Evaluation of the Efficiency Face to the NOx Emissions from European
Gas-Fired Heat Process Equipment
Bent Karll (DGC); MJ. Fourniguet; A. Quinqueneau (Gaz de France);
P. Breithaupt (Gasunie); O. Jonsson (SGC); P. Navarri (CETIAT)
July 1999, R9913
Summary
In the frame of the THERMIE Contract STR-397-95-FR entitled "Evaluation
of the efficiency face to the NOx emissions from European gas-fired
heat process equipment", tests have been performed by GAZ DE
FRANCE, CETIAT, DGC, GASUNIE and SGC on 35 European industrial sites
in order to depict what the European industry using natural gas
as an energy source actually looks like in 1997, the levels of efficiency
and nitrogen oxides (NOx) emissions currently being achieved. These
35 industrial sites were chosen among the three following sectors
: steam or water boilers, engines or turbines and industrial processes
(food processing industry, metallurgy, ceramic, paper and textile
industries). The partners focused on relatively new installations
or newly retrofitted which were equipped with low NOx technologies.
To create an open database between the Partners, a common EXCELŽ
sheet has been defined and used to report the results for the three
sectors concerned including principally the following items
- General background on the site : it includes
the description of the installation, technical characteristics of
the furnace, the boiler or the engine, operating scenarios, gas
total rating, and depending of the type of installation power density,
rated electric power or production rate.
- Description of the equipment : it includes, if available,
the control system of the heating equipment and the low NOx techniques
identified.
- Description of the measurement techniques : In order to
compensate for the lack of international standard, this part has
been particularly detailed. It includes the description of flue
gas analysers (CO, CO2, O2, NOx, CH4, UHC, N2O,
VOC...), metering and pressure and temperature probes in terms of
measurement principle, supplier, measurement rang and accuracy and
gas calibration. It precises the position of the sampling points
and the type of the sampling line.
- Results : The operating conditions (atmospheric data, type
of natural gas burnt during the test and measurement period) are
given before the results themselves (complete flue gas analysis
and determination of combustion and process efficiencies).
The results show that the situation in terms of NOx emissions and
efficiency is quite different from one country to another and for
one installation to another.
Boilers
For this test campaign, the firetube boilers efficiencies (steam
or hot water) range from 78% to 97% which is in accordance with
previous and published results. The NOx emissions range from below
50 mg/m3(n) at 3% O2 to 140 mg/m3(n)
at 3% O2 which corresponds to low NOx boilers. To generalise
these results, we could say that efficiencies between 80% to 90%
and NOx level between 80 to 100 mg/m3(n) at 3% O2
are achievable for a majority of modern or future boilers. However
the excess air ratio as the power density have a great influence
on the performances of the boilers and thus has to be taken into
account in the analyse of experimental data. Moreover, the study
was based mainly on low gas input and hot water boilers : for more
powerful steam boilers, the conclusions should be adapted.
Concerning low NOx technologies, it appears that internal flue
gas recirculation and gas staging tend to be the most promising
techniques to reduce the NOx emissions from boilers. Future R&D
work should be carried out to optimise these techniques and to spread
them in the industrial field. The external flue gas recirculation
is also an effective technique which may be applied in the future
to be able to reduce the NOx emissions of existing installations.
Engines and turbines
The tests have shown that lean-burn gas engines, in the range from
0.5 - 5 MWe, operate with an electrical efficiency of
approx. 39-41% and a total efficiency of 85 - 95% (ref. to net calorific
value). The relative NOx emissions are 0.8 g NOx/kWhheat
and power
As a side-effect of the engine development toward higher efficiency,
the emissions of unburned hydrocarbons (UHC) have generally increased.
The composition of UHC is similar to that of the natural gas, i.e.
approximately 90% (vol.) methane. Methane is a strong greenhouse
gas, and emissions of UHC thus cause a reduction in the CO2
reduction benefit obtained by cogeneration.
Relatively high emissions of formaldehyde and acetaldehyde have
been detected in the engine exhaust, and this is a matter of concern
since the species may present a health risk.
Turbines are generally larger in rated power than the engines and
the electric efficiency is somewhat lower. The total efficiency
of heat and power is similar to that of the engines.The NOx emissions
from turbines have been reduced in recent years using low NOx burners.
The level can be lower than that of the engines. The turbines heat
supply temperatures are higher than for the engines, and this is
important in many industrial applications where steam is needed.
Industrial processes
The sector of the thermal industrial processes is very large and
the selection of sites is in comparison too small. In particular,
no very high temperature process has been investigated, such as
glass melting or white ceramic furnaces where the temperature could
be above 1500°C. Moreover, the oldest installation dated from 1992
and we tried to focus on low NOx technologies.
However we can clearly observed the expected trends, as the influence
of the combustion air temperature and of the process temperature
on NOx emissions and combustion efficiency. We can indeed remind
that the most common way to reduce the CO2 emissions
in natural gas thermal heating equipment consists of increasing
the combustion efficiency by recovering the flue gas energy to preheat
the combustion air. Using low NOx technologies, it is possible to
keep the NOx emissions under 400 mg/m3(n) at 3%O2,
even with preheated combustion air. The low NOx technologies have
yet to be adapted to the industrial processes when the impact between
the flame and the load to be treated has a big influence on the
final product (in term of quality in particular). Some efforts have
to be made to optimise or adapt the low NOx technologies (as examples,
extremely high air staging or flameless oxidation techniques) in
regards of a large variety of situation and to spread them in all
the industrial fields.
The process efficiency vary a lot from one installation to another.
In general, low temperature processes have lower process efficiency
(as well as the combustion efficiency and the NOx emissions). For
a rational use of energy research should be performed to raise them.
As a conclusion, it seems necessary to continue such a survey on
Low NOx and high efficiency Best Available Technologies in the different
industrial fields and R&D works in terms of comprehension and
optimisation of new low NOx techniques and of implementation of
these techniques in new or retrofitted installations.
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