INVESTIGATIONS OF POTENTIALITIES OF BIOMASS GASIFICATION AT HTAG SYSTEM

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1 The Royal Institute of Technology in Stockholm INVESTIGATIONS OF POTENTIALITIES OF BIOMASS GASIFICATION AT HTAG SYSTEM WITH LOW OXYGEN CONCENTRATION SUSPOWER PROJECT Sławomir Kakietek July, 25 Author, Institute of Power Engineering, Augustówka 5 Street, Warszawa, Poland, tel , fax , s.kakietek@ien.com.pl

2 2 Abstract This report presents research performed at the KTH High Temperature Air Combustion (HTAC) facility within the SUSPOWER Project. The influences of temperature and oxygen concentration on biomass and refuse derived fuel (RDF) behaviour during combustion are presented. The ignition delay, mass loss, emission of NO x and concentration of CO, CO 2 and O 2 for three different temperatures 1 C, 8 C and 6 C for three concentrations of oxidizer i.e. 5, 1 and 21% within different characteristic times of combustion from 15 to 3 seconds for biomass and RDF pellets were investigated. It was shown that for very high oxidizer temperatures the differences in O 2 concentrations do not have a major influence on mass loss as compared to lower temperature operation. According to actual knowledge the obtained NO x emissions decreases while decreasing the concentration of O 2, however a significant increase in nitric oxide emissions was noticed when increasing the temperature of oxidizer (21% of O 2 ) from 8 C to 1 C (approximately from 4 mg/nm 3 to 1 mg/nm 3 for RDF pellet). This behaviour is in agreement with general knowledge about NO x emission but diverged from other recently reported results. Further investigations are suggested to clarify this point, and combustion system aspects like injection port location may play a role. However this phenomenon was not observed for low concentration of O 2, e.g. 1%. The increase the temperature from 8 C to 1 C did not cause an increase of NO x emissions, representing a major advantage of HTAC systems.

3 3 Acknowledgement I m very grateful to everyone who was contributed to my stay in Sweden, at KTH in Stockholm. Many thanks to my supervisor, Sylwester Kalisz Ph.D., which introduced me into laboratory secrets and arrange many other, important big and small things. I would like to thank the European Community Research Programme for the financial support and for opportunity to meet many people from all over the World. My thanks to Juliette, Sentor, Anna, Anders and Nabil for technical and personal support. Without them this project would not have been so instructive and rich in developing new areas of science and not only. Many thanks also for my Polish colleagues from my company, especially for Dr. Tomasz Golec who allowed me for four weeks leave of absence and Krzysztof Groszewski who takes care of my business in my company. One more time: Thank You!

4 4 Table of contents ABSTRACT... 2 ACKNOWLEDGEMENT. 3 TABLE OF CONTENTS INTRODUCTION OBJECTIVE DESCRIPTION OF TEST FACILITY 5 4 DESCRIPTION OF METHODOLOGY OF INVESTIGATIONS. 6 5 RANGE OF INVESTIGATIONS 7 6 RESULTS MASS LOSS IGNITION DELAY EMISSION OF NO X SUMMARY LITERATURE. 15

5 5 1. Introduction Gasification and pyrolysis are considered to be promising technologies and alternatives due to potential economic and environmental advantages (higher efficiency, lower emissions, low production cost, low cost of gas cleaning etc.). Whiteley [1] expects that gasification and pyrolysis will be predominant technologies for biomass cogeneration by the year 21. During the past few decades many new gasification processes have been developed. Significant progress was made in the area of solid fuel gasification, solid fuel feed system, syngas quality etc. In spite of this there is still a need to improve and develop new technologies. One of the most promising technologies is High Temperature Air and Steam Gasification (HTAG) [1-5]. One of main feature of HTAG is to use high-temperature air/steam as gasification agents of the solid fuels for maximum economical efficiency, low environmental impact, and possibility to gasify different kinds of fuel (biomass, wastes) with opportunity of obtaining high calorific syngases. Regarding the EU policy, in the area of renewable sources of energy, mainly the biomass and wastes fuels should be considered as a future fuel for gasification. Thus there is still a need to carry out the investigations, both in the area of HTAG systems and behaviour of such fuels, as for instance biomass, in the high temperature oxidizer. 2. Objective The main objective of the project to investigation the behaviour of biomass and refuse derived fuel (RDF) during high temperature combustion with different composition of oxidizer. The ignition behaviour, mass loss and emission of NO x were mainly considered during this short research trial. 3. Description of test facility The scheme of the test facility is shown at Figure 1. The rig is more or less 1 meter in length horizontal combustion chamber with its inner diameter about.1 m. During the heating up state the fuel gas (1) and air (2) feed the gas burner (3) which through the combustion chamber (4) heats the ceramic honeycomb (5). The flue gases from the honeycomb (5) through the second part of the combustion chamber (6) go to facility s outlet (7). During the experiment state (when the gas burner is off) the air or mixture of air and nitrogen (2) is heated by already hot honeycomb (5) to reach the proper temperature which is measured by

