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Flexible premixed burners for low-cost domestic heating systems

Final Report Summary - FLEXHEAT (Flexible Premixed Burners for Low-Cost Domestic Heating Systems)

The FLEXHEAT project was aimed at developing low-cost, fuel-flexible, efficient and low-polluting premix burners for domestic heating appliances (floor-standing boilers, wall-hung boilers, stoves). The major targets of the project are summarised here:
- Development of 1:3 power modulating atmospheric metal premix burners without water cooling, optimised burner material composition and pollutants emissions lower than 50 mg/kWh (state-of-art atmospheric burners operate in a partially premixed mode with high NOx-emissions, while in fully premixed mode they do not allow a power modulation).
- Development of 1:5 power modulating fully premixed metal burners, optimised burner material composition and pollutants emissions below 40 mg/kWh over the entire power modulation range (state-of-art burners with these characteristics are mainly manufactured with very high cost materials).
- Achievement of enhanced fuel flexibility, i.e. capability of handling different gases as feedstock (e.g. natural gas, LPG), with special focus on direct use of low heating value resources (biogas, low methane-containing natural gas coming from Western Balkan regions, blast furnace gases, …). Furthermore, the developed burners should guarantee the usual durability targets for such appliances of at least 30 000 hrs and 200 000 thermal cycles.
- A fully-validated modelling and design platform, including simultaneous CFD modelling of flames and finite-elements modelling of burner stress and strain, primarily aimed at tailoring the burner geometry, the slits and holes shape and distribution pattern. Pollutants emission, flame stability and especially thermoacoustic effects (noise analysis) are also included.
- Model validation to be performed in ad hoc-designed experimental test rigs as well as in real-life experimental set-ups (validators), assembled and tested by the end-users, where the achievement of the above objectives would be proven.

Besides the technological aims of the project and the obvious corresponding impact on environment, competitiveness and employment, the project also aimed at stabilising and reinforcing the research potential of the participants from the Western Balkan both in the research community (universities) and in industry.

The workplan was structured in the following six work packages (WPs) and the additional WP 7 on project co-ordination:
-WP1: Burner material development
WP2: Modelling
WP3: Burner mantle design and manufacturing
WP4: Burner prototyping and testing
WP5: Validator 1 assembly and testing
WP6: Validator 2 assembly and testing.

Concerning the activities on the burner material development (WP 1), an investigation of the properties requirements, imposed to the constituent burner material by the specific application of atmospheric and premix burners, led to a final selection of three promising commercially available materials: 1) a reference stainless steel EN 1.4016 2) an advanced stainless steel AK SR 18, and 3) an advanced Ni-based alloy Supermet. Detailed oxidation resistance characterisation was performed, with the major result that the relatively low cost AK SR 18 material was the most promising material from the oxidation resistance point of view. The high cost material Supermet has the superior thermomechanical stress characteristics, which is of great practical importance in perforated burner mantle production. Thus, it was clear that compositional design of innovative metal sheets should be a trade-off between oxidation and mechanical stress resistance at high temperatures, trying to maintain a potentially low cost composition.

New / advanced materials were prepared through addition of alloying elements to the basic SR 18 composition, which would form second phase hardening. The aim of such additions was to improve the materials mechanical properties at high temperatures. Two solutions were applied:
1) moderate additions of Ni and Al, which were expected to improve the thermal stability of the conventional stainless steel, and
2) Ti and Al additions were employed as a cheaper solution.

Based on the characterisation of the newly prepared materials, the material that was finally chosen for the burner mantel production is based on the commercially available AK steel 18SR, with addition of alloying elements Ti and Al to the basic 18SR composition, in order to improve the materials mechanical properties at high temperature. The new material, named 18SR_TiAl, was defined and prepared in the POLITO laboratories.

The modelling activities, planned in the scope of the WP 2, included investigation of the available chemical kinetic mechanisms for different fuels and their reduction, simulation and the parametric optimisation of the overall burner performance, coupling of the CFD and the finite element modelling (FEM) for the thermo-mechanical stresses of the perforated burner mantles, and modelling of thermoacoustic instabilities.

The expected end results of these activities were:
- an optimised design for the atmospheric burner concerning the air entrainment allowing fuel flexibility and power modulation;
- an optimised burner mantle pattern design minimising thermo-mechanical stresses and allowing the use of lower cost materials for both burner types.

Concerning the burner mantle and burner construction design, which was the contents of the WP 3, the first evolution stage burners were designed by an ad-hoc analysis performed by BCT. Two different perforation patterns for the fan assisted burner and a third perforation pattern for the fully premixed atmospheric burner were designed.

