Value Added Products. Tulane University New Orleans. August 2003 Summit Conference. Charting the Future Direction of Bioenergy Technologies

Transcription

1 Value Added Products Tulane University New Orleans August 2003 Summit Conference Charting the Future Direction of Bioenergy Technologies AUTOMATING SOLIDS HANDLING & THERMAL PROCESSES James E. Welp Black & Veatch Corporation Peter Brady Alpine Technology. Inc.

2 ABSTRACT Solids handling and thermal processes have the best performance with consistent and steady feed systems. Operators know that an inconsistent feed reduces capacity, increases operating costs, reduces quality of product or increases emissions, and requires additional operator attention. In order to achieve a consistent and steady state system, the solids handling process should be automated. Two obstacles must be overcome before automation becomes accepted. The first is the realization that solids processing is a mass flow not a volume flow process. Since most of the plant is rated in terms of gallons per minute, it is difficult to switch to tons per hour for a solids handling process. The second is the awareness that there may finally be good reliable instrumentation to make automation possible. Over the past twenty years, many attempts have been made to automate thickening and dewatering systems. Most of these systems failed because the instrumentation was not reliable enough to provide the operator the confidence that it was worth the effort to maintain the instrument for automatic control. By automating the solids handling process, the operator would be able to get the right solids for the application. In some drying applications, if the feed sludge is too dry, it is difficult to control dust in the unit. A 23 percent solids cake may be more desirable than a 28 percent cake. The same may be true for thermal oxidizers where the best operation is near the autogenous combustion point. If the autogenous combustion is at 25 percent solids, higher cake solids make it harder to control and may increase emissions. The automation system needs to provide the right mass flow and solids concentration for the application. This paper will focus on mass flow control to provide consistent feed to thickening, dewatering, and thermal processes and how these systems may be automated. Case studies of current applications will be presented. The approach used in this paper may be used at other treatment plants in automating their solids handling ant thermal processes.

3 OBJECTIVES Lowest operating cost Maximum throughput Consistent solids Least headaches Liquid Processes Primary sludge flow and concentration o Blanket levels WAS flow and concentration o Blanket levels Oil and grease flow and concentration (?) Feed arrangement, mixing, and storage Thickening flow and concentration o ph, SVI Polymer flow and concentration Speed, differential speed, torque, dam adjustment Thickened solids concentration, centrate quality Digestion flow and concentration o VS percent, VS reduction Temperature, gas production, mixing Dewatering flow and concentration o ratio of primary to biological solids, ph, VS percent Polymer flow and concentration Speed, differential speed, torque, dam adjustment Dewatered solids concentration, centrate quality Thermal Processes Mass flow o Mass concentration o VS percent, calorific value (?) Temperature, pressure, differential pressure, auxiliary fuel use, oxygen concentration, cooling air flow, water spray(s) flow o Product quality Emissions Costs Polymer

4 Electricity Auxiliary fuel, chemical Hauling, tipping fee Product revenue Maintenance STEADY STATE What is steady-state? It may depend on the processes indicated in the following table. Steady State Conditions for Various Processes Process Steady State Condition Land Application / Landfill Constant mass flow Thickening Constant mass flow, constant total solids, and constant SVI Digestion Constant mass flow, constant total solids, and constant biodegradable volatile solids Dewatering Constant mass flow, constant total solids, and constant primary to biological solids ratio Dryer Constant mass flow and constant total solids Thermal Oxidation Constant mass flow, constant total solids, and constant calorific value Since the operator has little control over the SVI, biodegradable volatile solids content, or the calorific value, the emphasis of this paper will focus on developing a system to provide a constant mass flow rate at a constant total solids percent.