6 6 thermocouple (12). The special constructed basket with biomass, which is not shown at this scheme, is put in by special screw (8), first into the small bucket (9) when it is constantly cooled by nitrogen (1), and second during the investigation to the combustion chamber (6) in this manner that it is visible through glass opening (11). The composition and temperature of flue gases after biomass combustion is measured by thermocouple (13) and gas analysis probe (14). After certain time of experiment the basket is taken out first to the small bucket (9) and after certain time, which is necessary because of cooling purposes, is removed from the rig. Figure 1 Scheme of the HTAC facility at KTH Through the glass opening (11) the ignition phenomena can be observed by digital camera, which is not highlighted in the scheme. 4. Description of methodology of investigations The methodology of investigations focus mainly on the second state of experiment (see chapter 3) during which the constant flow of high temperature air or its mixture with nitrogen (for dilution purposes) heats up the solid fuel sample causing its ignition and combustion. The investigated fuel sample before each measurement is weighed. After setting proper temperature and flow of oxidizer it is put to the combustion chamber for certain amount of time. During the experiment two temperatures are measured, before and after the sample with solid fuel (see Figure 1 in Chapter 3), and composition of flue gases is also monitored. During the whole experiment the sample is observed by digital camera. After suitable time of experiments the sample is taken out from the combustion chamber and immediately cooled by nitrogen in the small bucket during the next 5 minutes. After this the sample is taken out from

7 7 the rig and weighted. On this basis the mass loss is estimated. In return the ignition delay is measured as a function of several parameters: an optical visibility of flame which has the shortest possible delay in relation to real time of ignition, increase of temperature after the solid fuel sample and increase or decrease the amount of CO, CO 2 and O 2 at the outlet of combustion chamber. The NO x emission is measured by gas analyzer. On the basis of such measurement the behaviour of biomass and RDF during high temperature air combustion with different concentration of O 2 is determined and investigated. 5. Range of investigations The investigations were carried out mainly for wood pellets, for three temperature of oxidizer 6, 8 and 1 C, and for three its concentrations 5, 1 and 21% for characteristic times of combustion ranged from 15 to 3 seconds (5 minutes) as it is shown in Table 1 (blank spaces with X concern the experiments performed within SUSPOWER project, the blanks spaces with # denote experiments which has been already investigated before and they are brought forward for clarity and integrity). Table 1 Range of investigations for wood and RDF pellets Temperature and O 2 concentration 1 C 21% 1 C 1% 1 C 5% 8 C 21% 8 C 1% 8 C 5% 6 C 21% 6 C 1% 6 C 5% 15 X X X 3 # # 45 # X X X X X 6 # # # # # # X X X 12 X X X X X X 18 # # # # # # # X X 3 X X X X X X X X X X X X X X X X X # X X - experiments performed within Suspower project for wood pellets X - experiments performed within Suspower project for additional investigations of NO x emission for wood pellets X - experiments performed within Suspower project for additional investigations of NO x emission for RDF pellets Some preliminary experiments were carried out also for RDF pallets to compare the mass loss, ignition delay and NO x emission for two different kind of fuels.. During the measurements particular amounts of air and nitrogen, two temperatures before and after the basket and flue gas concentrations (O 2, CO, CO 2, and NOx) were measured and

