The Vermont Gasification Project

The Vermont Gasification Project, located at the McNeil Power Station in Burlington, Vermont served as a development platform for the field verification of the SilvaGas process. The process underwent initial development at Battelle’s Columbus Laboratories as a part of the U. S. Department of Energy’s Biomass Power Program. The technology is specifically designed to take advantage of the unique properties of biomass, such as high reactivity, low ash, low sulfur, and high volatile matter. Unlike other gasification processes, the process indirectly heats the biomass using heated sand to produce a medium Btu gas (having a heating value approximately half that of natural gas) that has a wide range of potential uses.

Project Background

Initial developments at Battelle began in 1980 and continued through 2000. These efforts included the integration of the 10 ton per day process research unit gasifier with a gas turbine power generation system, the first such demonstration in the world. In parallel, gas cleanup technologies for conditioning the product gas for gas turbine applications were developed and demonstrated.

The development efforts at Battelle demonstrated the flexibility of the process through operations with a wide variety of biomass feedstocks. These included not only woody biomass, but energy plantation crops, grasses, biomass residue materials, biomass sludges, and prepared municipal solid waste. This extensive data base, developed after over 20,000 hours of testing, provided the basis for the construction of a commercial scale demonstration facility. In 1994, the project entered into a cooperative agreement with the U. S. Department of Energy to construct such a facility.

Project Overview

The commercial scale demonstration plant based on the SilvaGas process was constructed in Burlington, Vermont. Construction of the facility was completed in 1997. The commercial scale demonstration of the SilvaGas process at the McNeil Generating station was designed for 200 tons per day of biomass feed (dry basis) but was operated in excess of 300 tons per day (dry basis). Burlington Electric Department’s (BED) McNeil station was selected as the site for this demonstration plant. BED has a long history with biomass based power generation. The McNeil station, at 50 MW, is one of the world’s largest wood fired power stations. The McNeil station uses conventional biomass combustion technology, a stoker grate, conventional steam power cycle, and particulate removal using ESP’s. The gas produced from the gasifier was used as a co-fired fuel in the existing McNeil power boilers.

The program was conducted in three phases, (1) design, (2) construction and initial operation, and (3) parametric testing and demonstration of gas cleanup.

Project Partners

The partners in the development of the process at the McNeil site were FERCO, Burlington Electric, Battelle, the US DOE, and the National Renewable Energy Laboratory (NREL). Battelle engineers invented the process and conducted the initial developments under contract to the US DOE. The US DOE and NREL provide valuable technical support to the program along with program management.

The development program at McNeil had as its primary objective the demonstration, at commercial scale, of the SilvaGas gasification process and associated support technologies. These technical goals paralleled the Department of Energy’s goals for bioenergy and the progress made by the Vermont Gasification project establishes the technology as a key element for the future utilization of biomass as an energy source. Gasification of biomass in the SilvaGas process provides flexibility for the production of a complete slate of products including gaseous fuels, heat, power, liquid fuels, and chemicals in a virtual “biomass refinery”.

The scale of the Vermont plant was selected so that commercial scale process equipment could be utilized to eliminate so called “pilot plant compromises” in the design. The design of the plant included all key process systems, with the exception of heat recovery.

Program Results

The first operation of the Vermont gasifier in full steam gasification occurred in August of 1999. During the startup period, numerous design and operational changes to the plant were necessary to improve the performance of process auxiliary systems. No problems with the core process were encountered throughout the startup period, but a number of improvements to process auxiliary systems were made. These included materials handling, solids separation, and product gas scrubbing.

Immediately following the initial around-the-clock test period, a series of “parametric” tests were undertaken to evaluate key process parameters. In addition, these tests allowed for the performance of the system to be monitored over extended around-the-clock operating periods. The Vermont gasifier is a demonstration facility and, as such, is only operated for specific experimental campaigns, rather than in a “production mode”.

