Process Enhancements to Improve the Operation of an Existing Heat Drying Facility

The existing heat drying facility is over 20 years old and has process limitations. The product silo and truck loading areas utilize older dust collectors, which result in higher energy consumption and increased emissions. The existing control system consists of a Programmable Logic Controller (PLC) and relay that are functionally limited. The performance of both venturi scrubbers is limited by the amount of feed water available, which results in particulate carryover and fouling of the regenerator thermal oxidizer (RTO). The current facility also utilizes a nonrecirculating exhaust gas configuration that limits production, wastes heat energy, and increases operation and maintenance (O&M) costs. The city of Largo is currently implementing key process enhancements to the venturi scrubbers, dust collectors and the biosolids pelletizing configuration to improve performance and energy recovery. The new supervisory control and data acquisition (SCADA) system will be provided to control the existing facility using a logically-structured operator interface that is configured to operate the facility from the main control panel or any other workstation. The three (3) existing dust collectors on the product silos and truck loading area will be replaced with a manifolded ductwork system and a common dust collector. The existing venturi scrubbers will be replaced in-kind, the water supply lines will be upgraded to provide proper water flow, and the tangential velocity to the venturi scrubbers will be increased to reduce particulate carryover. The nonrecirculating exhaust gas pelletizing configuration will be upgraded to recirculate exhaust gas after the existing venturi scrubber and immediately ahead of the RTO. The process modifications will substantially benefit the facility. These improvements are expected to reduce emissions, decrease electrical and natural gas consumption, and increase heat energy recovery. Based on the current annual production of 2,800 tons, these modifications are expected to save the city of Largo approximately $110,000 per year.