Efficient Conversion of Solid Biomass into Gaseous Fuel

Transcription

1 Efficient Conversion of Solid Biomass into Gaseous Fuel Dr. Aysha Irshad Dept. of Chemical Engineering, University of Engineering & Technology, Lahore, Pakistan Prof. Gordon E. Andrews, Dr. Herodotos N. Phylaktou Prof. Bernard M. Gibbs School of Chemical & Process Engineering, University of Leeds Presented by: Prof. Gordon Andrews 12 th ECCRIA Conference, Cardiff University, Cardiff, UK 5th-7th September 2018

2 Importance of biomass as fuel Renewable source Carbon neutral if sustainability is maintained School of Chemical and Process Engineering, University of Leeds, UK 2 The use of biomass for heat usually involves two stage combustion Two stage combustion systems include moving grate systems pellet and chip biomass boilers log boilers sometimes called gasification boilers

3 Primary air Secondary air School of Chemical and Process Engineering, University of Leeds, UK 3 Gasification zone 1 Moving grate two stage combustion continuous fuel addition Oxidation zone 2 Moving grate systems Can use wood chips or pellets or logs Electricity production using steam turbine Two stage combustion with rich primary or underfire air and overfire or secondary air to complete the combustion. Used for generation of electricity in the 1 50 MW range.

4 Solid biomass combustion system Solid biomass combustion systems are usually two staged Rich combustion stage in which gasification reaction results in the formation of CO & H 2 School of Chemical and Process Engineering, University of Leeds, UK 4 O 2 sensor Primary air Secondary combustion stage where excess air reacts with gases from gasification stage to burn them completely Drying Gasification Air Fan Overall excess air is controlled via oxygen sensor, in most units primary air is usually a fixed ratio of the overall excess air Disadvantage of water cooling of gasification zone Fuel added typically once per day Secondary air combustion Ash Secondary air hole at throat between gasifier zone and secondary combustion zone

5 School of Chemical and Process Engineering, University of Leeds, UK 5 TGA analysis

6 School of Chemical and Process Engineering, University of Leeds, UK 6 Objective To optimise the gas yield and thermal efficiency of the first stage of two stage biomass combustion. Thermal Efficiency = Energy in Gases From the First Stage Gasification rich combustion Energy in the original biomass on daf basis This is sometimes called the CGE combustion gas efficiency In this work we are using the heat of rich combustion to generate the temperature and to operate in the temperature region that TGA analysis shows that 80% of the volatiles are released from biomass o C, which will undergo rich combustion to generate CO and H 2 plus Hydrocarbons if there is inefficiency in the rich burning.

7 School of Chemical and Process Engineering, University of Leeds, UK Chimney The Cone Calorimeter with controlled atmosphere air box 12 th ECCRIA The European Conference on Fuel and Energy Research and its Applications 2018 Oct. 5-7 Cardiff University, Wales

8 School of Chemical and Process Engineering, University of Leeds, UK 8 1 Test biomass placed on the Load cell here 3 Air flow set to achieve rich combustion. Gas composition is CO, hydrogen and hydrocarbons. Effectively this is an upward flow gasifier insulation 2. cooling jacket on load cell 3. Load cell Cone calorimeter insulated Confined atmosphere air box Air supplied through two pipes in Bottom of the compartment 12 th ECCRIA The European Conference on Fuel and Energy Research and its Applications 2018 Oct. 5-7 Cardiff University, Wales

9 School of Chemical and Process Engineering, University of Leeds, UK 9 Ash rich zone gasification with 70 kw/m 2 radiant heat flux. Air flow 9 g/m 2 s which is a HRR of 27 kw/m 2 5 sticks of ash

10 Experimental setup of Cone calorimeter The cone calorimeter is common equipment in fire research used to determine HRR by oxygen consumption Incident heat flux from conical heater is variable Wood and biomass samples were placed in a sample holder 100mm x 100mm x mm, an insulation of mm was placed underneath the biomass that made total height of sample holder 40mm. A number of experiments were performed to achieve the gasification conditions in the cone calorimeter enclosure School of Chemical and Process Engineering, University of Leeds, UK 10

11 School of Chemical and Process Engineering, University of Leeds, UK 11 Gas sampler with 20 sample holes Pine wood sample arranged in sample holder Thermocouples inserted into the wood a different distances from the heat source.

12 Efficient Conversion of Solid Biomass into Gaseous Fuel A. Irshad, G.E. Andrews, H.N. Phylaktou, H. Li and B.M. Gibbs School of Chemical and Process Engineering, University of Leeds, UK 12 Pine wood Dry ash wood Eucalyptus wood A range of 9 biomass have been studied Included two supplied to me on a trip to China a few years ago. White wood pellets Sunflower shell pellets Grade B torrified wood pellets 12 th ECCRIA The European Conference on Fuel and Energy Research and its Applications 2018 Oct. 5-7, Cardiff University, Wales China biomass black China biomass skin Corn cobs

13 School of Chemical and Process Engineering, University of Leeds, UK 13 Near steady State burning Most volatiles are released at this temperature of the wood at the top surface. Volatiles are being released from the wood below the surface for a long time after the 600s test period shown here. Temp. vs time of pine wood gasification at 70 kw/m 2 at Ø m = 2.8

