Deliverables
This study was performed on various types straw samples originating from Denmark and Spain. The biomass samples contain low to high amounts of alkali metals, chlorine, sulphur, and silicon. Moreover, the Columbian coal COCERR was analysed. The untreated biomass and coal samples were analysed by chemical analysis and by XRD. Knudsen effusion mass spectrometry (KEMS) and high pressure mass spectrometry (HPMS) were used to study the alkali metal, chlorine, and sulphur release in combustion systems. The mechanism of the alkali metal, chlorine, and sulphur release was specified.
SO2 release:
At a temperature of 600 °C the total amount of sulphur being released differs from the results of the chemical analysis of the samples prior to combustion. The reason for this discrepancy might be seen in the high amount of potassium occurring in some of the samples and therefore samples with a high ratio of K/S. A high ratio of K/S leads to the formation of K2SO4 during the combustion process capturing large amounts of initially organic bound sulphur. The sulphur bound into the sulphate can, due to the low vapour pressure of K2SO4 at the prevailing temperature, not be released into the gas phase anymore. Therefore, a high ratio of K/S leads to a reduction of gas phase sulphur. In this context, Sulphur capture by Ca and formation of CaSO4 is considered to play a minor role due to slower kinetics of formation.
At 800 °C among the K/S ratio the ratio of K/Si needs to be considered as well. Silica is known for it's ability to absorb alkalis. A high amount of Silica and therefore a low ratio of K/Si therefore leads to a large amount of potassium silicate being formed during the combustion process. The potassium on the other hand that is captured by Silica is not available for the formation of K2SO4 anymore and therefore a large amount of Si lowers the active ratio of K/S especially in samples with a low K/Si ratio.
At 1100 °C in addition the sulphur is being released during char combustion deriving from inorganic compounds, such as K2SO4 and CaSO4.
KCl/HCl release:
The KCl (NaCl) release is more affected by the chlorine content of the samples than by the K (Na) content. Therefore, the highest potassium levels can be expected in high chlorine samples. The observation of the HCl release during combustion reveals that, as well as the KCl release, the release of HCl can strongly be related to the Cl content in the samples. Most of the HCl is being released during the primary combustion phase and only small amounts during char combustion.
Co-combustion:
Concerning the SO2 release, the co-combustion experiments underline the results from the SO2 release experiments. The amount of SO2 released from the samples increases with the amount of coal in the mixture. But the increase in the SO2 release is far higher than the overall content of sulphur in the samples would suggest, because the silica in the coal leads to an enhanced capture of potassium and major parts of the potassium sulphate formation is inhibited. This effect is strongest with samples with a very high potassium content and high ration of K/S respectively.
The KCl release of the mixtures corresponds to the amount of biomass in the mixture. The higher the amount of biomass in the mixture, the higher the KCl release.
Conclusions
High amounts of Potassium lower the SO2 release by formation of Potassium sulphates.
Silica promotes the release of SO2 by potassium capture inhibiting the K2SO4 formation.
Release of Potassium rather dependent on Cl content than Potassium content of the samples. Negative influence on SO2 release by Co-combustion due to high Silica content of the coal. Reduction of alkali release by Co-combustion due to alkali uptake by Silica.
A TGA/DSC Instrument (Thermo-Gravimetric analysis/Differential Scanning calorimetry) was applied to characterize the biomasses that will be investigated in the HIAL project. The measurements provide information about the total mass loss of the ash and some information concerning the ash melting as a function of temperature. This information is the basis for an improved understanding of the transformation of important ash components during heating.
The results of the STA measurements appeared to be reproducible in most cases verifying the value of the method for studying the melting behaviour and ash transformation of the ash samples. The STA curves could be explained to some extent by the chemical analysis of the studied ash samples. Furthermore, a more detailed study and calculation of the melting data is needed in order to obtain a clear view of the interactions between the different ash constituents during the thermal treatment of the specific ash samples. Work on a more detailed interpretation of the STA data proceeds.
The STA results are part of the work for an improved understanding of the transformation of K, S and Cl during biomass grate combustion. This information can be used to minimize deposit generation and SO2 emission from biomass fired boilers.
A Time Of Flight High Pressure Mass Spectrometer (TOF-HPMS) has been set up. The HPMS was manufactured by the Central Workshop of the Research Centre Jülich. High Pressure Mass Spectrometry is a highly suitable method used for hot gas analysis in the range from room temperature to 1500 °C at atmospheric pressure. The HPMS technique can deliver real-time-, on-line analysis and specification of detected alkali metal vapours. For this reason, the HPMS technique is ideally suited for determining the alkali and other condensable species. The integrity of the sampled high temperature alkali- laden gases is preserved during the free-jet expansion since chemical reactions are effectively quenched and condensation is prohibited. The non-equilibrium nature of the free-jet expansion and the subsequent formation of a molecular beam allows reactive and condensable species to remain in the gas phase at temperatures far below their condensation point for long periods of time in comparison to reaction rates. Using the mass spectrometer comprehensive detection of all gas phase species can be fulfilled.
