Scholarly article on topic 'Oxy-biomass Ignition in Air and Relevant Oxy-combustion Atmospheres for Safe Primary Recycle and Oxy-burner Development'

Oxy-biomass Ignition in Air and Relevant Oxy-combustion Atmospheres for Safe Primary Recycle and Oxy-burner Development Academic research paper on "Chemical engineering"

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Abstract of research paper on Chemical engineering, author of scientific article — Ignacio Trabadela, Hannah Chalmers, Jon Gibbins

Abstract Results for ignition behaviour of pulverised biomass fuels in a 20 litre (L) spherical combustion chamber are presented and discussed. Four types of biomass currently used in UK utility pulverised fuel boilers have been tested for ignition behaviour in air, so at 21%v/v O2, and also, to assess relative performance under oxy-fuel combustion conditions, in a 21%v/v O2, balance carbon dioxide (CO2) balance mixture (21Oxy) and a 25%v/v O2 mixture (25Oxy) respectively. Peak pressures (Pmax) during constant volume ignition and combustion with 2500J and 5000J igniters were measured and recorded. The pressure ratios (P/R), defined as the ratio of the maximum pressure (Pmax) to the pressure at the start of ignition (P0) for each test are reported. A P/R above a threshold of 2.5 is taken as an indication of positive ignition. All four biomass types ignited nearly as readily in 25Oxy as in air at a range of fuel concentrations. Ignition was much less readily achieved in 21Oxy for all fuel concentrations and peak pressures were also generally lower. Results were more erratic with 2500J igniters compared to 5000J igniters, suggesting a relatively stronger ignition source is required with these biomass samples than with pulverised coals previously tested; this is tentatively attributed to larger particle sizes and higher moisture contents. Implications for pulverised fuel oxy-fuel combustion applications are: 1) a primary recycle (PR) stream with 21%v/v O2 would give improved pulverised fuel (PF) milling safety when compared to air firing but reduced ignitability in the burners; 2) a 25%v/v O2 primary stream would approach air behaviour in mills and burners. These preliminary results suggest that approximately 25%v/v O2 may give air-like performance in oxy-fuel pulverised coal plants using oxy-biomass.

Academic research paper on topic "Oxy-biomass Ignition in Air and Relevant Oxy-combustion Atmospheres for Safe Primary Recycle and Oxy-burner Development"

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Energy Procedia 63 (2014) 403 - 414

GHGT-12

Oxy-biomass ignition in air and relevant oxy-combustion atmospheres for safe primary recycle and oxy-burner development

Ignacio Trabadelaa*, Hannah Chalmersb, Jon Gibbinsa

aInstitute for Materials and Processes, School of Engineering, King's Buildings, University of Edinburgh, EH9 3JL, UK. bInstitute for Energy Systems, School of Engineering, King's Buildings, University of Edinburgh, EH9 3JL, UK.

Abstract

Results for ignition behaviour of pulverised biomass fuels in a 20 litre (L) spherical combustion chamber are presented and discussed. Four types of biomass currently used in UK utility pulverised fuel boilers have been tested for ignition behaviour in air, so at 21%v/v O2, and also, to assess relative performance under oxy-fuel combustion conditions, in a 21%v/v O2, balance carbon dioxide (CO2) balance mixture (21Oxy) and a 25%v/v O2 mixture (25Oxy) respectively. Peak pressures (Pmax) during constant volume ignition and combustion with 2500J and 5000J igniters were measured and recorded. The pressure ratios (P/R), defined as the ratio of the maximum pressure (Pmax) to the pressure at the start of ignition (P0) for each test are reported. A P/R above a threshold of 2.5 is taken as an indication of positive ignition. All four biomass types ignited nearly as readily in 25Oxy as in air at a range of fuel concentrations. Ignition was much less readily achieved in 21Oxy for all fuel concentrations and peak pressures were also generally lower. Results were more erratic with 2500J igniters compared to 5000J igniters, suggesting a relatively stronger ignition source is required with these biomass samples than with pulverised coals previously tested; this is tentatively attributed to larger particle sizes and higher moisture contents. Implications for pulverised fuel oxy-fuel combustion applications are: 1) a primary recycle (PR) stream with 21%v/v O2 would give improved pulverised fuel (PF) milling safety when compared to air firing but reduced ignitability in the burners; 2) a 25%v/v O2 primary stream would approach air behaviour in mills and burners. These preliminary results suggest that approximately 25%v/v O2 may give air-like performance in oxy-fuel pulverised coal plants using oxy-biomass.

