Using fuel cells in...
converting waste to energy
Background
Waste management is a necessity wherever people live, but traditionally waste is viewed as a problem, something
that pollutes and needs reducing or mitigating. What if this mindset could be changed such that we value our waste
as a source of bioenergy? Waste contains significant quantities of energy and, if we could harness this close to where
the waste is generated and convert it to a useable form, it could provide both a distributed source of electricity and
contribute to solving the global problem of waste.
Biogas is widely produced from processed municipal waste using anaerobic digesters and then is usually combusted.
However, biogas can be fed to a fuel cell to produce electricity and heat.
KEY BENEFITS
Using biogas in a fuel cell is the most efficient way to extract
the energy stored within it. Although carbon dioxide is emitted,
biogas is a renewable energy source and therefore any electricity
produced by the fuel cell would be considered carbon dioxide
neutral. Waste treatment facilities are by nature usually located
close to population centres, and using them for the distributed
production of electricity avoids transmission losses. Fuel cells
operate very quietly and cleanly, and are an ideal means for
generating power close to where people live.
Biogas can be either directly converted into electricity using high
temperature fuel cells or processed further into hydrogen, which
is a universal fuel cell fuel. Certain fuel cell types can in fact
produce excess hydrogen when running on biogas and this hydrogen could be diverted for sale to fuel cell electric
vehicle (FCEV) owners. In other words, a fuel cell plant generating heat and power from biogas would also produce
a saleable by-product that would enhance the commercial viability of the project (more details are in the case study
overleaf). Distributed, small-scale production and supply of hydrogen would support local clusters of FCEV, and is
widely believed to be the most effective way to establish fuelling infrastructure for these vehicles.
Fuel Cell Types Used In This Application Today
MCFC
Molten carbonate fuel cells (MCFC) are the technology of choice for electricity and heat generation in waste-toenergy applications. MCFC operate at high temperatures, around 650°C and there are advantages associated with
this: firstly, the high temperature helps drive the chemical reaction and this removes the need for specialised
catalysts; and secondly, the higher temperature also makes the cell less prone to carbon monoxide poisoning than
lower temperature systems. As a result, MCFC systems can operate on methane – the major constituent of biogas –
without the need for external reformers.
Orange County Sanitation District
Orange County Sanitation District (OCSD) provides wastewater and sewage
treatment for the coastal areas of Orange County, California, USA. In 2011 it started
a collaborative waste-to-energy project with Air Products and FuelCell Energy, the
leading company in this market which has sold MCFC-based systems to a number
of customers.
In 2011, FuelCell Energy installed a fuel cell plant at OCSD’s facility that runs on
anaerobic digester gas (ADG) and produces electricity, heat and hydrogen – what is
referred to as ‘trigeneration’. The fuel cell provides 250 kW of power for use at the treatment facility, a significant
benefit in itself, but it also supplies hydrogen to an Air Products hydrogen refuelling station situated a mile from
the plant. For Air Products, the world’s largest hydrogen producer, this project is its first venture into hydrogen
production from biogas. The company is a market leader in hydrogen fuelling stations, with more than ten stations
operating in California state. The OCSD fuel cell plant generates enough hydrogen for 25 to 50 fuel cell vehicle
refuellings per day.
In this installation, the heat from the fuel cell reaction is fed back into the fuel cell to power the fuel reforming
process (converting the biogas to carbon dioxide and hydrogen). This produces an excess supply of hydrogen,
which is diverted for sale in the hydrogen station, but it also has another benefit. Fuel cells need to be cooled
and the cooling equipment places a load on the fuel cell, reducing the electrical output. In this case, however, the
reforming process uses heat and this keeps the fuel cell cool, increasing the electrical output of the overall system.
Images: The Sanitation District (ASCE); sanitation plant (Register); fuelling a Hyundai ix35 at OCSD (Air Products); OCSD fuel cell system (Air Products)
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MAY 2012
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