FEBRUARY 2005 ISSUE
backgrounder
Welcome to the New Generation of Recycling
Bottles, newsprint and plastics are one thing. A logical next step in the
Kyoto Accord world, however, is quite another — fossil carbon recycling.
Editor's
Note: The following article is one in a series generated by the APEGGA
Environment Committee, in an effort to inform and widen the debate on environmental
issues. Opinions expressed are not necessarily those of APEGGA.
BY DOUG HEATON,
P.ENG.
With the Kyoto Accord now a reality, a carbon constrained world is taking on new meaning. Recycling of conventional resources is a well established concept, but recycling of fossil carbon is still relatively unfamiliar.
Fossil carbon recycling can occur through the use of biogas from landfills, wastewater treatment plants and animal feedlots.
Large scale biogas power plants are commonplace but many biogas sources are frequently too small to be cost effective – farms and municipal landfills, for example. The need for efficient, cheap generation equipment in the 250-750 kW range is urgent and has spawned a serious interest in developing biogas-fuelled fuel cells, microturbines and Stirling engines.
Fuel Cells
Fuel cells come in many configurations: alkaline, phosphoric acid, molten carbonate, proton exchange membrane and solid oxide. They require pure hydrogen at the interface of the stack, irrespective of the fuel source and this contributes to their high cost.
Proton exchange and solid oxide systems have largely supplanted the other technologies for utility application and continue to be promising for combined heat and power, and transportation application.
Fuels need to be reformed to hydrogen and were originally limited to electrolysis of water or reforming of natural gas. However methanol, naphtha, ammonia and hydrogen sulphide are all being studied as hydrogen carriers.
Isopropanol (rubbing alcohol) is being looked at as the fuel source for small-scale fuel cells for use in cell phones and laptop computers. Penn State University is investigating membrane microbial fuel cells using waste itself rather than the methane it releases as a viable power resource.
Microturbines
A new generation of small (less than 500 kW) gas turbines is showing potential for the biogas industry, with lower maintenance costs than its larger brethren and low gas pressure requirements. Microturbines are an emerging biogas energy recovery technology option, especially at smaller feedlots, where larger electric generation plants are not economically feasible.
Biogas microturbine projects have come online, demonstrating the risks and benefits of small-scale applications, and can be used where the gas output is too low for larger engines and conventional turbines.
Over 100 waste fuel microturbine projects are already operational. They have enjoyed considerable popularity for small-scale power generation and the 30 kW Capstone unit has been applied at landfills and wastewater treatment plants in many locations. As larger units become available, they will become applicable to feedlots and farms.
Microturbines are susceptible to contamination, particularly particulates, hydrogen sulphide and siloxanes. Front-end cleaning is mandatory to minimize corrosion and erosion and this is especially important with biogases.
Siloxanes are semi-volatile organosilicon compounds that convert to inorganic siliceous deposits within a combustion chamber. A carbon filter is provided for removal as a package by microturbine vendors to ensure performance guarantees.
Stirling Engines
Stirling engines are an emerging technology. They are commercially available as 55kW units and are projected to be available in 150-300 kW sizes by 2006-2007. A unit consists of a set of pistons, heat exchangers and a regenerator.
The engine is filled with a working gas such as hydrogen, nitrogen or helium. The pistons are arranged to create both a change in volume of the working fluid and a net flow of the fluid through the heat exchangers. Heat is absorbed from an external source in the hot end, creating mechanical energy, and rejected in the cold end to the environment.
The working fluid is contained inside the engine at all times, meaning the cycle is closed. This enables a Stirling engine to operate cleanly and quietly with no combustion products coming into contact with the engine's working components and no release of high-pressure gases.
Stirling engines present serious competition to chemical-mechanical and electrochemical technologies. The characteristics of the Stirling engine make it a promising approach for CHP generation from biogas.
Because it is a temperature difference that makes a Stirling work, it is even being studied by NASA for deep space application.
Doug Heaton, P.Eng., is principal sustainable development consultant at Pyecombe Consulting Services. He has over 40 years experience in the plastics, pharmaceuticals, synthetic crude oil, landfill gas, biogas, and conventional and alternative power generation industries.