Development of a Feedstock-to-Product Chain Model for Densified Biomass Pellets
D. McPherrin, A. Pollard and D. Strong
Department of Mechanical and Materials Engineering
Queen’s University at Kingston
Introduction
The densified biomass market has been rapidly growing, despite several issues surroundingcommercially available white pellets. Torrefied, densified biomass black pelletsareconsidered as a potential improvement due to their increased energy density and hydrophobicity. Torrefaction is a process where biomass is heated in an oxygen free environment to approximately 2600C-3000C, which causes the more volatile, lower energy components of the biomass to evaporate, leaving an energy dense and hydrophobic product. These improvements are valuable as they reduce shipping and handling costs. Critically important is the reduction in the capital expenditure required for the conversion of coal fired power plants to co-fire biomassbecause of the increased similarity between coal and black pellets. However, the commercial production of black pellets is similar to that used for white pellets, while only marginally improving bulk density and material handling and storage issues. The Q’Pellet is a spherical pellet under development that should allow for higher bulk density and reduced production of fines, allowing for lower cost shipping and handling.
Methods
A flexible spreadsheet model has been created to permit the comparison of white pellets, black pellets, and the Q’Pellet. The spreadsheet enables production scenarios to be created from the acquisition of raw biomass through to delivery to the end user the “feedstock-to-product chain”. Cash flow analysis, combined with sensitivity analysis, can then be used to determine the economic viability of the created production scenario. Internal rate of return (IRR) was used to compare the economic performance of the models. IRR is the discount rate that will set the present value of a cash flow to zero, with a 10% IRR being a typical minimum value for investment. Greenhouse gas (GHG) emissions for the chosen scenario are also modelled to allow for comparison.
GHG emissions are lower for the Q’Pellet for the two main sources of GHG emissions for typical white pellet production – natural gas used to dry the biomass and fuel used totransport the pellets to end user. By utilizing off-gasses from the torrefaction process as fuel for process heat, GHG emissions from natural gas can avoided. The bulk density of the Q’Pellet is higher than both white or torrefied pellets, and so more energy can be transported per shipload, thereby reducing fuel usage.
Results
A case study of a plant built in Williams Lake, B.C. was used to compare the three types of pellet. The results of the economic model are shown in Table 1:
Table 1: Case study IRR for the three processes
Q'Pellet / Torrefied Pellet / White Pellet15.8% / 11.8% / 14.3%
The results of the GHG model are shown in Figure 1:
Figure 1: Case study GHG results
Conclusion
Economic and GHG modelling of a case study were performed to allow for comparison between Q’Pellets, torrefied pellets and white pellets. The Q’Pellets had the highest modelled IRR, 15.8%, compared to the white pellets at 14.3%. The Q’Pellets also had the lowest GHG emissions, 6.9 kgCO2eq/GJ, compared to 20.6 kgCO2eq/GJ for white pellets.