© 2014TheAuthors.Publishedby Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Peer-review under responsibility of the Organizing Committee of GHGT-12

Keywords: Oxy-biomass; pulverised fuel; ignition; safety; burner.

* Corresponding author. Tel. (UK): +44 -776-546-7701, Tel. (US):+1-347-276-5154 E-mail address: i.trabadela@ed.ac.uk

1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/3.0/).

Peer-review under responsibility of the Organizing Committee of GHGT-12

doi:10.1016/j.egypro.2014.11.042

Nomenclature

21Oxy: 21 volume % oxygen in 79 volume % carbon dioxide. 25Oxy: 25 volume % oxygen in 75 volume % carbon dioxide. 300xy: 30 volume % oxygen in 70 volume % carbon dioxide. 400xy: 40 volume % oxygen in 60 volume % carbon dioxide.

304L: Grade of stainless steel similar to 304 type with low carbon content useful in corrosive environments. ASU: Air separation unit . bar: Pressure unit.

bar (a): Absolute pressure unit in bar. Ba(N03)2: Barium nitrate. Ba02: Barium peroxide.

CCS: Carbon capture and sequestration/storage. CCT: Carbon capture technologies . C02: Carbon dioxide.

CPU: Compression and purification unit used downstream in oxy-fuel combustion.

E: Energy, in the context of ignition energy, units J.

FNPT: Female National Pipe Thread.

IEA: International Energy Agency.

J: Joule, SI unit for energy.

kPa: kilo Pascal, 103 Pascal, pressure unit.

kPa (a): Absolute pressure unit in kPa.

Kst: Maximum rate of pressure rise of dust ignition assuming "cubic law", st for "Staub" (dust in German), units in

bar m/s or kPa m/s.

L: Litre, volume unit.

m3: Cubic metre, SI unit for volume.

M48: Standard external Metric thread and fastener/bolt size. N2: Nitrogen. 02: 0xygen.

02/ C02: 0xygen and carbon dioxide mixtures used in oxy-fuel. P0: Pressure at start of ignition for pressure ratio calculation. Pa: Pascal, SI unit for pressure. PF: Pulverised fuel.

Pmax: Maximum absolute pressure during dust ignition or peak pressure, bar or kPa.

Psig: Pound per square inch gauge, pressure unit.

PR: Primary recycle in oxy-fuel combustion.

P/R: Pressure ratio (dimensionless).

PTFE: Polytetrafluoroethylene, commercially Teflon.

R-20: 20 litre spherical ignition chamber designed and built at the University of Edinburgh. rpm: Revolutions per minute, rotational speed unit. T0: Absolute temperature at the start of the experiment. Tb: Absolute temperature of the burnt gas.

TWh: Terawatt-hour, 1012 watts per hour, electricity generation or electrical energy unit. Zr: Zirconium.

%v/v: Volume per volume percentage concentration.

1. Introduction

Combining oxy-fuel technology with biomass as fuel (oxy-biomass) is a way to reduce carbon dioxide (C02) emissions from power generation. C02 removed from the atmosphere as biomass grows can be permanently stored away from the atmosphere providing the potential for 'negative' emissions with oxy-biomass (and other combinations of biomass combustion/use with C02 capture). Different types of biomass are employed in ignition tests but usually properties of biomass are chosen to match combustion conditions of conventional pulverised fuel (PF) as closely as possible due to oxy-burner specifications and boiler requirements. Co-firing of biomass with coal is also useful from the perspectives of maximising use of available resources, improving plant fuel flexibility and reducing cumulative C02 emissions to atmosphere.

