Environmental Engineering Reference
In-Depth Information
In addition, signifi
cant efforts continue to be made to convert wastes to fuels. In the
following we describe how some of the important features of this technology have
already been utilized for converting wastes to useful energy.
Local governments and businesses, which dispose of enormous quantities of
industrial wastes, have started to examine practical methods to reduce wastes and
to convert waste to energy by applying thermal recycling technology. These systems
are already in operation in some areas. Power generation using waste as the fuel
(one type of thermal recycling technology) is now considered to be a new energy-
producing process. In Japan, the outline of “New Energy Introduction Plan” issues
in 1994 set target levels for the waste utilizing power generation at 2 GW in 2000
and 4 GW in 2010. In the United States solid wastes alone have the potential to
produce about 20 GW electricity with only 30% plant efficiency.
High temperature air, preheated to 1000˚C and higher, has rarely been utilized
in industrial practices for waste treatment and energy recovery. At such high tem-
peratures most fossil fuels autoignite. If the heat recovered from the exhaust gas is
used to preheat the combustion air, and the waste (organic portion of municipal,
industrial, farm, and hospital solid wastes) is burned with this preheated air, then
the enthalpy of the exhaust gases is said to be effectively used to increase the
temperature in the combustion region. Recent studies at NKK Corporation, Tokyo
Institute of Technology (TIT), and other institutions reveal many benefits of utilizing
high temperature air for the thermal decomposition of several different kinds of
wastes and coals using a regenerative high temperature air facility designed by NFK.
5
very, prevention on
the formation of dioxins and furans can be made by rapid cooling of the exhaust
gases to low temperatures,
It is to be noted that, with some proper measures of heat reco
although the
TIT group had not addressed this issue at
6
the time of this writing.
Pyrolysis and gasification of solid fuels and wastes using elevated temperatures
has been used for a long time. In former times, coal was gasified with the aid of
gasifying material such as gas or steam using elevated temperatures. It should be
noted that for pyrolysis the material is heated in the absence of oxygen. The TIT
studies used high temperature air to gasify coal and solid wastes. They used a two-
chamber arrangement with provision for feeding the waste from the side and hot air
from the top in the top chamber. The two chambers were connected together with
a throat, which also had ceramic balls. The syngas formed and the slag flowed over
the ceramic balls into the bottom chamber. In this chamber the slag was collected
and the syngas was removed from one side of the chamber. Any loss of ceramic
balls into the bottom chamber had to be replenished from the top chamber via an
opening for the solid fuel feed. Gases of low to medium calorific value have been
produced from the gasifier using over 1000˚C high temperature air. In these studies
the high temperature air was obtained from a regenerator facility manufactured by
NFK, Japan. The NKK and TIT demonstrations are device application of high
temperature air to pyrolyze or gasify the solid fuels or wastes. If the wastes fuel or
coal utilizes both high temperature and steam (or some other gasifying material), it
is appropriate to consider the overall gasification and steam-reforming reactions.
The high temperature air gasification and steam reforming with wastes may be
represented by the following very simple reaction:
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