Page 331 - Advanced thermodynamics for engineers

320 CHAPTER 14 CHEMICAL KINETICS

is that found as gas, oil or coal. These are all hydrocarbon fuels with the quantity of hydrogen
decreasing as the fuel becomes heavier. The combustion of any of these fuels will produce carbon
dioxide.
Obviously, the policy that should be adopted is for the amount of CO 2 /kW, or CO 2 /mile to be
reduced. This can be achieved by using more efficient engines, or changing the fuel to, say, hydrogen.
The most efficient power station, using conventional fuels, in terms of CO 2 /kW is the combined cycle
gas turbine plant running on methane. Such plant can achieve thermal efficiencies higher than 50% and
claims of efficiencies as high as 60% have been made. Large marine diesel engines also achieve
thermal efficiencies of greater than 50%, and other smaller diesel engines can achieve around 50%
thermal efficiencies when operating at close to full-load. It is presaged that the inter-cooled regen-
erated gas turbine will also have efficiencies of this order.
The hydrogen-powered engine produces no carbon dioxide, but does form NO x . However, the
major problem with hydrogen is its production and storage, particularly for mobile applications.
California has now introduced the requirement for the zero emissions vehicle (ZEV) which must
produce no emissions. At present the only way to achieve this is by an electric vehicle. It has been said
that a ZEV is an electric car running in California on electricity produced in Arizona! This is the major
problem that engineers have to explain to politicians and legislators: the Second Law states that you
cannot get something for nothing, or break-even. Many ideas simply move the source of pollution to
somewhere else.

14.6.5 CLEAN-UP METHODS
The clean-up methods to be adopted depend upon the pollutant and the application. As stated above,
the exhaust of a power station can be cleaned-up using desulfurisation plant to remove the sulfur
compounds. It is also possible to remove the grit from the power station boilers by cyclone and
electrostatic precipitators. These plants tend to be large and would be inappropriate for a vehicle,
although investigations to adapt these principles to diesel engines are continuing.
All petrol-fuelled cars being sold in the USA, Japan and Europe are fitted with catalytic converters
to clean up the gaseous emissions. This currently requires that the engine is operated at stoichiometric
mixture strength so that there is sufficient oxygen to oxidise the unburned hydrocarbons (uHCs) while
enabling the carbon monoxide (CO) and NO x to be reduced. This means that the engine has to be
operated under closed loop control of the air–fuel ratio over a significant operating range, and this is
achieved by fitting a l sensor (which senses the fuel–air equivalence ratio) in the exhaust system. The
error signal from the sensor is fed back to the fuel injection system to change the mixture strength. The
need to operate the engine at stoichiometric conditions at all times has a detrimental effect on the fuel
consumption, and investigations into lean operation (reducing) catalysts are being undertaken. These
include the use of exhaust gas recirculation and more complex combustion chambers to enable the
engine to run smoothly at weak mixtures. This has two effects: first, the temperatures achieved in the
cylinder are not as high as with stoichiometric combustion and, second, it is possible to achieve low
powers without throttling the engine as much.
At present it is not possible to use catalytic converters on diesel engines because they always
operate in the lean burn regime. This causes a problem because there is no mechanism for removing
the NO x produced in the diesel engine combustion process, and it has to be controlled in the cylinder