Analysis of feedstock requirement for the expansion of a biomass-fed district heating system considering daily variations in heat demand and biomass quality

Abstract

Evaluating the feedstock requirement of biomass-fed district heating systems is important when planning their installation or expansion. Public acceptance, which favors or deters the utilization of biomass, could be impacted by feedstock supply logistics. Feedstock requirement of district heating plants depends on the heat demand from the consumer and biomass quality characteristics, which vary over time. Previous studies on utilizing biomass for district heating mostly assumed average feedstock quality characteristics and aggregated heat demand values without considering their daily variations. While using average quality characteristics provides the average feedstock requirement of the plant, the actual requirement may vary significantly from the average value when daily variations in quality characteristics are considered. Not including these variations could lead to infeasibilities in meeting the heat demand of the plant. This study assesses the feedstock requirement of expanding a district heating plant considering daily variations in biomass quality characteristics and heat demand. A Microsoft Excel-based simulation model with one-year planning horizon and daily time steps is developed for the assessment. The costs and CO2-eq emissions of using different feedstock types are evaluated with and without trucking limitations. The developed model is applied to a real biomass-fed district heating plant at the University of British Columbia, Canada. The capacity of the heating plant is planned to be expanded from 6 MW to 18 MW. Three feedstock types, namely, wood residues, pellets and briquettes, and a mix of them are assessed. The results suggest that when enough trucking capacity is available, the plant would receive 2–9 truckloads of wood residues or 1–3 truckloads of pellets/briquettes. The expansion could reduce the total CO2 emissions by about 11,000 tonnes compared to those emitted by the 6 MW (current) biomass-fed heating capacity and natural gas (for a total of 18 MW) when sufficient biomass is available to the plant. However, when trucking is limited, using only wood residues could cause biomass shortage for almost half of the year and result in more emissions due to natural gas consumption. Utilizing energy dense feedstock, such as pellets and briquettes, could mitigate the effects of limited trucking, though, the total fuel cost could increase by 76% and 58%, respectively, compared to using only wood residues. Using a mix of wood residues and pellets/briquettes could achieve a reasonable trade-off between emission reduction and unit fuel cost.