**4. Conclusions**

The present study was aimed at a more effective use of straw as a fuel for energy production and combined complex experimental study and mathematical modelling of the processes developing when co-firing wheat straw with a solid fuel pellets (wood or peat). In order to assess the electric field applicability for additional control of the main flame characteristics, the electric field effects on the processes developing downstream the combustor were studied and analyzed.

The co-firing of straw with wood or with peat pellets results in the enhanced thermal decomposition of the mixture, which is determined by the mass share wt.% of straw in the mixture, and approaches its peak value if the straw mass share in the mixture is about 20–30%.

The field-induced ion current in the space between the electrodes is responsible for the field-enhanced reverse axial heat/mass transfer of the flame species, which provides the enhanced heating and thermal decomposition of biomass pellets. Increasing the current up to 2–3 mA decreases the average axial velocity at the flame base (70 mm from the secondary air supply nozzle) decreases from 0.36 <sup>m</sup>·s<sup>−</sup><sup>1</sup> to 0,24 <sup>m</sup>·s<sup>−</sup><sup>1</sup> for a 30% straw mixture with wood and from 0.49 <sup>m</sup>·s<sup>−</sup><sup>1</sup> to 0.15% for a 30% straw mixture with peat. In addition, it enhances the swirl intensity close to the flame axis by increasing the axial flow swirl number from 0.03 to 0.5.

The field-enhanced reverse axial heat transfer causes an increase of the average value of the weight loss rate from 0.23 <sup>g</sup>·s<sup>−</sup><sup>1</sup> to 0.25 <sup>g</sup>·s<sup>−</sup><sup>1</sup> during the self-sustained burnout of a straw/ wood mixture and from 0.24 to 0.26 <sup>g</sup>·s<sup>−</sup><sup>1</sup> for a straw/pea<sup>t</sup> mixture with a correlating increase of the volume fraction of the combustible volatiles entering the combustor. For the straw/wood mixture, the volume fraction of CO at the flame base increases from 75 <sup>g</sup>·m<sup>−</sup><sup>3</sup> to 98 <sup>g</sup>·m<sup>−</sup>3, for the straw/pea<sup>t</sup> mixture–from 73 <sup>g</sup>·m<sup>−</sup><sup>3</sup> to 85 <sup>g</sup>·m<sup>−</sup>3.

*Energies* **2019**, *12*, 1522

The enhanced release of the volatiles correlates with the increased heat output from the device. For the straw/wood mixture, the heat output from the device increases from 3.2 kW to 3.5 kW, for the straw/pea<sup>t</sup> mixture−from 2.9 kW up to 3.1 kW. For the straw/wood mixture, the produced heat per mass of burned pellets increased from 13.9 MJ·kg−<sup>1</sup> to 14.3 MJ·kg−1, for the straw/pea<sup>t</sup> mixture−from 11.4 MJ·kg−<sup>1</sup> to 12.3 MJ·kg−1.

The field-enhanced combustion of volatiles at thermo-chemical conversion of the mixtures is confirmed by the increase of the CO2 volume fraction in the products, with dominant increase of CO2 at the primary stage of volatiles burnout, when CO2 increases from 13% to 14% in the straw/wood mixture, whereas in the straw/pea<sup>t</sup> mixture from 11.4% up to 14.4%.

The results of the mathematical modelling show that the action of the electric body force at Pe > 0.5 leads to a contraction of the flame diameter and enlarges the flame length because of the decrease of the average value of the flame temperature due to the axial expansion of the flame reaction zone and to the correlating increase of the maximum flame temperature at the center. Moreover, if the electric field is applied to the flame, the flame vorticity enhances, which strengthens the mixing and combustion of the volatiles.

The results of the numerical study explain the e ffect of the electric field applied to the combustion flame flow by the influence of the Lorenz force on the electrons produced in the flame, but the experimental results show the integral e ffect of the field-induced motion of the positively and negatively charged species in the flame. The mathematical model needs further development, nevertheless, the main combustion parameters obtained by the numerical simulation can be compared to the experimental ones at a higher bias voltage, when the induced current is exceeds 3 mA, and when the field enhanced axial flow of the combustible volatiles is observed, which altogether reduces the combustion e fficiency and the total heat collected during the complete combustion of the pelletized biomass mixture samples.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1996-1073/12/8/1522/s1, Figure S1: The diagram of a schematic description of the experiment, Figure S2: The technological scheme of the equipment and measurement instruments, discussion about the measurement data acquisition and accuracy, the experiment repeatability and uncertainty.

**Author Contributions:** Conceptualization, M.Z. and H.K.; Data curation, I.B., A.K., R.V., S.V., H.K. and U.S.; Formal analysis, I.B., A.K., R.V., M.Z, S.V., H.K. and U.S.; Funding acquisition, M.Z.; Investigation, I.B., A.K. and M.Z.; Methodology, I.B.; Project administration, M.Z.; Resources, R.V. and M.Z.; Software, H.K. and U.S.; Supervision, I.B. and M.Z.; Validation, I.B. and M.Z.; Visualization, I.B., A.K., H.K. and U.S.; Writing—original draft, M.Z. and H.K.; Writing—review & editing, I.B. and A.K.

**Funding:** This research was funded by European Regional Development Funding gran<sup>t</sup> number 1.1.1.1/16/A/004.

**Acknowledgments:** The authors gratefully acknowledge the European Regional Development Funding, project No.1.1.1.1/16/A/004.

**Conflicts of Interest:** The authors declare no conflict of interest.
