Emissions from in-use residential wood pellet boilers and potential emissions savings using thermal storage

Effective biomass energy utilization for residential space heating relies on having combustion systems with both high efficiency and low emissions. European technology in wood pellet boilers was introduced into the United States over the past decade. However, these systems need to be designed and operated to minimize the emissions of air pollutants, particularly airborne particulate matter. One approach to minimize emissions is to improve system efficiency and limit the number of boiler cycles that include start-up and shut down periods where emission rates are higher by utilizing thermal storage as part of the boiler system. In this study, emissions from two 25 kW European-designed, but U.S. manufactured wood pellet boilers (PB and WPB) with thermal energy storage (TES) were measured in actual home operation using the EPA CTM-039 stack sampling method. These measurements allowed the estimation of the emissions reductions due to the presence of TES. PB had much higher emissions than WPB because PB had frequent local oxygen deficit-induced non-uniform combustion, which highlights the significance of periodic onsite oxygen tuning after the boiler installation. Particulate emissions were dominated by PM_2.5 and the particles mainly consisted of low melting point, alkali compounds such as K₂SO₄, KCl, Na₂SO₄, CaCl₂, etc. Both PM_2.5 and polycyclic aromatic hydrocarbons (PAHs) emissions increased linearly with CO because they are products of incomplete combustion. Optimum boiler operating conditions were found with 12% flue gas oxygen content for both systems to achieve minimum CO emissions, which is 2% higher than the manufacturer's set-point of 10%. The potential emissions reductions by using a system with TES instead of a non-TES system were estimated under three scenarios. The results showed both significant gaseous and particulate matter emissions reductions that demonstrate that modern, high-efficiency wood pellet boilers with TES systems can produce heat with lower total emissions compared to non-TES systems.