Barrier Issues to the Utilization of Biomass

Abstract

The Energy & Environmental Research Center (EERC) has completed a project to examine fundamental issues that could limit the use of biomass in small industrial steam/power systems in order to increase the future use of this valuable domestic resource. Specifically, the EERC attempted to elucidate the ash-related problems-grate clinkering and heat exchange surface fouling-associated with cofiring coal and biomass in grate-fired systems. Utilization of biomass in stoker boilers designed for coal can be a cause of concern for boiler operators. Boilers that were designed for low-volatile fuels with lower reactivities can experience problematic fouling when switched to higher-volatile and more reactive coal-biomass blends. Higher heat release rates at the grate can cause increased clinkering or slagging at the grate due to higher temperatures. Combustion and loss of volatile matter can start much earlier for biomass fuels compared to design fuel, vaporizing alkali and chlorides which then condense on rear walls and heat exchange tube banks in the convective pass of the stoker, causing noticeable increases in fouling. In addition, stoker-fired boilers that switch to biomass blends may encounter new chemical species such as potassium sulfates, various chlorides, and phosphates. These species in combination with different flue gas temperatures, because of changes in fuel heating value, can adversely affect ash deposition behavior. The goal of this project was to identify the primary ash mechanisms related to grate clinkering and heat exchange surface fouling associated with cofiring coal and biomass-specifically wood and agricultural residuals-in grate-fired systems, leading to future mitigation of these problems. The specific technical objectives of the project were: • Modification of an existing pilot-scale combustion system to simulate a grate-fired system. • Verification testing of the simulator. • Laboratory-scale testing and fuel characterization to determine ash formation and potential fouling mechanisms and to optimize activities in the modified pilot-scale system. • Pilot-scale testing in the grate-fired system. The resulting data were used to elucidate ashrelated problems during coal-biomass cofiring and offer a range of potential solutions. This multitask project began in October 2000 and was completed with the preparation of this final technical report. This report documents project activities and summarizes results on a task basis. The focus of the discussion concerns Task 2-Modification of the Pilot-Scale Combustion System; Task 4-Laboratory-Scale Testing and Fuel Characterization; and Task 5-Pilot-Scale Testing and Reporting. Task 2 involved modification of an existing pilot-scale pulverized coal-fired combustion system to permit its use in a grate-fired configuration without preventing its continued use in a pulverized fuel-firing configuration. Shakedown testing of the pilot-scale grate-fired simulator was completed to ensure the modified system (fuel preparation and feed, combustor, and flue gas analyze...