Professor K. Yoshikawa, Tokyo Institute of Technology, Japan & Dr A. Sanyal, International Environmental & Energy Consultants Inc, USA
The commercial means of waste treatment are currently mass landfilling and incineration. In countries such as Japan, where land is at a premium, most combustible waste is incinerated. However, on a global-scale landfilling remains the most popular way of waste treatment. However, the mood across the world is to prevent or limit the landfilling of wastes. Thus, a third option for waste treatment needs to be found.
Most of the waste-based power generation plants are large-scale units treating over 200 tonnes a day. However, obtaining permits for the construction of such big plants is becoming more and more difficult due to growing public opposition. An obvious alternative is small-to-medium-scale power plants, which is more likely to receive local approval. Until now, these small-to-medium-scale waste power generation plants have not been considered to be as economically feasible as conventional waste power generation plants, which employ a combination of large scale incinerators, boilers and steam turbines.
Japan’s Tokyo Institute of Technology is focusing on the development and commercialization of small-to-medium-scale power generation facilities, employing new technologies for the utilization of solid wastes, including biomass, as new energy resources. These technologies cover total solutions ranging from pre-treatment to final energy production.
The pre-treatment of wastes involve several different processes , such as crushing, drying and deodorizing. Now an innovative technology called a Resource Recycling System (RRS) has been developed that can perform these three pretreatment functions in a single process, utilizing high-pressure saturated steam. This technology is characterized by low energy consumption for drying. Figure 1 shows the operating principle of RRS.
Figure 1: Diagram of the operating principle of the Resource Recycling System (RRS)
Solid wastes, such as municipal solid waste (MSW), plastics, food residues, animal manure and sewage sludge, are fed into the reactor. Saturated steam at 200-230 à‚°C, 16-30 bar is injected into the reactor for about 60 minutes. The reactor blades rotate for about 10-30 minutes to achieve uniform mixing. The product is then discharged after extracting the steam.
The raw wastes of various sizes are converted into a powdery material, although the moisture content remains almost the same as that of the raw material. The powdery product is subsequently dried by ambient air or by air blowing. Drying is carried out using natural energy, which results in low energy consumption. The product is almost odourless.
Figure 2: Commercial plant based on RRS system
The product can be used for co-firing with coal for power generation or as part-substitute for coal for cement manufacture.The extracted steam is condensed and the condensed water is discharged or recycled as boiler feed water after appropriate treatment.
Fuel Gas Production from Solid Fuel
The RRS product can be utilized as a fuel for small-to-medium scale power generation. For this purpose, a small distributed generation type system has been developed based on solid fuel gasification known as the STAR-MEET (STeam/Air Reforming type Multi-staged Enthalpy Extraction Technology) that can be installed at distributed locations to minimize the cost of transportation and treatment of solid wastes. Figure 3 shows a diagramtical representation of a typical STAR-MEET system.
Figure 3: Commercial plant based on STAR-MEET system for power generation from chicken manure
Solid fuels are fed into a fixed-bed pyrolyzer using a continuous feed device. Thermal energy for the pyrolysis of the solid fuels is supplied by the partial combustion of char at the bottom of the pyrolyzer or melting furnace. Residual ashes are extracted from the bottom of the pyrolyzer in the form of calcinated ashes or from the melting furnace in the form of molten slag. Pyrolysis gas contains hydrogen (H2), carbon monoxide (CO), methane (CH4), nitrogen (N2), carbon dioxide (CO2), oxygen (O2), light hydrocarbon and tar.
In the reformer, the tar and soot components are reformed with high temperature steam in the following endothermic reactions.
CnHm + nH2O nCO + (n+m/2)H2 (1)
C + H2O H2 + CO (2)
These reactions take place at a temperature of over 800 C. To sustain this temperature, high temperature steam is employed, as well as high temperature air for partial combustion of the pyrolysis gas. The main components of the reformed gas are H2, CO, CO2, N2, CH4 and gaseous hydrocarbons, such as acetylene (C2H2).
High temperature steam and air are produced from a high efficiency heat exchanger, with hot gas from a furnace burning part of the fuel gas. The thermal energy of the reformed gas is used for generating saturated steam and hot air for the pyrolysis stage. Impurities, such as HCl and H2S, in the reformed gas, are removed in the purifier, which is a scrubber, wet and/or dry type such as a dust filter or an impurity adsorption device. The recovered flyash (mainly soot) is fed back into the pyrolyzer. The condensed water from the moisture in the solid fuels and the steam supplied for reforming is treated and discharged. Finally the purified gas is used as a fuel for an engine with a power generator and for a low-BTU gas-burning furnace with a heat exchanger.
Power Generator for low-BTU gas
Heating value of gaseous fuels produced in the STAR-MEET systems is low some tenth of that of natural gas. There are almost no established energy conversion methods for such low-BTU gases. Therefore, a dual-fuel diesel engine has been developed for burning such low-BTU gases.
At the start of the plant operation, the engine is fueled by light oil only. Then the produced low-BTU gas is gradually mixed into the combustion air, and in the steady state operation, 20-30 per cent of the total thermal input is supplied by light oil and 70-80 per cent is supplied by the low-BTU gas. With this method, it is possible to keep the electrical output constant by controlling the amount of light oil supply even if the heating value of the gas fluctuates.
The appropriate treatment of chicken manure produced in poultry farms has become very important in Japan because of regulation on the treatment of livestock manure and potential transmission of bird flu. Power demand in chicken farms are also rising for lighting, ventilation and so on.
A STAR-MEET commercial facility has been installed in a poultry farm for generating a fuel gas by high-temperature air reforming of the pyrolysis gas produced from chicken manure to drive a dual-fuel diesel engine for production of electric power and thermal energy.
In May 2007, 2200 hours of successful continuous operation of the facility was achieved, which confirmed the technology and its commercial viability. The photograph of this facility is shown in Figure 3. The treatment capacity of dried chicken manure is two tonnes/day, and the electrical output is around 100 kW. The poultry body heat is effectively used for drying the manure. The heating value of the reformed gas reaches 3971 KJ/Nm3 (950 KCal/Nm3). This value is approaching the same level as that of the heating value of wood chips. High-temperature air reforming effectively suppresses tar formation, and there is no significant condensation of tar or dust in the downstream components even after this long-term continuous operation.
Two novel waste treatment technologies based on hydro-treatment and gasification have been developed and commercialized, and they are very different from the conventional incineration-based waste treatment technology. In general, the economical feasibility of new energy resources tends to be relatively unattractive, but in the case of wastes revenues can be earned firstly by treating the wastes instead of landfilling and by selling the products, such as electricity, steam, hot water and fuels).
Their attractive economics aside, employment of these technologies will contribute to the reduction of global warming and promote a more recycle-orientated society.