By M.J. de Wet Pr. Eng. NRGen Advisors, Stellenbosch
In agri-industrial heating applications, replacing coal with wood chips as a viable sustainable biofuel is a real solution.
Climate change activists and environment-conscious food consumers are putting huge pressure on agri-industrial food and dairy processors, and producers using coal for animal husbandry heating purposes, to switch from fossil fuels (coal, gas, HFO) to renewable biofuel. It’s now proven that wood chips produced from unwanted biomass (waste wood, invasive alien trees, old orchards) can successfully replace fossil fuels.
Resource surveys carried out in the Western Cape by Africa Biomass Company [ABC] and NRGen Advisors (as audited by the CSIR), confirm that sufficient biomass chips are available to replace coal used by agri-industries in that province. The surveys confirm that approximately 150 000 t.p.a of unwanted invasive alien trees in the riparian zones of the Breede, Zonderend and Berg rivers exist for harvesting for the next 20 years and beyond. By harvesting these trees, water runoff will increase, the risk of bush fires is reduced and the original fynbos and palmiet systems will return.
Previous attempts to replace coal with wood failed
Several past attempts failed to lessen the negative effects of coal. The most common consequence of these attempts are that the moisture in the wood boiled (vaporised/condensed) and mixed with the sulphur in the burning/smouldering coal to form caustic acids which corroded the metallic components of coal-fired heaters and the chain grates of boilers.
Pellets made from cleaned (debarked, and often washed) SA-pine worked well as biofuel because of its dryness and easier materials handling characteristics. White or soft wood pellets are however too expensive to compete with lignite coal but can successfully compete with natural and methane rich gas.
The manufacturing of hardwood pellets is experiencing technical (mainly wear) challenges. The hard woods of SA, especially from the more arid areas, are hard and abrasive. Add to this the fact that pellet making requires the milling down of the wood to a dry (< 10% moisture) meal before compressing it into a pellet, making the final product too expensive to compete with Heavy Furnace Oil [HFO] and coal.
The breakthrough with wood chips for industrial heating applications
Africa Biomass Company [ABC], Worcester, and Calore Sustainable Energy [CSE] of Paarden Eiland, Cape Town, have pooled their technologies to establish a wood-chip-fired heating solution, using dry wood chips. CSE have the technology at hand to stop-and-start and modulate their wood-fired burners as the heat load demands. The CSE burners can automatically control the infeed of wood chips onto a unique igniting system and can modulate the flame of the burners to narrowly follow the temperature setpoint of, for example, a broiler house to be heated.
ABC in turn can supply ‘dimensional’ chips to the required specifications for the optimal operation of the CSE burners including:
- Moisture content @ 15 ± 1%
- Particle size distribution to fit the infeed apparatus
- Net calorific value @ 15.5 ± 0.5 GJ/t
The sulphur content of industrial fuels
One of the major disadvantages of coal and heavy furnace oils are their high sulphur content. Sulphur dioxide (SO₂) is emitted when sulphur-containing fuels are burnt in the presence of oxygen. See Table 1.
Thermal energy cost comparison between wood chips and lignite coal
Table 2 compares the thermal energy cost of biomass chips supplied by Africa Biomass Company with lignite coal in the Western Cape and the North West Provinces:
From Table 2, wood chip energy is marginally more expensive than its coal-fired equivalent in North West, but more than 26% cheaper in the Western Cape. However, when burner and heat exchanger efficiencies are considered, the wood-fired application is more cost effective. Refer to Table 3. When accounting for the environmental costs related to SO₂ emissions and the high volumes of ash to be disposed to certified landfill sites, or certified end-users, wood chips are a logical choice.
Zooming in on broiler house heating
Physical trials by ABC & CSE on broiler house heating conclusively proves that the efficiency of the modern wood-fired systems outperformed the traditional coal-fired heaters by far, as illustrated in Table 3.
Conclusions
Biomass chips is a viable alternative to coal, especially in the poultry industry for decentralised heating of broiler houses, as well as for central hot water boilers for hatcheries and broiler houses with central heating systems. It competes with coal economically and outperforms coal in all environmental aspects.
Biomass can give the broiler farmers of the Western Cape a competitive edge and will contribute millions of cubic meters of additional runoff into the Breede, Zonderend and Berg river systems due to the harvesting of invasive alien trees from the riparian zones of these rivers.
The environmental benefits of using a renewable biofuel are so overwhelming, that coal will most likely be phased out over the next couple of years in the poultry industry.