An important consideration for plant safety is that the oxygen (02) level in transient primary recycle (PR) has to be controlled to prevent fires in the PF milling stage. However, increasing 02 content from flue gas in PR has the potential to improve plant flexibility. For example, since 02 is available from the air separation unit (ASU) in oxy-fuel when compared to air combustion, 02 can be supplied to boiler at different points: PR, secondary recycle (SR) or direct injection. The higher partial pressure of C02 from flue gas in PR, due to recycling for boiler temperature control, provides an extra margin for 02 addition. Burner design has to be adapted to oxy-firing conditions and for air to oxy-fuel combustion switching when the power plant moves to C02 capture regime. Experimental apparatus that is able to provide data on flame speed is, therefore, useful. Research in this area is relevant to deploying oxy-fuel technology more efficiently, for example, in the US FutureGen 2.0 and the UK White Rose projects.

Biomass use in large utility PF boiler for power generation is increasing. By 2050 bioenergy could provide 3,100 TWh of electricity, i.e. around 8% of world electricity generation, according to the International Energy Agency (IEA) [1]. Biomass can co-fired with coal in PF boilers or, usually with some derating, on its own. Currently only conventional air-firing is used, but in the future carbon capture and storage (CCS) may be applied to biomass utilisation in order to obtain potentially negative emissions [2]. There are three main capture technologies (CCT) that might be used: pre-combustion, post-combustion and oxy-fuel [3]. The work reported in this paper is focused on oxy-fuel combustion for C02 capture from biomass, and specifically PF safety and PF ignition in oxy-fuel atmospheres in the mill and boiler respectively.

0xy-fuel operation is based on replacing the nitrogen (N2) that would be present in air combustion with C02 that is recycled from the combustion flue gas [4], with the oxygen (02) being added at relatively high purity from an air separation unit (ASU). Whereas in air firing the oxygen content is a constant 21%v/v, in oxy-fuel firing the concentration can be controlled at essentially any desired value. In general, however, the aim will be to obtain similar performance in oxy-fuel plant as in air-fired plant, and certainly not to reduce safety margins due to a greater propensity for combustion at excessively high 02 concentrations in the primary recycle stream (PR) used to transport the fuel into the boiler from the mills.

In oxy-fuel power plants the PR composition can vary significantly from one plant to another depending on a range of factors including chosen recycle strategy and interactions with other plant components. Fires in pulverised fuel (PF) mills are relatively common events in air-fired PF power plants. An important consideration in assessing combustion safety at power plants burning PF is, therefore, the potential for suspended PF ignition (colloquially known as a 'puff1 in the UK) during milling (particularly during mill shut-down), which could lead to overpressurisation of mills and/or pipework. Improved understanding of PF ignition under different conditions (e.g. 02/C02 concentration) and for different biomass types would determine which oxy-fuel power plant operating options provide process safety that is, at least, equivalent to conventional primary air PF milling in air-fired plants.

Combustion of biomass in the boiler in 02/ C02 atmospheres would ideally also occur in a similar manner to air combustion, without excessive ignition delays as the fuel enters the boiler but also without overly intense combustion near the burner tips. The C02 recycle in oxy-fuel combustion increases the partial pressure of carbon dioxide downstream for ease of treatment at compression and purification unit (CPU) before transport and

sequestration [4] but its increased heat capacity may also tend to delay PF ignition if the O2 content is not raised to compensate. The challenge is to find an optimum between PF safety considerations (ideally low O2) and air-like ignition characteristics.

2. Background and methodology

2.1 Theory

Dust ignition theory in 20 litre ignition chamber has been comprehensively described by Cashdollar and others elsewhere [5-8]. As noted by Cashdollar [6], when it can be assumed that during an explosion there is a limited change in the number of moles of gas on combustion, the ideal gas law can be arranged to a more simple expression for a rapid combustion process in a constant volume:

= Ik Equation (1)

Pmax is the maximum absolute pressure during dust ignition or peak pressure, P0 is the pressure in the chamber at the start of the dust ignition experiment, Tb is the absolute temperature of the burnt gas and T0 is the absolute temperature at the start of the experiment [6]. Equation [1] can be rearranged into the "cubic law" as derivation shown by Cashdollar [6]:

