| dc.description.abstract | Background
The sustainability of first generation bioenergy has been researched intensively due to problems resulting from land use change. There is a growing interest to use agricultural residues for bioenergy production as they can be a sustainable feedstock available at low cost since residues are regarded to be a by-product.
Many studies on the potential to use residues for energy purposes have been conducted but results vary widely. The differences arise from a poor understanding of the factors affecting residue potentials. Especially the amount of the produced residues that can be removed without negatively affecting soil productivity and the demand for residues by competing uses is usually estimated
rather than calculated.
South Africa has a heavy CO2 footprint due resulting from its coal dependence; coal produces 88% of the country’s electricity. Using agricultural residues for bioenergy production could be an option to move away from this dependence and
reduce its CO2 footprint. Besides, converting biomass into transport fuels can help improve the energy security since the country is now fully dependent on oil imports.
Goal and Scope
This master thesis consists of both a literature review and case study for South Africa. The focus in both parts is on understanding how the supply and cost of residues is affected by the factors and how this varies under different conditions.
Only for the case study actual potentials and cost are calculated. This thesis assess 1) the theoretical potential: the total amount of residues produced. 2) the sustainable potential: the amount of residues that can be removed from the land
without decreasing soil productivity 3) the technical potential: the sustainable potential minus the demand for residues by competing uses and 4) the supply cost at farm gate. This is done for maize stover, wheat straw, sugar cane tops and trash and sugar cane bagasse.
Methodology
The calculation of the amount of residues produced (theoretical potential) is straight forward and is done based on the crop yield, residue-to-product ratio,area under cultivation and moisture content HHVdry. The calculation of the sustainable potential is less straight forward. The
removal of residues is considered sustainable as long as the soil productivity is not reduced; two criteria are chosen to ensure this. 1) a residue cover of 2 tonne/ha must be present to control erosion; leaving more residues has only a marginal effect. 2) Enough residues (accounting for both above and below ground residues) must remain in the field to maintain a 2,0% SOC in the top 20 cm of
the soil. This amount was dependent on local conditions and was modelled using the Rothamsted Organic Carbon Model. Summarizing, a minimum of 2 tonne residues/ha must be left in the field and depending on the local conditions an
additional amount of residues may be required to maintain 2,0% SOC.
Regarding the technical potential, the demand for residues by competing uses was calculated based on the size of the livestock population and estimates of the percentage of the livestock using residues and the duration of the winter period
when the livestock can not graze on the pastures.
The supply cost at farm gate are calculated based on the direct cost and the indirect cost. The former are the extra cost the farmer faces for harvesting the residues and the latter the compensation for removed nutrients. The harvest cost of conventional harvest where crop and residues are harvested separately are compared with innovative harvest methods which harvest both the crop and residues in a single pass | |