A stainless steel cone, 35 mm high with a 108° interior angle and an orifice diameter of 0.3 mm is used for molecular beam sampling. This cone is fitted to a water-cooled stainless steel flange connecting HPMS recipient and cone. This front orifice is connected with the flow channel tube by moving the furnace towards the HPMS. The theoretical gas flow through the orifice at the given temperature and gas composition has been calculated and the flow reactor operated with some excess gas flow to the calculated value to prevent ambient air from entering the gas stream. The protrusion of the orifice into the flow reactor prevents condensation on the sampling cone and excludes interactions between gas stream and orifice.
The HPMS system consists of a three-stage differentially pumped vacuum system. Sampled gases entering the system undergo a free-jet expansion. The core of the expanded gases is extracted by a conical skimmer (1mm diameter) at the entrance to stage two forming a molecular beam. For better beam performance, this skimmer is constructed to be moveable along the recipient's main axis. A shutter positioned upstream the stage three entrances is installed to have the possibility of blocking the beam for background scanning. The molecular beam than enters the ionisation region inside the third stage by passing another small hole (1.5mm).
For ionising the molecules in the ion region, a laser beam is used. The ionising laser beam enters the stage 3 orthogonally to the molecular beam. A Continuum II 8010 pulsed Nd:Yag laser (fundamental wavelength 1064nm) is necessary for this ionisation. It is used to generate vacuum ultraviolet (118nm) photons which represent the ninth harmonic of the Nd:YAG fundamental laser frequency. This ionising radiation is produced by focusing the third harmonic of the fundamental frequency (355nm) into the centre of an isolated gas cell.
A mixture of Xe/Ar gas inside the gas cell is then used to triple the 355 output of the laser to attain the desired wavelength of 118nm.. A LiF lens between recipient and gas cell focuses the laser beam on the middle of the molecular beam. The main benefit of this Ionisation scheme is that background species and major product gases such as O2, H2O, CO, CO2, He, N2 all have ionisation potentials above the threshold ionisation energy and are therefore not ionised and detected. This allows us to focus on minor and trace products.
After the molecules of the molecular beam are ionised they are focused by the appropriate ion optics into the flight tube of the TOFMS. TOFMS is based on the principle that different masses with the same initial kinetic energy will have different velocities and therefore arrive at the detector at different times. At the end of the flight tube, the ions are reflected back toward the ionisation region and onto a multi-channel plate detector. Signal from this plate are sent to an oscilloscope along with a trigger pulse from the Nd:YAG Q-switch, which is interfaced with a personal computer.
The most important feature of the new system is the use of laser ionisation instead of an electron impact ioniser to ionise the sampled gases. This will provide a stable low energy source with a very narrow energy spread compared to the ionising electrons generated from a metallic filament. The low energy will reduce fragmentation of ionised species and generally simplify the resulting mass spectra. This technology will be useful for studying alkali metal release during biomass gasification and combustion, as hydrocarbons released during biomass gasification tend to obscure the detection of trace amounts of alkali metal containing species.
The scientific objective of the HIAL project is to understand the influence of fuel composition and combustion conditions on the release of alkali metals, S and Cl to the gas phase during combustion of straw fuels with a high content of alkali. Partners with the use of various equipments will study the release of these elements experimentally. In order to obtain comparable results it is necessary to provide well-characterised samples of straw and to distribute representative samples to the partners.
12 samples of Spanish and Danish straw have been sampled by EHN and Tech-wise and analysed by Ensted Power Plant Laboratory.
Chemical analyses include moisture, ash, heating value and the major elements C, H, N, Cl, S, P, Si, Al, Fe, Ca, Mg, K and Na. Ash compositions have been calculated on the basis of elemental composition.
The samples represent a broad range of elemental composition of high alkali biofuels and provide a good basis for the experimental study of alkali release in combustion processes.
Four key samples have been selected for study by all partners in the consortium: Danish wheat straw with a typical composition. Spanish winter barley with a high content of K and Cl. Spanish oats with a low content of K, Cl and Si and the Spanish crop Carinata with a high content of K and S and a low content of Si and Cl.
The objective of HIAL work package WP-2 is to investigate the combustion chemistry and the release of alkali metals, S and Cl, using both experimental and theoretical work. The theoretical computation started with the elaboration of gas-phase reaction mechanisms and numerical simulations of homogeneous combustion, and continues with numerical simulations of perfectly stirred reactors and laminar flat flames.