[M Vj = 4.84 - l) PmaxSu Equation (2)

where Kst is the maximum rate of pressure rise normalised to the volume of the ignition chamber, dP and dt are differential of pressure and time respectively, V0 is the volume of the ignition chamber normalised and Su is the burning velocity related to the unburnt gas ahead of the flame formation. In addition, pressure ratio (P/R) is defined [6-8] as:

p/R = Equation (3)

If P/R is above 2.5 then ignition can generally be considered to have happened for this set of experiments. Alternatively, ignition has also been defined to have occurred if the P/R is above 2 and if the Kst as described by Cashdollar [6] and Man and Gibbins [7] is above 1.5. However, in these current experiments the former appeared to be a more representative measure, especially given the need to 'overdrive' the R-20 test using a 5000J igniter to ensure positive ignition, instead of the more conventional 2500J maximum previously used with coal samples [7]. Dust ignition theory is applied for dust explosions prevention in industry [9] and in underground coal mine safety [10].

2.2. R-20 ignition chamber at the University of Edinburgh

The ignition chamber R-20 is a 20 litre spherical pressure vessel designed and built at the University of Edinburgh (Figure 1) with unique capabilities. It is made of stainless steel 304L in two hemispheres, held together by bolted lock rings. When assembled these parts form a perfect spherical test cavity with suitable wall thickness for withstanding 50 bar (a) or 5000 kPa (a) pressure and beyond if needed for other type of tests. In Figure 2, the lower hemisphere of R-20 is shown with the O-ring seal, space for 16 M48 12.9 grade bolts, a PTFE or Teflon igniter

mounting for low friction positioning of the ignition source at centre of the chamber and a round dispersion nozzle located in a 1 inch FNPT port at the bottom of the chamber. Biomass dust is dispersed through the nozzle with a blast of the gas mixture released using an electronically-controlled valve from a gas sample cylinder reservoir.

Figure 1. 20 L ignition chamber at the University of Edinburgh (R-20)

Figure 2. Bottom hemisphere of R-20 ignition chamber

Prior to each ignition test, R-20 is evacuated with a vacuum pump; the rapid flow of gas into the evacuated chamber then provides the dust dispersion. A high shock resistant strain gauge pressure transducer is used to monitor the evacuation of the chamber prior to the addition of the test gas. Two additional transmitters are used to record gas cylinder reservoir and vacuum line pressures. On the top of the ignition chamber a borosilicate viewport is placed in another 1 inch FNPT port for possible imaging or temperature measurements using infrared pyrometers. The high shock resistant strain gauge pressure transducer is also used to measure peak pressure (Pmax) and pressure rise rate inside R-20 during the ignition test (see section 2.4 below for igniter details). Pressure data (and other sensor outputs if required) is sampled using National Instruments LabVIEWTM software in a desktop computer, from which data is processed and stored.

2.3. Biomass samples

Four different types of commercially-relevant biomass have been tested in air and O2/CO2 atmospheres with R-20. These are identified in this paper as thermally treated Wood pellets, Miscanthus pellets, White Wood pellets and Cereal co-product. Additional details are restricted due to commercial sensitivity but, since identical samples are being used for air and oxy-fuel combustion conditions, the main objective of comparative assessment can still reliably be achieved. The use of thermally treated biomass is important from an experimental perspective since, because it is more friable and less fibrous, it can be used to give a check on the adequacy of dust dispersion in the chamber with untreated biomass sample. All four types of biomass have been grounded and milled to representative commercial biomass PF size ranges using an ultra-centrifugal Retsch ZM 200 apparatus with a high speed rotor mill at 8000 revolutions per minute (rpm) and a 0.25 mm stainless steel ring sieve. Mill operating periods were controlled to prevent overheating.

2.4. Igniters

The pyrotechnic chemical igniters employed are manufactured by Fr. Sobbe (Germany); a detailed published description is available [7]. Their chemical composition is 40 wt. % zirconium (Zr), 30 wt. % barium nitrate (Ba(NO3)2) and 30 wt. % barium peroxide (BaO2). After ignition (in the centre of the chamber) these chemicals become part of the solid process residues as barium and zirconium oxides. Due to minimum increase of gas volume from igniter breakdown, their effect on post ignition pressures can generally be neglected. However, peak pressures measured in R-20 ignition tests are normalised to take into account pressure rise due to the igniter by averaging a set of blank tests for each type and new batch of igniters. Igniters are commercially available in the 500-10000 J energy (E) range. It is generally preferable to use the lowest energy igniter that will give reliable ignition to avoid 'overdriving' the chamber. In this case 2500 J igniters were found to be inadequate for use with the biomass samples under all conditions (notably the 21Oxy tests) and 5000 J igniters were therefore also used.

