Biomass is one of the most abundant natural and renewable resources for power generation, as well as being versatile and having a relatively high calorific value. Furthermore, as we produce more and bio-waste (biodegradable garden and park waste, food and kitchen waste from households, restaurants, caterers and retail premises and comparable waste from food processing plants), new technologies are needed for its disposal and recovery.
Some industrial and agricultural waste can also be assimilated to bio-waste. Biomass resources in fact include agricultural residues, animal manure, wood waste from forestry and industry, remnants from food and paper industries, municipal green wastes, sewage sludge, dedicated energy crops such as short-rotation (3-15 years) coppice (eucalyptus, poplar, willow), grasses (Miscanthus), sugar crops (sugar cane, beet, sorghum), starch crops (corn, wheat) and oil crops (soy, sunflower, oilseed rape, iatropha, palm oil).
However, these new technologies need to be environment-friendly, health-compatible, and should result in small investment and management costs. Many technologies have been developed to convert biomass and organic waste into energy from direct combustion and anaerobic digestion through to gasification and pyrolysis.
Organic waste treatment
This where the Italian environmental engineer, Laura Sanna has been focusing her efforts in recent years. “Maim Engineering s.r.l. has worked since 2005 in the field of renewable energy, running many interesting experiments and research activities on slow, humid and catalytic pyrolysis for the treatment and recovery of organic waste,” she explains.
Laura has worked for over ten years in the design of facilities for waste treatment and energy production, and lately has been working on a mathematical model supported by Maim Engineering S.r.l. and funded through a research grant by the Regional Government of Sardinia to develop a conceptual design of the process and the prediction of pyrolysis products. At the same time, a small scale plant has been built for demonstration and experimentation purposes.
“By operating the demonstrative plant, it was possible to define process parameters which can be used to design industrial scale plants,” says Sanna, who reveals in a recent scientific paper that the demonstrative plant, designed by Maim Engineering s.r.l. and built by AIT s.r.l., can process 20 kg/h of biomass, and produces a syngas, mainly composed of hydrogen, carbon monoxide and light hydrocarbons.
“When burnt, this syngas produces little emissions, so that it is not harmful to the environment, and provides an efficient mean for energy recovery from organic waste such as manure, as well as any other biomass,” comments Sanna.
Slow, humid and catalytic pyrolysis, which takes place after the biomass cracking reactions, produces a fuel syngas. This syngas is made primarily of hydrogen, carbon monoxide and light hydrocarbons. Dry biomass is fed into a heated rotary kiln designed to minimize the intake of air.
An automatic control system adjusts the amount of gas to keep the reactor at the constant temperature of 450°C and at the pressure of 1 atm. The syngas is extracted by a fan which keeps the system at atmospheric pressure, with a gap of few mbar. It is then injected into a calm area, where inert and bio-char are discharged through a rotary valve.
Afterwards, it goes through a cyclone dust separator, which removes a dust consisting of a fine carbonaceous powder. It is then cooled with water in a Venturi device, where the temperature drops almost instantly from 450°C to 45°C. It is then washed into a scrubber, where some pollutants are removed (dust, acid gas or tar). The clean gas goes through a flow device, is accumulated in a tank and then used in an internal combustion engine for power generation. A portion of the depurated syngas is also used to heat the reactor.
In an industrial-scale plant, the hot wastewater from the Venturi scrubber (40°C), containing some solid residuals, can be sent to a clarifier. There, the clean water is separated from the sludge. The sludge will be removed from the bottom of the clarifier using a pump, and sent back to the hopper of the reactor. The clean water is cooled down and sent back to the washing system.
To improve the production of H2 and CO, some catalysts are used and water is added to the dry biomass (30% in weight of the dry biomass). The off-gas flows at 700°C in a pipe coaxial to the reactor.
Steam and catalysts are added to facilitate the process. Compared to similar processes at the same temperature, this gives an increased gas flow rate, and a syngas which is richer in hydrogen and carbon monoxide; in fact, the results are comparable with higher-temperature pyrolysis. This also reduces the production of biochar, and no bio-oil is produced.
While a variety of materials such as animal waste (manure), agro-industrial waste (olive sansa), sewage sludge, and plastic were tested by Sanna, most of the tests were carried out with chicken manure, a biomass which is readily available in Sardinia. “Chicken manure disposal is a relevant issue in many European countries, due to the constantly increasing demand for chicken products. In north-eastern Italy in particular, the problem of disposal of chicken excrements is worsening, putting both the environment and public health at risk,” she says.
Minimal environmental impact
Sanna’s experiments revealed that the environmental impact of this pyrolytic plant is very small. In the tests using chicken manure as feed, solid residual wastes comprising biochar and inert elements only accounted for a combined total of 20-23% of the weight of input biomass.
The Italian researcher recognises that pyrolysis does have some environmental impact from off-gas emissions from both from the heating system and moto-generators, although none of these exceeded current statutory limits under Italian environmental law.
Visual impact is also minimal. In an industrial scale plant, it is mainly due to a syngas storage tank, and a chimney for off-gas emissions which may have height greater than 4 m.
The noise impact is mainly due to moto-generators, fans, and water circulating pumps. From a calculation made for an industrial facility with power of 3.6 MW, the noise would not exceed 60 dB(A), in compliance with the Italian and European regulations for an installation in an industrial area.
“This study demonstrates how we can produce a high hydrogen rate and a high low-heating value of syngas from all biomasses used. Off-gas composition and the absence of unburned material, particulate, and dioxins, shows the potential of the process even from an environmental point of view,” concludes Laura Sanna.
[Editor's note: This article was kindly prepared and submitted by Laura Sanna and edited by Toby Price for REM. She has worked for more than 12 years as a researcher, consultant, and freelance in many environmental engineering fields: waste treatment and energy production (design of composting facilities, waste water treatment plants, landfill's capping with biogas recovery, and gas cleaning systems); environmental laws compliance; technology assessment; site characterization and environmental impact assessment]
[Inset: Courtesy of Maim. Pilot pyrolisis plant in Sardinia]
For additional information:
Contact Laura Sanna at Maim Engineering (laura at maim dot it)