A comprehensive reaction mechanism has been elaborated to describe the gas phase reactions during the combustion of high alkali biomass fuels. The C/H/O sub-mechanism was updated using the latest thermodynamic and kinetic data. The reliability of the mechanism was tested against measurements published in the literature and investigated using the methods of uncertainty analysis. SOx and NOx reaction blocks were recently developed in cooperation between the Eötvös University and the Departments of Chemistry, and Fuel and Energy of the Leeds University and these were used in our calculations. A sub-mechanism that describes the degradation of KCl and its reaction products in combustion systems was developed. Kinetic and thermodynamic data for the relevant species were collected or estimated. The whole reaction mechanism consists of 97 species and 521 reversible reactions. Exploratory simulations were carried out for the description of the combustion of typical straw pyrolysis products.
The large concentration of Cl, K and S in straw contributes to biomass boiler operational problems such as increased levels of boiler ash deposit formation, deposit induced super-heater corrosion and emission of SO2, HCl and aerosols from the stack on. The processes in a grate fired boiler chamber can be divided in two areas, the primary grate combustion zone and the freeboard. K, Cl and S can remain in the grate combustion zone and be contained in the bottom ash or be transported to the freeboard of the boiler by particle entrainment and evaporation of volatile species. The objective of the HIAL Workpacked-2, Task 2.2 is to investigate the release of Cl, K and S from straw to the gas phase at conditions that resemble the local combustion conditions on a grate of biomass-fired boilers. The experimental release investigation revealed that both the combustion temperature and the ash composition had severe impact on the extent of Cl, K and S release to the gas phase at grate combustion conditions. Based on the investigated HIAL bio-fuels the following conclusions may be drawn:
Potassium:
The potassium release increased with applied combustion temperature for all samples regardless of ash composition. Up to 900°C between 20 and 60% of the total potassium had been released to the gas phase. At temperatures above 900°C, the relative potassium release increased almost linearly with temperature until 60 to 90% had been released at 1150°C. The local temperature on the grate in grate-fired boilers is fluctuating, but frequently above 900-1000°C. This illustrates that significant amounts of the fuel potassium are released to the gas phase during grate firing. The ash composition in form of the content of chlorine and silicon relative to potassium proved to have the highest influence on the potassium release behaviour. Evidence was found in the SEM-EDX analysis that potassium was partly incorporated into silicate structures at 650°C and nearly complete incorporated at 900°C. In a silicate lean biomasses the SEM-EDX investigation indicated that phosphor were able to retain potassium in the form of K3PO4. However, as calcium also forms phosphates and none of the investigated bio-fuels contained significant amounts of phosphor relative to potassium and calcium, the effect is probably minor in the overall release picture. In general, it is expected that the potassium release in grate-fired boilers would obey the following qualitative trends with respect to ash composition:
K/Si < 2 and low chlorine content Þ low K release
K/Si < 2 and high chlorine content Þ high K release
K/Si >> 2, at all chlorine levels Þ high K release
Chlorine:
For the samples with substantial chlorine content the release occurred in two steps. Between 30 and 60% was released in the first step below 500°C. The remaining chlorine was released in the second step between 650 and 800°C. On the contrary, for the samples with relatively low chlorine content, more than 80% was released in a single step below 650°C. Nevertheless, at combustion above 800°C chlorine was exclusively released to the gas phase. The composition of the ash or the absolute chlorine content did not affect this fact. The combustion temperature in the fuel-layer in grate fired boilers is typically significantly above 800°C, which implies that most fuel chlorine would be released to the gas phase during combustion in CHP plants.
Sulphur
In contrast to the chlorine and potassium release, the sulphur release appeared more sensitive to ash composition than combustion temperature. In case the bio-fuel contained relative high amounts of calcium relative to silicate, the sulphur release was lowest and unaffected by combustion temperature. On the contrary, if silicate was present in excess the sulphur release increased heavily with combustion temperature. In general, other ash forming elements such as Cl, K and P are indirectly coupled to the sulphur release. For instances a higher chlorine content results in higher potassium release, which will leave more silicate to react with calcium and more sulphur is released. None of the biomasses show a sulphur release of less than 40% of the total fuel sulphur at any combustion temperature. This is probably related to the occurrence of substantial amounts of volatile organic sulphur in the biofuels. Again, the sulphur release in grate fired boilers is expected to be controlled by the following qualitative trends with respect to ash composition:
Ca/Si > 1 and Ca/S >> 1 Þ low S release
Ca/Si << 1 and Ca/S >> 1 Þ high S release
K/Si > 2 and low chlorine content Þ low S release (at moderate temperatures)
This study has provided information on the influence of temperature and fuel composition on gas phase release of K, Cl and S during combustion of high alkali biomasses. The information is an important step towards an understanding the SO2 emission and ash generation process in straw fired grate boilers.
Searching for OpenAIRE data...
There was an error trying to search data from OpenAIRE
No results available