THE PRINCIPLES OF GASIFICATION HAVE BEEN
WELL-KNOWN FOR MORE THAN 200 YEARS,
BUT BIOMASS GASIFICATION HAS NOT SEEN
COMMERCIAL SUCCESS UNTIL RECENTLY.
Existing Technology, Scaled for Future
The biomass gasification system serving Dockside Green was
developed and built by Vancouver-based Nexterra Systems Corp.
The plant itself is owned by Dockside Green Energy LLP, a ‘micro
energy utility’created by Terasen Energy Services, Vancity Capital,
Windmill Developments and Corix Utilities, with additional financial
support from BC Hydro, the provincial government and the city
of Victoria.
The system takes urban wood fuel and converts it to low-emission synthetic gas, or ‘syngas.’The syngas is directed through
an oxidizer and then a boiler, which in turn provides hot water
that is piped to Dockside’s various buildings and used for space
heating and domestic hot water (90 degrees F). The solution provides a cleaner, quieter alternative to traditional combustion with
lower emissions, greater fuel flexibility and higher turndown.
Gasification differs from conventional combustion because it
uses just 20 percent to 30 percent of the oxygen needed for complete fuel combustion. The process consists of heating wood in an
oxygen-starved environment until volatile gases (carbon dioxide
and hydrogen) are released from the wood. The gases are mixed
with air in a secondary combustion chamber, the oxidizer, where
they are burned to complete the combustion process. Hot flue
gas leaving the oxidizer can then be directed into energy recovery equipment or fired directly into boilers to produce hot water,
steam and/or electricity.
The gasifier at Dockside is based on a fixed-bed, updraft
design. Biomass fuel, sized to 3 inches in diameter, is bottom-fed
into the center of a cylindrical, refractory-lined gasifier.
Combustion air, steam and/or oxygen are introduced into the
base of the fuel pile. Partial oxidation, pyrolysis and gasification
occur at 1,500-1,800 F, and the fuel is converted into syngas and
noncombustible ash.
The fuel-bed temperature is tightly controlled to prevent the
bed from exceeding the fuel’s ash melting point. This prevents
the ash from forming ‘clinker,’ which is a challenge for conven-
Courtesy Nexterra Systems Corp. Photo Bob Matheson.
The Dockside Green biomass plant is housed in an architecturally designed
building that blends in with the neighborhood.
Dockside’s biomass gasification plant is undoubtedly an anchor
for the development’s sustainable pedigree. Another unique
component of the community that sets it apart is its on-site
sewage/wastewater treatment plant. One hundred percent of
the treated effluent and reclaimed water at Dockside is used
in toilets, rooftops gardens and irrigation systems for the
entire site. The minimal amount of sludge left over from the
sewage treatment process is compacted and can be used as
fuel in the biomass gasification plant.
Leaving Nothing to Waste
Nexterra recently began testing biosolids (wastewater residuals) supplied by Metro Vancouver as a new fuel source at its
Product Development Centre in Kamloops, B.C. This new fuel
has the potential to provide a renewable energy source for
drying, heat or power generation at wastewater treatment
facilities.
According to the U.S. EPA, there are more than 16,000 wastewater treatment facilities in the U.S., with many owned and
operated by municipalities. Traditional methods for the disposal of biosolids involve trucking to landfills or using as land
spread. However many municipalities would like to discontinue these practices due to rising fuel and disposal costs, greenhouse gas emissions and concerns about landfill capacity.
tional combustion systems. Instead, the ash remains granular, free-flowing and is discharged intermittently through the gasifier base
into a single ash bin. The mineral-rich ash generated at the
Dockside facility is actually collected by the fuel supplier and
recycled as compost.
The principles of gasification have been well-known for
more than 200 years, but biomass gasification has not seen commercial success until recently. Scaling the technology to handle
larger-capacity loads is an area that has shown great promise.
While Dockside’s system generates 7 MMBtu/hr of net usable
heat, a 72 MMBtu/hr system using the same gasification technology
is operating on University of South Carolina’s (USC) Columbia
campus.
The USC biomass gasification cogeneration facility converts
wood residue supplied by local sawmills into clean renewable
energy and significantly reduces campus greenhouse gas emissions.
At peak capacity the plant generates 60,000 lb/hr of steam to heat
the campus, as well as 1. 38 MW of electricity sold to the grid. On-site
power generation is also planned for Dockside Green in the future,
using syngas produced at the biomass gasification plant and conveying it into a high-efficiency internal combustion engine (see
sidebar).
Gasification also offers flexibility both in terms of the fuel
used – Dockside’s system can handle anywhere between 10 percent
and 55 percent moisture content – and the reduced need for
complex after-treatment systems, due to the technology’s inherent cleanliness.
When operating at peak capacity, the Dockside plant requires
delivery of approximately one truckload of wood fuel every two
days. (The system also has a natural gas boiler to provide backup
service during scheduled maintenance intervals.) This fuel is
material that would otherwise be destined for a landfill: wood
waste derived from land-clearing activities, municipal tree trimmings, the remains of construction two-by-fours and used pallets.
There is more than an ample supply of this locally sourced wood,
