Powering an Engine Generator with a Biomass Gasifier
The Energy & Environmental Research Center is developing distributed biomass gasification systems for a variety of applications, the most common of which is electricity production. The EERC has investigated system designs that employ internal combustion engines, high-temperature fuel cells and microturbines.
This article summarizes our experience with internal combustion engines. Future articles will discuss our experience with high-temperature fuel cells and microturbines.
The system designs originated during World War II, when the shortage of petroleum fuel in Europe sparked a wave of innovation on ways to use locally available biomass, primarily chunk wood, to fuel internal combustion engines for transportation. Those designs are still being employed to power internal combustion gensets.
Biomass is gasified in a downdraft gasifier, which is then wet-scrubbed to condense tars and reduce syngas temperature for the internal combustion engine. A series of filters is used to reduce particulates before the engine generator. A downdraft gasifier is often employed because it produces the lowest amount of tars of all the gasifier types.
This system is simple and has worked to produce power from syngas for more than 50 years. However, even after a half-century of development, key techno-economic barriers have yet to be resolved that would make it commercially viable.
The first is environmental. While downdraft gasifiers produce the lowest amount of tars of all the gasifier types, they still produce tars. These have to be scrubbed out or they eventually plug the piping. The problem is that scrubbing the tars does not get rid of them. It produces a tarry effluent, much like oily water, that must be disposed of. Oftentimes, this can be a deal breaker for a small commercial entity hoping to convert its waste wood into power.
Another issue is that the syngas has high nitrogen dilution when air is used for the gasification process. This reduces the energy content to approximately 110 Btu per standard cubic feet (scf). Most modern engine generators are rated for 300–1,000 Btu per scf. Trying to operate the engine generator on 110 Btu per scf derates the engine by more than 75 percent. This means a 100-kilowatt (kW) engine generator will produce only approximately 25 kW when run on syngas alone. It also significantly reduces the reliability of the system.
Any drop in syngas quality shuts down the generator, requiring manual restart. This requires an operator to constantly watch the system, which significantly increases the operating costs of a distributed-power system. One method to overcome this latter barrier is to cofire the engine generator with petroleum fuel. When this is done, both engine reliability and power increase. However, the economics must now account for the cost of petroleum fuel, and the overall system is not as “green” as it would have been with just biomass alone.
While these barriers are significant, the EERC is working with Cummins Power to develop an engine generator designed to operate on low-Btu syngas. If this project is successful, one of the largest barriers for this type of system in 50 years will be overcome.
Author: Phil Hutton
Research Manager, Energy & Environmental Research Center