8 8 acquired. Moreover for each experiment optical visualization, which was performed by digital camera, was undertaken. The proximate and ultimate analysis of investigated fuels is highlighted in Table 2 and 3. Table 2 Wood pellets analysis (as received) Name Unit Value Name Unit Value Diameter mm 8 Carbon % 47,1 Density kg/m3 671 Hydrogen % 6,6 Moisture % 7,7 Nitrogen %,2 Volatile % 74,7 Oxygen % 45,42 Ash %,7 Sulfur %,1 Calorific Value MJ/kg 18,72 Net Calorific Value MJ/kg 17,27 Table 3 RDF pellets analysis (as received) Name Unit Value Name Unit Value Diameter mm 8 Carbon % 61,5 Density kg/m3 472 Hydrogen % 9, Moisture % 2,9 Nitrogen %,2 Volatile % 81,9 Oxygen % 22,92 Ash % 5,8 Sulfur %,9 Calorific Value MJ/kg 28,63 Net Calorific Value MJ/kg 26,7 For the comparison purposes between wood and RDF pellets the same fuel/air ratio and total amount of flow was set (velocity). Thus the amount of RDF sample must have been decreased from 3g (suitable for wood) to 2g. 6. Results 6.1 Mass loss The results show different combustion behaviour of wood pellets depending on temperature and O 2 concentration in air. Figures 2 4 highlight the mass loss during experiments. The mass loss is much greater at the beginning of combustion (times till 6 s) for high temperature as compared with lower temperatures (Figure 2). This difference deepens when the concentration of O 2 is decreasing. After certain time of combustion (times 18-3s) there are still some differences for given concentration of O 2 when decreasing the temperature, however these differences are not so pronounced except the lowest level of O 2 i.e. 5%. This result clearly arises that for high temperature of oxidizer the combustion process is more

9 9 intensified. Even when the oxygen concentration is changed from 21% to 1% the mass loss for 1 C Mass loss [%] C 8 C 6 C a) Mass loss [%] C 8 C 6 C b) Mass loss [%] C 8 C 6 C c) Figure 2 The comparison of mass loss for different temperature and O 2 concentration is still the same (times 45 or 6s) while for 8 C it changes from 52 to 23%, respectively (Figure 2). Even for 5% of O 2 the mass loss for 1 C is a little lower for the same times as compared with 21% of O 2. This phenomena declines when increasing the time of combustion, since the process of combustion for lower temperature is delayed (later ignition) and eventually it has to be finished. of O 2 then for 21%. It means that the oxidizing for 21% has

10 % O2 1% O2 5% O2 a) 21% O2 1% O2 5% O2 b) Mass loss [%].. Increase of mass loss [%] Figure 3 The comparison of mass loss for different O 2 concentration for 1ºC been already finished before. The same happens for lower temperatures. In every case the process of oxidation is rapid after the ignition. When ignition occurs the noticed phenomena of high temperature air combustion still exists but it is not so visible. This behaviour is of course caused by heat energy released from solid fuel chemical energy. Even so for 5% of O 2 the HTAC phenomena again grow in significance. The state of the combustion of volatiles and char (unfortunately experiments do not allow to separate this two phenomena) of solid fuel can be reasonable determined not only by mass loss but also by increase of mass loss (right graphs at Figures 3 4). This parameter show in what step time the combustion process was more intensified. For instance looking at Figure 3 b) it is clear that for 18 seconds this process is more intensified for 1 and especially for 5% of O 2 then for 21%. It means that the oxidizing for 21% has been already finished before. The same happens for lower temperatures % O2 1% O2 5% O2 a) b) 21% O2 1% O2 5% O2 Mass loss [%].. Increase of mass loss [%].. Figure 4 The comparison of mass loss for different O 2 concentration for 8ºC

11 6.2 Ignition delay The ignition which is described in this chapter was determined on the basis of optical analysis.

On the basis of such investigations the ignition delay time was determined for different temperature and O 2 concentrations as highlighted at Figure 5.

Additionally the ignition for RDF pellets is much faster and the flame behaviour is different then for wood pellets.