2.5. Experimental procedure and parameters measurement

After the previously-evacuated R-20 ignition chamber was filled with dust dispersed from the bottom of the chamber and air or premixed 21Oxy or 25Oxy mixtures the igniter, placed in the Teflon mounting at the centre of the chamber, was electrically activated with a 0.4s delay, in order to facilitate biomass dust cloud formation before ignition. Peak pressure (and pressure rise rate, but not used here) inside the chamber were then recorded, with ignition taken as positive, for a pressure ratio (P/R) above 2.5.

3. Results and Discussion

Pressure ratios are shown for all four biomass samples at a range of dust loadings in air, 210xy and 250xy respectively and with 2500J igniters (Figures 3,5,7,9) and 5000J igniters (Figures 4,6,8,10).

Figure 3. R-20 P/R vs. conc., Thermally treated Wood Pellets (2500J) for oxy-biomass combustion

Figure 4. R-20 P/R vs. conc., Thermally treated Wood Pellets (5000J) for oxy-biomass combustion

□ Miscanthus Pellets Air

10 OMiscanthus Pellets 21 Oxy A

9 A Miscanthus Pellets 25 Oxy A

8 A A A

7 .a '■3 «6 □ □ B

9> ( □

V) & A it 4 u £ 3 □ □ A □ □ □

2 i & fa h © A O O 6 8

100 200 300 400 Biomass Concentration, g/m1 500 600

Figure 5. R-20 P/R vs. conc., Miscanthus Pellets (2500J) for oxy-biomass combustion

11 □ Miscanthus Pellets Air

10 OMiscanthus Pellets 21 Oxy

9 A Miscanthus Pellets 25 Oxy □ A O A □ O A

8 7 § A □ A 8 □ O

«6 ai □

41 <hl J 3

•f A aj4

3 a n

2 1 a o Q

1 100 200 300 400 Biomass Concentration, g/m3 500 600

Figure 6. R-20 P/R vs. conc., Miscanthus Pellets (5000J) for oxy-biomass combustion

Figure 7. R-20 P/R vs. conc., White Wood Pellets (2500J) for oxy-biomass combustion

11 □ 12 W Wood Pellets Air

10 012 W Wood Pellets 210xy

9 A12 W Wood Pellets 250xy □ n □ A

8 7 p « 6 0£ tU c h. J 3 □ □ > □□ § A 8 A O □ O

£ 3 n

2 1 b ° A

I 100 200 300 400 Biomass Concentration, g/m1 500 600

Figure 8. R-20 P/R vs. conc., White Wood Pellets (5000J) for oxy-biomass combustion

11 □ Cereal Co-product Air

10 O Cereal Co-product 2! Oxy i □

9 A Cereal Co-product 25 Oxy A □ A □ A □

8 7 o *-S « 6 S£ □ A □ A

X A ¡u4 M r

2 I § I 8 8 O

i { 100 200 300 400 Biomass Concentration, g/m3 500 600

Figure 9. R-20 P/R vs. conc., Cereal Co-product (2500J) for oxy-biomass combustion

Figure 10. R-20 P/R vs. conc., Cereal Co-product. (5000J) for oxy-biomass combustion

For all biomass samples tested it is apparent that ignition in the 21Oxy mixture is significantly more difficult than in air and also that pressure ratios if any combustion does occur are lower. With 2500J igniters none of the samples ignite in 21Oxy. Ignition can be achieved in some cases with 5000J igniters, but then only at higher fuel concentrations than with air. There are clear implications here for mill safety; 21Oxy would appear to be safer than air with respect both to the likelihood of ignition (e.g. due to smouldering dust accumulations in the mill or to frictional heating from damaged components) and to the elevated pressures that an internal 'puff' would cause.

