Zero Emission Power Plant of Highest Efficiency

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1.3.1.1
Graz Cycle – a Zero
Emission Power Plant of
Highest Effi ciency
Franz Heitmeir
Wolfgang Sanz
Herbert Jericha
Institute for Thermal Turbomachinery
and Machine Dynamics
Graz University of Technology, Graz,
Austria
http://www.ttm.tugraz.at
franz.heitmeir@tugraz.at
wolfgang.sanz@tugraz.at
81 81
1.3.1.1-1 Introduction
In the last hundred years the concentration of some greenhouse gases in the
atmosphere has markedly increased. There is a wide consensus in the scientifi c
community that this seems to infl uence the Earth surface temperature and thus the
world climate.
Therefore, in 1997 the Kyoto conference defi ned the goal of global greenhouse
gas emission reduction of about 5% in the next years compared to the 1990 emission
level. CO2 is the main greenhouse gas due to the very high overall amount emitted
by human activities, and about one third of the overall human CO2 emissions are
produced by the power generation sector. In the European Union (EU) there is a
strong pressure on public utilities and industry to reduce the CO2 emissions by power
generation1. In 2003 the European Parliament passed a directive on emission trading.
In 2005 emission allowances were assigned to about 10,000 companies in 25 countries
within the EU which cover about 46% of the overall EU CO2 emissions. Companies
which do not need their full amount can sell it to companies which need more than
assigned. As emission allowances become scarce, they will have an increasing value.
First estimates varied between 10 and 20 €/ton CO2 (12 and 24 $/ton CO2) by 2010,
but in June 2005 European Union Allowances (EUA) were already being traded at 23
€/ton CO2 ( $28/ton CO2).
So there is a strong driving force to develop commercial solutions for the capture
of CO2 from power plants. The main technologies are as follows2:
- post combustion CO2 capture, e.g. by washing of exhaust gases using
amines;
- pre-combustion decarbonization of fossil fuels to produce pure hydrogen or
hydrogen-enriched fuels for use in conventional power plants;
- chemical looping combustion; and,
- oxy-fuel cycles with internal combustion of fossil fuels with pure oxygen.
The authors believe that oxy-fuel cycles are a promising technology. The
combustion with pure oxygen leads to a working fl uid consisting mainly of steam and
CO2, which allows an easy and cost-effective CO2 separation by steam condensation.
Further advantages are the great variety of fuels which can be used (natural gas, syngas
from coal or biomass gasifi cation, etc.) and the low NOx generation, since nitrogen is
only introduced by fuel bound nitrogen or as a residue in the oxygen to the combustion
chamber. The generated NOx as well as other gases are removed together with CO2, so
that no pollutants are emitted to atmosphere. On the other hand oxy-fuel cycles need
the development of new turbomachinery components and have to bear the high efforts
for oxygen supply. Oxygen needed in a large amount for this kind of cycles can be
generated by air separation units (ASU) which are in use worldwide with great outputs
in steel making industry and even in enhanced oil recovery. The largest air separation
plant already in operation for some years in the Gulf of Mexico produces nitrogen
for the injection in the gas dome of a large oil fi eld off-shore3. Fortunately, the new
working fl uid of steam and CO2 allows new power plant cycles of highest effi ciency,
so that the additional efforts for oxygen supply can be largely compensated. Among
them the Matiant Cycle, the Water Cycle, and the Graz Cycle are the best known4
.
History
The authors believe that the so-called Graz Cycle has the potential of highest
effi ciency. The basic principle was developed and published by Jericha in 19855
.
He presented a power cycle without any emissions which was based on the internal
combustion of hydrogen with oxygen in stoichiometric ratio as obtainable from solar
power plants. Thermodynamically this steam cycle was an integration of a top Brayton
cycle and a bottom Rankine cycle. In the nineties the hydrogen technology lost its
impetus, so that the Graz Cycle was adopted for the fi ring of fossil fuels6. At this
time cooperation with Japanese companies and research organizations led to the name