BioCycle July 2006, Vol. 47, No. 7, p. 52
Project at a Washington state food processing plant generates steam, primarily for heating wastewater, that saves thousands of dollars in fuel annually.
Christopher J. Snider and Pat Worthington
A FIRST-OF-ITS-KIND biogas utilization system built for a Washington state food processing plant could help create momentum toward similar smaller-scale green energy projects. Implemented for Penford Food Ingredients of Richland, Washington, the system uses biogas produced at its waste-water pretreatment facility as fuel for a dual-fuel boiler. The boiler produces plant steam, primarily for wastewater heating, resulting in thousands of dollars in fuel cost savings annually.
It is common for large municipal or regional wastewater treatment plants to utilize the biogas produced by these systems. The anaerobic processes in these large systems typically produce biogas flow rates ranging from several hundred to several thousand standard cubic feet per minute (scfm). The gas may be used to fire a boiler for process heating or as fuel for internal combustion engines or gas turbine generators that power combined heat and power applications.
The large gas flows in these systems can easily support the costs of biogas recovery because the large initial capital investment is quickly offset through savings in utility costs. However, it has historically been more challenging to create an economic justification to utilize waste gases produced by smaller public and private wastewater treatment systems in the 100 scfm flow range and smaller. Prior to the current period of rising energy prices, these gases have typically just been waste flared.
But those days may be over. Between 2000 and 2005, gas prices have increased approximately 76 percent and 91 percent in the commercial and industrial sectors respectively. These steep increases – combined with an interest in utilizing “green” energy sources – have caused an upsurge in smaller biogas utilization projects.
Smaller projects pose special challenges, though, including the need to minimize initial capital and operations and maintenance (O&M) costs. Customized process controls are among the new cost competitive technologies available because they can greatly reduce initial capital/O&M costs. These “smart process control” strategies are capturing owner’s interests because they can yield excellent returns for small industrial wastewater anaerobic digester biogas systems.
WORKABLE SOLUTION FOR PENFORD FOODS
Penford Food Ingredients is an international food processor focused in the starch business. The Richland plant processes raw starch feedstock into a refined starch product widely used in the food industry. As part of its continual process improvement, Penford Food Ingredients commissioned an energy conservation study which considered all energy uses in the entire plant and identified several energy savings measures. The plant selected biogas recovery coupled with new steam boilers. Energy savings have been realized since the project was completed in late 2005 – these savings drop straight to the bottom line. Burns & McDonnell Engineering Company, based in Kansas City, conducted the study and provided turn-key design and construction services for the project.
The system recovers and uses biogas produced by the plant’s industrial wastewater pretreatment system as fuel in a dual-fuel steam boiler. What makes this utilization system unique is its small size and ability to combust the biogas at a variable flow rate (modulated burner firing based upon fuel availability).
The variable biogas fuel flow rate presented special challenges because no gas storage volume existed in the plant’s anaerobic digester. This innovative yet simple system design eliminated the need for an expensive storage and compression system for the biogas. This new biogas combustion system design is an exciting development in biogas utilization systems because gas storage has been eliminated from the cost equation – this opens the door for small and large energy recovery projects across public and private industry currently waste flaring small and large quantities of a waste gas.
HOW THE SYSTEM WORKS
The project involved the installation of two new 150 psi boilers – a 2,700-MBH Bryan water tube boiler, and a second smaller Bryan 1,200-MBH water tube boiler. The larger dual fuel boiler operates as the lead boiler with digester gas as its first choice fuel, while the smaller boiler serves as a make-up boiler. Both boilers are equipped with state-of-the-practice remote controlled burners manufactured by Germany-based Weishaupt Corporation. The burners are also equipped with oxygen trim which optimizes efficiency.
The custom programmable logic controller (PLC) for the burner and system control is the heart of the package. The Seimens PLC is programmed with a custom logic that compensates for variable digester gas flow and variable steam header pressure. The biogas flow varies as a function of the wastewater influent flow and composition into the digester. The need for digester gas storage is eliminated by remotely modulating the firing rate of the burner thereby combusting the digester gas as it’s produced. The smaller make up boiler is designed to simply maintain steam header pressure. Controlling the firing rate and utilizing the biogas in this manner is quite unique according to the burner manufacturer and system supplier, Weishaupt and Proctor Sales, Inc.
Automatic fuel switching from biogas to natural gas in the same burner is remotely controlled by the custom PLC as heating demand increases. The fuel is switched from digester gas to natural gas if either the adjustable steam header set point drops below a given level for a period of time or the digester gas pressure drops below a given level while the burner is at low fire. This design allows for the maximum amount of digester gas to be utilized while maintaining the plant’s steam demand. Similar control logic can be applied in many applications where a waste fuel is available and currently flared.
The wastewater flow rate is approximately 140 gallons per minute. The wastewater enters a preacidification (PA) tank for pH, nutrient adjustments, and steam heating. The steam produced by the biogas-fueled boilers is used to heat the influent wastewater in the PA tank. The PA tank heating was formerly supplied by an old inefficient and O&M intensive 500 hp boiler which was determined during the energy study to be oversized for the application. The biogas is produced in the Biopaq® upflow anaerobic sludge blanket bioreactor (UASB), and passes through a knockout tank. The gas pressure to the burner fuel train is produced by a backpressure regulating valve located at the flare.
CONSTRUCTION AND COMMISSIONING
Construction was completed in approximately nine months by a local mechanical contractor. The project was essentially a standard boiler installation job with a relatively sophisticated yet simple control system. Commissioning was completed over a period of several weeks which allowed for complete system prove-out and efficiency adjustments. The project design engineer was on-site for much of the commissioning which allowed for a tremendous amount of information sharing between the system designer, supplier, and owner.
The intensive dialogue and information exchange during the commissioning process proved to be incredibly valuable and resulted in a highly robust system. “This project involved a true commissioning – so many of the projects anymore aren’t really commissioned.” noted Dave Parker, senior controls and PLC programmer at Proctor Sales, Inc.
Since start-up, the system has performed as designed. Penford Food Ingredients has adjusted some of the set points to enhance operational performance. The total system cost was approximately $300,000 and the simple payback is approximately three years. The cumulative savings are a testament to the value of energy usage studies and waste gas recovery projects.
The Penford project greatly reduced the required level of initial capital investment, due to the elimination of biogas storage and compression systems. That aspect, combined with the level of controls flexibility, has gone a long way toward demonstrating the economic viability of small- to mid-size “green” energy projects.
Chris Snider, PE, RG and Pat Worthington, PE are project managers at Burns & McDonnell Engineering Company, Inc., headquartered in Kansas City, Missouri. Visit www.burnsmcd.com for more information.
July 25, 2006 | General
Energy Savings And Economic Viability Of Smaller Projects
BioCycle July 2006, Vol. 47, No. 7, p. 52