Fuel Blending: Maximizing Benefits for Biogas Operations

Cost-conscious energy asset managers should look to a natural gas blend to maximize market opportunities afforded by biogas.
By Eva Garmendia | August 29, 2017

As an organic process, biogas production is inherently variable, dependent not only on feedstock, but also on ambient conditions such as temperature. For energy asset owners who want to deliver dependable heat and power in commercial or industrial energy supply applications, this inherent variability potentially represents a significant challenge. One solution for spark ignition reciprocating engines is fuel blending, which effectively supplements and precisely matches the variations in biogas availability with additional natural gas. Maintaining a constant output from the engine, irrespective of variations in biogas availability, ensures consistent energy supply even if biogas production falls to zero.

Ensuring constant energy availability from the gas engine can boost biogas production economics. For instance, consistent temperatures are required to optimize biogas production in commercial biomass digester applications. Falling temperatures within the digester require additional boilers to be ignited. In these instances, supplementing digester gas with natural gas to maintain the engine at full power may save money, and enable more flexible operation without having to fire additional boilers to heat the digester and ramp their output up or down.

Similarly, supplementing biogas with natural gas to keep the engine at full power and avoid sourcing electrical supplies from the grid may be an equally valid economic approach.

Maximizing Bioenergy Use, Operating Flexibly
The key benefit of a fuel-blending system is that it allows operators to maximize their bioenergy usage while maintaining consistent power output. Alternatively, asset owners may require maximum use of all the available biogas to produce an associated amount of energy. A fuel-blending system provides the flexibility to either match and follow the required load profile, or use the available biogas fuel to maximize the amount of energy they can produce. Key to this adaptability is the engine management system.

Using an on-engine solution that is integrated into the engine controls and automates the fuel-blending process, Siemens has the ability to dynamically blend natural gas and biogas as a standard, factory-designed and supplied option for gas engines. This design avoids the use of an off-engine system, and the associated complexity and space that this type of system would require. This approach uses a twin gas-feed system on the engine, one for each fuel, which reduces the need for mechanical and control integration and saves space. Blending is then completed using a standard carburetor and intake, a common fuel/air delivery method for large, spark-ignition gas engines.
The controls are incorporated into the gen-set and/or cogeneration management system, which allows the engine to react and adjust characteristics, such as ignition timing, in response to the changing fuels.

The engine management system’s sophisticated mapping technology continuously extrapolates, constantly changing biogas and natural gas parameters. It does this to meet set performance criteria, and, moment-by-moment, automatically adjusts the amount of each fuel introduced into the combustion chamber to deliver the correct yield rate, as determined by the user. Proprietary valve technology ensures fuel supplies are maintained from zero to 100 percent of either fuel.  In addition, although the supplied fuels must be within certain broad parameters and standards, the system is able to cope with some natural variation in the quality of biogas. Different kinds of biomass entering the digester might result in biogas of different qualities. By dialing in preset outputs, the fuel blending system is able to accommodate some variation in the energy value in the fuel.

From an end-user perspective, the system will automatically call for a blend of the available fuels to achieve certain preset goals, depending on the parameters programmed into the system, seamlessly delivering the energy required without user input. From a control perspective, the system offers the end user limitless control options. The mix ratio can be controlled directly by using the customer’s own algorithm and a programmable logic controller, or by Siemens’ control logic to provide either power-optimized or fuel-optimized operation, or another desired mode.

One unique characteristic of this solution is the ability to seamlessly switch from 100 percent of one fuel to 100 percent of another or any combination in between, while maintaining 100 percent power output.

Fuel Flexibility: Business Agility
The fuel-blending system was developed for Siemens SGE-SL (SFGLD) and SGE-HM (HGM) engines with outputs from 250 to 1067 kWe and 520 to 1350 kWe, respectively, running at 1,200 rpm, 1,500 rpm and 1,800 rpm.

These engine platforms are fuel-flexible and robust, as it was originally developed for use with more challenging fuels. It is manufactured by Siemens’ engine business at the Zumaia Operations Center in Spain. The system is capable of starting the engine on either fuel, and can then rapidly transition to any combination of fuel mix, from zero to 100 percent of either. However, the system is able to achieve this while maintaining the as-designed emission levels and engine performance, in terms of efficiency and energy output. For example, NOx levels are consistently low across all operational modes, while power delivery can be maintained at maximum levels. Service intervals are also unchanged from a standard biogas-fueled unit.

Maintaining the predictability and the consistency of the engine performance, while being able to blend two fuels, increases user flexibility in engine operation and optimization. This allows the engine to operate independently of any restriction that may apply to the primary fuel. Fuel blending serves as a complementary system, and offers end users a means to manage their operations more effectively, and react to changing conditions at the site, irrespective of fuel production and the economic environment. Ultimately, fuel blending allows users to capitalize on the use of biogas and maximize its use and value in today’s competitive and environmentally sensitive energy market.

Author: Eva Garmendia
Product Line Manager, Siemens Engine Business