomass to generate energy, although not in the most efficient ways. Often, this sector of the population is strongly dependent on this energy source, because electricity is not available everywhere. This use of ‘bioenergy’ is frequently accompanied by its own set of problems. Open fires in the house used for cooking and heating often lead to (toxic) smoke inhalation. Volatile organic compounds along with particulate matter (soot) are the cause of many long-term illnesses, informally known as ‘hut lung’. This is a general term that refers to respiratory diseases, such as tuberculosis, respiratory infections and cancer. If the fire is not tended properly, it often leads to the loss of property and even life.

The challenge associated with this level of bioenergy use is to decrease the use of wood obtained illegally from plantations or protected trees and to provide communities with safe and affordable alternative biofuels. More efficient conversion techniques also need to be used. Various initiatives are trying to reduce the need for wood in rural communities by producing briquettes or pellets from wood waste generated by industry and foliage collected from around the houses. Many of these products have a low production cost and can provide an additional income for households. This article discusses biomass availability and possible conversion techniques with respect to all bioenergy users. On a small scale, this means that the cost factor of any conversion plays a significant role, while on a large scale the actual availability might be more important.

Conversion techniques

Biomass can be converted into different forms of energy using different conversion techniques.

Direct combustion

The simplest conversion technique is direct combustion, which produces heat. This is used widely in rural areas to heat living areas and cook meals. Unfortunately, this is also the most inefficient conversion technique and a great deal of energy is lost. Combustion can be made safer and more efficient by using more sophisticated reactors, such as closed combustion systems with well-designed air and flue gas ducts.

Gasification

Gasification is the combustion of biomass in an atmosphere starved of oxygen. It results in the production of heat and a mixture of combustible gases that can be compressed and stored for later use, for example, to cook meals. Certain reactor types can also power turbines, with the excess heat produced in the combustion chamber being used to produce electricity.

In Europe — mainly Scandinavia — these plants, which are known as combined heat and power plants, supply entire cities with heat (through underground pipes) and electricity. In South Africa, gasification is currently used by some industries to generate the heat and electricity they need. For example, sawmills use their wood residues to dry wood and run their machinery. Electricity produced in this way is not yet fed into the national grid.

Pyrolysis

The third major conversion technique is pyrolysis, which is the combustion of biomass in the absence of oxygen. Pyrolysis produces solid (often called biochar), liquid (bio-oil) and gaseous components.

A diagram showing the process of pyrolisis

Depending on the reactor configuration and control parameters, the ratio between these components can be altered. The bio-oil can be converted into bio-diesel to power engines or vehicles and the combustible gas can be compressed and stored, as for gasification. The simplest form of pyrolysis is commonly used in rural areas to make charcoal in earth mounds. Like open fires in the case of combustion, this is a rather inefficient conversion method that can be vastly improved by using, for example, steel reactors, that allow the reaction temperature and residence time to be controlled.

Availability of biomass

The largest wood product sectors in developing countries remain fuelwood and charcoal, which are used to provide energy for heating and cooking. Almost 90% of the wood harvested in Africa is used for fuel.

The demand for wood has a huge effect on forest systems. Deforestation now represents one of the most pressing environmental problems faced by almost all sub- Saharan African nations. Many sub-Saharan countries have had over three-quarters of their forest cover depleted and it is estimated that if current trends continue, many areas will experience a severe wood shortage by 2025.

The illegal logging of plantation forests and protected tree species for firewood or coal production poses an additional problem that further diminishes forest systems. It has therefore become vital to expand the application of energy conversion to a wider scope of biomass, such as harvesting and processing residues, agricultural residues or invasive plants. South Africa has a considerable number of invasive trees that were mostly imported from Australia (Acacia species) to act as wind breakers or to stabilise sandy ground. It was soon established, however, that these plants deplete the scarce water resources, spread too quickly and pose a threat to indigenous (slow-growing) flora. Most invasive tree species must now be cleared by law in order to prevent bush encroachment and water depletion. Table 1 shows the estimated biomass available from invasive trees in the Western Cape of South Africa. This biomass would potentially be available for energy conversion. It is particularly important to make these products and techniques available at an affordable price to the rural communities that depend on biomass for energy conversion.