11 Ignition delay The ignition which is described in this chapter was determined on the basis of optical analysis. The moment of ignition was defined by the very first any visible flame which appeared in the combustion chamber. On the basis of such investigations the ignition delay time was determined for different temperature and O 2 concentrations as highlighted at Figure 5. The ignition delay increases with decreasing temperature and decreasing concentration of O 2. The delay connected with decreasing O 2 is longer for lower temperatures of oxidizer then for higher ones. Additionally the ignition for RDF pellets is much faster and the flame behaviour is different then for wood pellets. Ignition delay [s] % 1% 5% O2 concentration [% ] 1C 8C 6C Ignition delay [s] % 1% 5% a) O2 concentration [% ] b) 1C 8C 6C Figure 5 Ignition delay for different temperatures and O= concentration, a)- wood pellets, b)- RDF pellets Figures 6-9 show some exemplar pictures of ignition and flame. At once one can ask, watching for instance the Figure 6, why the 9.95 s was chosen for ignition instead of s. The ignition phenomena is very complicated issue and many define its beginning differently. However at present stage of investigations assumed at the beginning thesis that ignition occurs when first flame becomes visible for man eye seems to be reasonable s (ignition) 26.6 s s s (typical flame) Figure 6 Temperature of air 1ºC, 21% of O 2, wood pellets

3 s (typical flame) Figure 7 Temperature of air 6ºC,

higher temperature of air is less visible then for

The flame appears but is not well visible for man

The flame behaviour for lower temperatures is also

The flame jumps, is less stable and its edgings

12 s (ignition) s s ( explosion ) 244 s s (typical flame) Figure 7 Temperature of air 6ºC, 21% of O 2, wood pellets The ignition moment for higher temperature of air is less visible then for lower. The flame appears but is not well visible for man eye. For lower temperature as shown at Figure 7, the ignition is sudden and unexpected. However the biomass pellets starts to burn before (see s at Figure 7) but there is no typical flame. The flame behaviour for lower temperatures is also different as compared with 1 C. The flame jumps, is less stable and its edgings change frequently its position. For high temperature the flame is smoother, stable and less bright s (ignition) s s s 27 s s (typical flame) s s Figure 8 Temperature of air 1ºC, 21% of O 2, RDF pellets

13 The ignition for RDF pellets is faster as shown at Figure 8 and 9.

Additionally the calorific value is much higher wherethrough the amount of released energy per mass unit is higher and the theoretic

It may be as well to add that at the end of combustion of RDF pellets there is still blue flame which would indicate still the combustion of

This may be caused by constrained some volatiles inside the pellets due to melting behaviour of RDF pellets.

This behaviour can be seen sometimes for particular types of hard coals. 25.99 s (ignition) 38.99 s 39.7 s 39.79 s 39.87 s 4.99 s 45.87 s 95.

13 13 The ignition for RDF pellets is faster as shown at Figure 8 and 9. The fuel is more reactive, mainly due to its chemical compositions and source of its generation (plastics, papers etc.). Additionally the calorific value is much higher wherethrough the amount of released energy per mass unit is higher and the theoretic temperature of combustion is also higher. It may be as well to add that at the end of combustion of RDF pellets there is still blue flame which would indicate still the combustion of volatiles (heavy hydrocarbons) as highlighted at Figure 8 for s. This may be caused by constrained some volatiles inside the pellets due to melting behaviour of RDF pellets. The RDF pellets melt and liquid material can bung the tides closing some volatile species inside the material. This behaviour can be seen sometimes for particular types of hard coals s (ignition) s 39.7 s s s 4.99 s s 95.3 s s Figure 9 Temperature of air 8ºC, 21% of O 2, RDF pellets For lower concentration of oxygen the flame is less bright and begins further from the surface of basket. 6.3 Emission of NO x Figure 1 shows the NO x behaviour during combustion for three temperatures of oxidizer and three concentrations of O 2. When decreasing the amount of O 2 the NO x emission decreases rapidly. When increasing the temperature of oxidizer the NO x levels increase. This behaviour