Similarly, a primary recycle stream of biomass in 21Oxy would probably eventually ignite and undergo internal flame propagation after leaving the burner even without O2 mixing from secondary streams, but only if the PF loading at the ignition location (likely to be around the periphery of an entering stream of suspended fuel) was generally higher than would be required with air (as suggested by the higher concentrations for ignition with 5000J igniters) and also with a greater delay after leaving the burner (based on the need for 5000J igniters instead of 2500J igniters with air).

For all biomass samples in the 25Oxy mixture ignition can be achieved with a 2500J igniter, although only for slightly higher dust concentrations than with air. Interestingly pressure ratios are somewhat higher in some cases with the 25Oxy mixture than with air when using a 2500J igniter, although they are very similar when using a 5000J igniter. This is tentatively attributed to more localised, but more intense, combustion of the biomass in the 25Oxy/2500J cases; this then avoids the quenching of the combustion by the presence of excess fuel. The latter self-quenching phenomenon can be seen in a number of cases (most notably the air data in Figures 3 and 5, but also in a general trend for falling P/R values beyond an initial rise to about 300g/m3). The greatest P/R values in the 25Oxy/2500J cases are, however, only slightly higher than values for air and so are unlikely to represent a significant change in safety level for milling operations. The likelihood of suspended fuel ignition in the mill is clearly also less in 25Oxy than air; higher concentrations are required for reliable ignition without overdriving. In the case of positive ignition sources were present in milling operations (i.e. analogous to the 5000J igniter tests), however, peak pressures would be very similar to those in air.

For burner operation, 25Oxy mixtures would appear to give nearly as ready ignition behaviour as air for all the samples. With 2500J igniters, while ignition occurs at slightly lower concentrations for all samples in air, the combustion may be more intense in 25Oxy mixtures. With the higher initial driving force from 5000J igniters there is intermittent ignition in air at around 100g/m3 lower concentration levels than in 25Oxy. These results suggest only a slightly increased ignition delay downstream of the burner for a PR stream using 25Oxy compared to air firing.

In general, increasing O2 content from 21 to 25Oxy enhanced PF ignitability with lower CO2 level, which acts as ignition extinguisher, having a significant effect too. Some negative ignition tests with 2500J energy gave positive ignition with 5000J igniters, particularly for biomass where lower fuel density and variable volatile content requires higher ignition energy. Most importantly, air ignition patterns were matched for all biomass tested when using 25 Oxy and 5000 J igniters. This has very important implications for oxy-burner design which varies from previous experience [7] where coal ignitability behaved similarly to air in 30 vol. % O2 in CO2 (30Oxy) atmospheres. Further investigations with higher O2 atmospheres and indicative measurement of flame speed from ignition are needed.

4. Conclusions

Similar trends for the relative ignition behaviour in 21Oxy and 25Oxy mixtures have been observed for a range of commercially-relevant biomass samples, including a thermally-treated wood sample. The indications are that ignition is significantly more difficult in 21Oxy than in air but close to air behaviour in 25Oxy. While further work with higher oxygen levels and a wider range of fuels is required to support these preliminary findings, the apparent insensitivity to biomass type and the very small differences between air and 25Oxy (with air being just slightly more 'active') suggests that a 25Oxy mixture might be chosen for biomass PF oxy-fuel plants in order to achieve (slightly

conservative) air-like safety levels in milling plant and air-like combustion behaviour in burners. This is a somewhat lower level than the 30%v/v or higher O2 level for air-equivalence noted for coals in a previous 20L study and generally observed in pilot scale coal oxy-fuel burner tests.

Acknowledgements

This work received funding support from Engineering and Physical Science Research Council (EPSRC) - E.ON Strategic Partnership as part of the Research Councils UK Energy Programme through Oxy-fuel Combustion -Academic Programme for the UK (OxyCAP UK) EP/G062153/1 grant. Input from George Cairns, Gordon Paterson and Douglas Carmichael, members of Technical Staff in the School of Engineering, University of Edinburgh, UK is also gratefully acknowledged.

References

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(Last accessed 12/09/2014)

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