Table 1: Estimated green biomass (t) of woody material of different diameter classes and foliage for three species of invasive Australian wattles in the Western Cape, South Africa

Component West Coast

Plains Agulhas Plains Eastern Cape

Plains Total Wood > 50 mm 169 450 764 869 5 757 356 6 691 675 Wood 25–50 mm 189 769 698 019 2 388 852 3 276 640 Branches < 25 mm 268 691 889 842 1 786 231 1 916 697 Foliage 191 856 499 546 1 225 295 1 916 697 Total biomass 819 766 2 852 276 11 157 734 14 829 776

Biomass preparation

Few gasification or pyrolysis reactors can process heterogeneously- sized fuel. Added to this, few reactors will reach their maximum conversion efficiency. The biomass therefore needs to be prepared, which in most cases requires work including intensive chipping, milling and subsequent compression to uniformly sized pellets or briquettes.

This is associated with equipment and labour costs and is not always feasible. An additional problem is that not all biomass can be pulverised and compressed equally well to give stable fuel pellets. Agricultural residues in particular can prove to be difficult because they often lack lignin, the natural binder present in wood, which stabilises pellets. Finally, the biomass should be as dry as possible before it is used for energy production. The conversion of wet biomass is very inefficient because much of the built-in energy is wasted on water evaporation. Optimum moisture content values are between 10 – 20% and therefore most biomass needs to be dried before it can be processed.

Biomass properties

The energy content is typically thought of as the most important property characterising a biofuel. It depends largely on the density of the biomass as well as the carbon (C), oxygen (O) and hydrogen (H) ratio. The higher the C content, the higher the energy content and the more O and H the biomass contains. More volatile components will be formed when biomass with a higher C content is combusted, which therefore lowers the energy content. The energy content of dry woody biomass is in the range of 18 – 20 MI/kg, with little difference between species.  Several other factors are equally important in deciding whether biomass is suitable for a certain form of energy conversion. These include:

• moisture content: The biomass should have a fairly low moisture content after harvesting. Ideally, air drying should be enough to reach the desired 10 – 20% moisture level in order to minimise processing costs
• size: The biomass should be homogenous in size (e.g. wood chips or residues, such as grape skins) or at least be easy to pulverise. Several reactor types (e.g. fluidised bed reactors) can be fed directly with chips or small particles, while others require pellets, briquettes or larger pieces of wood. The reactor type should be chosen according to the biomass that will be used and the pulverisation equipment that is available
• biomass composition: Apart from C, O and H, biomass contains nitrogen (N) and sulphur (S). Both can react with O to form toxic components in the flue gas, commonly termed nitrogen oxide (NO_x) and suplhur oxide (SO_x) These components have a severe negative impact on human health and the environment, e.g. in the form of acidic rain. For this reason, the N and S content should be as low as possible;
• ash content and composition: Ash is the inorganic part of the biomass that cannot be combusted and its content should be as low as possible. Wood typically has an ash content of < 0.5%, while many agricultural residues can reach 20 % and more.

The biomass/ash composition has a large impact on its possible further uses, e.g. as fertiliser. It also affects the conversion reactor. Metal oxides (silicon dioxide [SiO₂], aluminium oxide [Al₂O₃], iron (III) oxide [Fe₂O₃], etc.) lead to slagging (production of fused refuse from the metals), while alkaline metals (potassium chloride [KO], iron (III) chloride [FeCl₂], sodium chloride [NaCl], etc.) lead to corrosion of the reactor. The biomass/ash should therefore have a low Si, Fe, K, Cl, etc., content.

Conclusions

The conversion of biomass to energy can be done in various ways and on different scales, ranging from the processes in rural households to large industrial plants. In order to make it economically feasible, the following key questions have to be answered:

• What type of biomass should be used and how much of it is available?
• Which conversion technique will be used and what form of biomass can it handle?
• Does the biomass need any pre-processing?
• What environmental effect does conversion of the chosen biomass have?

In South Africa, woody biomass will have to be planted especially for energy conversion, or other residues will have to be used, due to a significant roundwood shortage. Alternative biomass sources, ranging from invasive plants to agricultural residues, appear to be a promising solution.

The physical and chemical properties — especially of the agricultural residues — have to be determined first, however, to make sure that they are suitable with regards to emissions and processability. This is part of ongoing research in our group and so far many suitable resources for energy conversion have been identified.

Martina Meincken  is a Senior Lecturer, Wood Physics and Bioenergy, Department of Forest and Wood Science, University of Stellenbosch.