The results of the evaluation of the first evolution stage burners can be summarised:
- The performance of the first evolution stage atmospheric burner was not sufficient. The target of achieving the fully premixed operation was fulfilled, due to which the first evolution stage burner reached the NOxemission targets. However, the power modulation targets were not met and the performance with limit gases and varying gas composition was not sufficient.
- The burner mantle and construction of the fan assisted burner fulfilled the project targets in terms of power modulation and emissions. Since the tested burner samples were of a lower nominal power (20 kW) compared to the required for Validator 2 (50 kW), an appropriate scale-up was needed to be done in the second evolution stage.

The results of the evaluation of the second evolution stage burners can be summarised:
- The experimental characterisation of the prolonged atmospheric burner showed that it was possible to operate it with all test gases. Experimental investigation of the three semi-circular and the three triangular atmospheric burner prototypes, representing the second evolution stage atmospheric burners, showed that they operate as fully premixed atmospheric burners and that they reached the NOx-emission targets. However, the power modulation targets were not met. An additional open issue for the second evolution stage burner prototypes was the a high frequency noise, emitted during their operation with the upper limit gas G21.
- The fan assisted burner of 50 kW fulfilled the project targets regarding the power modulation and emissions. UERLN experimentally investigated the temperature field over the burner surface mantel, which was uneven due to the burner length. Numerical simulation of the flow field in the fan assisted burner gave suggestions for possible improvements in the upstream flow distribution plate pattern.

The results of the evaluation of the third evolution stage burners can be summarised:
- Experimental investigation of the final evolution stage atmospheric burner showed that the burner fulfilled the power modulation target (1:2,5) with the nominal gas. The targeted NOx-emission was fulfilled already with the first evolution stage burner, while the detailed investigation showed that the CO emissions, which were above the targeted value below 6 kW, could be successfully regulated by adjusting the secondary air flow. This could be achieved by adjusting the gas appliance in which the burner is to be placed.
- The target power turndown ratio and emissions of the 50 kW fan assisted burner fulfilled the project targets.

In the scope of the WP 5, the long term tests of the newly developed atmospheric burner were conducted, where the validator 1 hosted the burner under development. The burner was targeted to operate as fully premixed, in order to reach the emission targets. As platform, the gas room heater ALFA-9 from ALFA PLAM was selected. In the existing product a partially premixed atmospheric burner with high NOxemissions was used. The existing product was produced with 15 000 - 30 000 pieces per year, pending on the market demand, and was basically distributed in the local market and in the market of the neighbouring Western Balkan countries. During different stages of the project ALFA PLAM delivered samples of the ALFA-9 appliance to MFB, UERLN and BCT, to be used in the experimental investigations.

Based on the results, obtained from the long term tests the following can be concluded:
- Long term tests of validator 1 showed that the main project goals were achieved: minimum agreed power modulating level of 1:2 was reached, pollutants emissions of CO and NOx lower than 50 mg/kWh were achieved.
- The NOx emissions at lowest power regime (3,4 kW) were about 30 % over the ELV. This was due to inadequate flow field around the burner for low power regime, as a consequence of performance of validator1 combustion chamber characteristics. This situation could be solved with reshaping of the combustion chamber.
- Some of the results with higher emission levels for CO and NOx, obtained during the long term tests of validator 1, were explained by serious changes in Serbian natural gas quality, which resulted in variations of primary / secondary air coefficients.

In the scope of the WP 6, the long term tests of the newly developed fan assisted burner were conducted, where the validator 2 hosted the burner under development. As a platform, a wall hung condensing boiler was selected. The nominal thermal power was selected to be 50 kW, which was suitable for larger houses and/or public buildings.

Based on the results, obtained from the long term tests the following can be concluded:
- Performance testing of validator 2 showed that the main project goals were achieved: power modulating level of 1:5 was reached, pollutants emissions of CO and NOx lower than 40 mg/kWh were achieved.
- Due to the low quality of local natural gas, which contains a lot of ballasts, and with a composition which varies in relatively short time period (hourly and daily) it was difficult to keep stabile operation regime over the whole proposed modulation range (1:5), particularly at maximum and minimum thermal outputs.
- Irrespective that low grade natural gas from drills in the surrounding of Zrenjanin was used in the experiments, one of the most important goals, i.e. NOx < 40 mg/kWh, was accomplished over the whole regime of fan-assisted burner operation.