Throughout the testing campaigns, the plant operation was smooth and reliable. The plant has been demonstrated to operate stably and reproducibly over a range of process conditions. Process availabilities of over 90% were demonstrated during the testing campaign and contribute to the commercial viability of the SilvaGas process.

Biomass feedstocks, including wood are heterogeneous materials. Inherent moisture contents can range from near 0% to over 50% depending on environmental, harvesting and storage conditions. Gas composition of the gas produced in the process was measured and found to be identical (on a dry basis) with that measured in the PRU. Product gas heating value remains stable regardless of changes in feedstock type, ash content, or moisture content. This characteristic of the process had previously been demonstrated during pilot operation at Battelle and has been verified through the current testing program in Vermont. The validation of this characteristic of the product gas is quite important in the overall commercialization of the process, because it provides the ability of the SilvaGas process to be adapted to both power and synthesis applications. Changes in wood moisture are well tolerated by the process. During the Vermont gasifier testing program, as feedstock moisture levels were changed from one level to another, the gasifier and combustor reactors responded within minutes to the changes.

The other factor entering into the use of a broad spectrum of biomass materials is the biomass particle size and / or shape. The SilvaGas process has demonstrated broad feedstock flexibility which provides the opportunity to utilize residue-type biomass along with other biomass materials to improve overall process economics.

In a further development of the SilvaGas process, a novel hot-gas conditioning catalyst (DN34) was developed that converts condensable products (tars) to lower molecular weight, and therefore, essentially noncondensable forms. This catalyst then allows for a much reduced loading on any scrubbing operation due to the elimination of the higher molecular weight materials. A DN34 based catalyst system was installed in the Vermont gasifier to further evaluate the DN34 catalyst performance at commercial scale. Test data generated during operation of this system showed that up to 90% of the “tar” produced in the gasifier is destroyed in the catalyst at normal operating temperatures. This result is consistent with prior tests conducted in the pilot plant.

Significant accomplishments for the program include:
• Product gas has been generated by the process and supplied to the McNeil station demonstrating the use of the gas as a co-firing fuel, and generation of power from
biomass gasification.
• Demonstration that the indirect gasification process achieves consistent results regardless of biomass type or process scale of operation.
• Process stability was demonstrated by around-the-clock operational periods utilizing wood feedstocks with moisture contents ranging from 10 to 50+%.
• Gas composition and production rates were identical with those projected based on pilot plant testing.
• The product gas heating value has been shown to be constant at 450 to 500 Btu/scf. Stability and reproducibility of the gas composition was demonstrated regardless of changes in feedstock moisture.
• Gasifier throughput was demonstrated to be over 150% of design corresponding to over to 500 tons per day (350 dry tons per day equivalent) in the 200 dry ton per day “design” facility. The ability to process such a significantly larger amount of biomass further improves process economics and provides process flexibility for commercial applications.
• Gasifier operation was stable enough to maintain the McNeil station at minimum load (about 7 MW) during an interruption to McNeil’s fuel supply.
• The system was able to be restarted within 30 to 45 minutes after plant shutdowns, demonstrating the ability of the process to provide for operational flexibility.
• Rapid response to feed rate changes demonstrated during operation
• Smooth operation of process observed with little operator attention required
• Gas cleanup results paralleling those measured in the process research unit were achieved.

Scaleup of technologies from PRU to commercial scale is generally considered to be the most risky step in a commercialization effort. The successful demonstration of the SilvaGas process in Vermont validated the technical feasibility of the technology on a commercial scale. The flexibility of the medium Btu gas produced in the SilvaGas process allows its use for:

• Direct use as a fuel gas that can be interchanged with natural gas or distillate oil
• Co-fired with biomass or fossil fuels for heating or power applications,
• Use as a fuel for advanced power generation cycles including turbines or fuel cells, and
• Use as a feed gas for synthesis applications such as production of Fisher Tropsch liquids, alcohols, and hydrogen.

Project Status

In 2002, after the completion of the DOE program and testing, the Vermont Gasifier Project was decommissioned.