14 14 is with good agreement with generally accepted opinion and knowledge. However, what is worth of underlying, whilst NO x increases seriously for 21% of O 2 when increasing the temperature from 8 C to 1 C, this phenomena is not observed for lower concentrations of O 2 (see Figure 1 a) and b)). This is very big advantage of HTAC, because for high temperature and lower concentration of O 2 then 21% one can get the same mass loss of solid fuel and very low NO x emission. In this case 1% is enough, however some optima can be also found for higher concentration of O 2. Recapitulating for lower concentration of oxidizer in the air the higher temperature of air can be used without risk of increase the NO x but with better burnout of fuel. This is very serious beauty of HTAC which makes this system very attractive and more efficient then conventional one NOx [mg/nm3] for 6%O C, 21%O2 1C, 1%O2 1C, 3%O2 Sample No [-] NOx [mg/nm3] for 6%O Sample No [-] a) 8C, 21%O2 8C, 1%O2 8C, 3%O2 b) 45 NOx [mg/nm3] for 6%O C, 21%O2 6C, 1%O2 6C, 3%O2 Sample No [-] Figure 1 Concentrations of NO x in mg/nm 3 recalculated for 6% of O 2 for RDF pellets The same behaviour as for RDF samples is also noticed for wood ones. However the levels of emission are lower in spite of the same amount of nitrogen in biomass. The reason of higher c)

15 15 concentration of NO x for RDF fuel lays in higher temperature of combustion which are caused by higher calorific value and more volatiles. 7. Summary In this report the influences of temperature and oxygen concentration on biomass and RDF fuel behaviour during combustion were presented. The ignition delay, mass loss, emission of NO x and concentration of CO, CO 2 and O 2 for three different temperatures 1 C, 8 C and 6 C for three concentrations of oxidizer i.e. 5, 1 and 21% within different characteristic times of combustion from 15 to 3 seconds for biomass and RDF pellets were widely investigated. Some results were highlighted in this report. It was shown that for very high temperature of oxidizer the differences in O 2 concentrations do not have such big influence on mass loss as for lower temperatures. According to actual knowledge the obtained NO x emissions decreases while decreasing the concentration of O 2, however the significant increase of nitric oxide emission was noticed when increasing the temperature of oxidizer (21% of O 2 ) from 8 C to 1 C (approximately from 4 mg/nm 3 to 1 mg/nm 3 for RDF pellets). This behaviour is in agreement with general knowledge about NO x emission. However this phenomenon is not observed for low concentration of O 2, e.g. 1%. The increase the temperature from 8 C to 1 C does not cause the increase of NO x emission. This is very serious advantage of HTAC which makes this system very attractive and more efficient then conventional one. 8. Literature [1] WHITELEY M., The future of CHP in the European market the European cogeneration study. Report no.: nxvii/4.131/p/ Overmoor, Neston, Corsham SN13 9TZ, United Kingdom: Energy for Sustainable Development (ESD) Ltd; 21. [2] TSUJI H, GUPTA AK, HASEGAWA T, KATSUKI M, KISHIMOTO K, MORITA M. High temperature air combustion: from energy conservation to pollution reduction. Boca Raton: CRC Press; 22. [3] TANAKA R. New progress of energy saving technology toward the 21st century; Frontier of combustion & heat transfer technology. Proceedings of the Conference: 11th IFRF Members Conference, 1-12 May [4] BLASIAK W, SZEWCZYK D, LUCAS C, TSAMBA AJ, RA.DI N. High Temperature Air/Steam Gasi.cation Technical Report No. 1: High Exergy Rate Gas Aided Low Grade Fuel Utilization Technology. Royal Institute of Technology, Stockholm, 22 March. ISRN KTH/MSE 2/16 SE ENERGY/TR. [5] YOSHIKAWA K. High efficiency power generation from coal and wastes utilizing high temperature air combustion technology. In: international symposium on advanced energy technology, Sapporo, Japan; 1998, p

HIGH TEMPERATURE AIR AND STEAM GASIFICATION OF DENSIFIED BIOFUELS

1 ST WORLD PELLETS NFERENCE SEPTEMBER 2-7, 2002, STOCKHOLM, SWEDEN HIGH TEMPERATURE AIR AND STEAM GASIFICATION OF DENSIFIED BIOFUELS C. Lucas*, D. Szewczyk, W. Blasiak Royal Institute of Technology (KTH)