5 Centrifuge Control Measured Variables Controlled Variables Desired Output Solids Feed concentration Centrate rate Centrate concentration Cake flow Cake concentration Centrate quality Maximize throughput Polymer Feed concentration Centrifuge Bowl speed Scroll speed Differential speed Torque Dam position (residence time) Vibration Amperage draw Bowl speed Scroll speed Differential speed Torque Dam position * Minimize polymer usage Minimize electric usage Minimize maintenance

6 Thermal Processing Control Measured Variables Controlled Variables Desired Output Dryer Recycle rate Temperatures Thermal oxidizerion Temperature and pressure fluidizing air, windbox, bed, freeboard, and HX, APC, and stack Differential pressures - bed, HX, and APC HX bypass damper position Fluidizing air flow rate Auxiliary fuel feed rate Scrubber water flow rate APC damper position Oxygen HX and stack Emissions stack Emergency cooling air/water spray HX bypass damper position Fluidizing air flow rate Auxiliary fuel feed rate Scrubber water flow rate APC damper position Maximize throughput Maximize quality Minimize operating costs including electric usage and auxiliary fuel consumption Maximize throughput Minimize emissions Minimize operating costs including electric usage and auxiliary fuel consumption Minimize maintenance

7 CASE STUDIES FURNACE REACTOR The Problem: With fluctuating cake feed characteristics to the furnace, the design heat loading can be exceeded. These fluctuations can include variations in cake percent dry solids, calorific value, percent inerts, and cake throughput loading. When the design heat loading is exceeded, the temperature will rise very quickly in the burning zone, especially with a fluid bed furnace. Safety control logic will then cause quench water to be pumped into the reactor. This results in an increase in emission gases, increasing system pressure, and burdening the fans, and downstream subsystems. The solution: Monitor and control the feed to the upstream dewatering equipment to provide optimum solids feed to the furnace. Monitor the operating parameters of the furnace, and automatically take the required corrective action. The Benefits: A proactive control system will work to avoid process conditions outside the design range. This will optimize system performance, and maximize the life of the equipment. The monitoring control system will take rapid corrective action to deal with any problems. AIR PREHEATER (RECUPERATOR) BYPASS The Problem: Due to fluctuations in sludge characteristics, and in upstream equipment performance, the cake being processed in the furnace can exceed its design heat loading. This can subject the preheater tubes to temperature excursions of 100 degrees F., or more. At the operating temperatures of a furnace, this temperature increase will reduce the strength of the metal by about 25% (see Fig.3). In addition, temperature fluctuations will also shorten the time to metal fatigue failure (see Fig. 3) The Solution: An air-side bypass duct with damper is incorporated with the heat exchanger. This bypass is designed to allow a portion of the air inlet flow to bypass the majority of the heat exchanger surface allowing tighter control over the recuperators performance. This bypass allows for manual temperature control of the air preheat under varying furnace loads.

8 In the operating condition where tubes are subjected to temperature, or pressure, outside the design range, a portion of the air inlet flow can be diverted around the heat exchanger so that air outlet temperatures can be reduced In another situation, should the heat exchanger 'overperform', say in a new clean condition, a portion of the air inlet flow can be also diverted around the heat exchanger so that air outlet temperatures can be reduced. This bypass is intended for trim control only, within about 10% of the design duty, and should not be expected to regulate temperature infinitely. Advantages of Bypass Control: Relative the heat exchanger operating temperatures, the ability to regulate the outlet air directly lowers the metal temperatures in the hot end of the unit. For the fluid bed, without the air side bypass, the only direct control of preheat is the water sprays to dilute the flue gas. If air preheat can be controlled directly, water spray use could be eliminated, or greatly reduced. When downstream cleaning equipment is considered, the reduction of flue gas mass flow is a positive, as well as reduction of system pressure drops. Figure 3 by monitoring, and optimizing process parameters. The air preheater bypass can be automatically controlled by an automation control system such as the Solids Expert. This results in improved performance over the lifetime of heat exchanger, monitor operating conditions to be proactive to problems, rather than reactive and maintain warranty conditions, Figure courtesy of ALSTOM Power Energy Recovery