14 Equilibrium calculations School of Chemical and Process Engineering, University of Leeds, UK 14 Pine wood CEA (chemical equilibrium with applications) programme by NASA was used to predict the equilibrium composition for the products of combustion, constant enthalpy and pressure system HP problem was chosen with Ø values from 0.5 to 8 with one temperature to start iterations Input to the software was, Elemental composition of the biomass Standard heat of formation of biomass calculated by method of Zainal et al. (2001)[2]. 2. Zainal, Z.A., Ali, R., Lean, C.H., and Seetharamu, K.N., Prediction of performance of a downdraft gasifier using equilibrium modeling for different biomass materials. Energy Conversion and Management, (12): p Equilibrium concentrations and adiabatic flame temperature of gaseous products as a function of equivalence ratio (Ø) for pine wood

15 School of Chemical and Process Engineering, University of Leeds, UK 15

16 School of Chemical and Process Engineering, University of Leeds, UK 16 Equivalence ratio (Ø) Hot gas efficiency HGE The equivalence ratio is the ratio of the stoichiometric air to fuel ratio to that of measured air to fuel ratio by mass A F Ø m = Stoichiometric A F measured Emission index EI ( g species /kg biomass ) or Yields = kg species /kg biomass EI is related to the volumetric specie concentration C and exhaust A/F ratio by mass HGE = [ {H.H.V of product gases+sensible heat of the gases}( MJ H 2 concentration H.H.V of the fuel ( Water gas shift equilibrium [ CO][ H 2O] K [ CO2 ][ H 2] kg biomass ) MJ ]x100 kg biomass ) EI = K C 1 + A F where K is a function of equilibrium temperature, here a value of 3.5 is used, which corresponds to T eq 1738 K [1]. K = Ratio of the molecular weight of gas component to that of exhaust sample 1. Chan., S.H., An exhaust emissions based air-fuel ratio calculation for internal combustion engines. Proc. Instn Mech Engrs, Part D: Journal of automobile engineering, : p

17 School of Chemical and Process Engineering, University of Leeds, UK 17 Table 1. Elemental analysis, Proximate analysis, CV and stoichiometric air to fuel ratio for biomass studied Biomass C % daf H % daf N % daf S % daf O % daf VM % daf VM % ar FC % daf H 2 O % Ar Ash % ar CV MJ/kg Stoich. (A/F)(g/g) Actual Daf actual Daf Pine wood Ash wood (dry) Ash wood (Wet) Eucalyptus Wood (Pakistan) Acacia Wood (Pakistan) Block wood Sycamore Wood White Wood processed pellets Grade B torrified wood processed pellets Sunflower Shell processed pellets Mountain ash raw pellets

18 School of Chemical and Process Engineering, University of Leeds, UK 18 Biomass C % daf H % daf N % daf S % daf O % daf VM % daf VM % ar FC % daf H 2 O % Ar Ash % ar CV MJ/kg Stoich. (A/F)(g/g) Actual Daf actual Daf China s biomass skin (China) China s biomass black (China) SPF ( Spruce, pine, Fir) raw SPF torrefied Grade B wood Grade B torrified wood Corn cobs (Pakistan) Wheat straw (Pakistan) Rice husk (Pakistan) Stoichiometric air to fuel ratio for these biomasses vary from 4.7 to 7.5

19 Results and discussion School of Chemical and Process Engineering, University of Leeds, UK 19 The cummulative mass of the gases flowing up the chimney from the rich burning gasification zone agrees very well with the loss in mass of the biomass. This means that the FTIR calibration is good and all the significant species have been determined.

20 School of Chemical and Process Engineering, University of Leeds, UK 20 Pine wood rich burning gasification Steady state 70 kw/m 2 radiant heat Ø m with time for pine wood at different air flow rates

21 School of Chemical and Process Engineering, University of Leeds, UK 21 Steady state Equivalence ratio varied by changing the primary zone air flow indicated Ø m is for the steady state period

22 School of Chemical and Process Engineering, University of Leeds, UK 22 Pine wood gasification 70 kw/m 2 radiant heat CO Adiabatic equilibrium Measured There is zero equilibrium THC so all these THC are rich combustion inefficiency HC and for efficient energy transfer they must make the second stage combustion as a significant part of the biomass energy is in these hydrocarbons. Inefficient transfer of these hydrocarbons to the burner/ engine / gas turbine reduces the overall thermal efficiency of the process.

23 UK standard for domestic water heating boiler is to have minimum thermal efficiency of 86%. Air flow 19.2 kg/m 2 s School of Chemical and Process Engineering, University of Leeds, UK 23 MJ/ Kg biomass HGE as a function of Ø m for pine wood 70 kw/m 2 radiant heat Trimethylbenzene Xylene Benzene Toluene Ethylene Acetylene Heating value as a function of time at Ø m = 2.8

24 School of Chemical and Process Engineering, University of Leeds, UK 24 MJ/ Kg biomass Sensible heat 1. Berend Vreugdenhil ERC/TNO Netherlands 12 th ECCRIA 2018 Th. 6A V.Lavrenov, Russian Acad Sci 12 th ECCRIA 2018 Th 5A Pedro Abelha ECN/TNO 12 th ECCRIA 2018 Th. 5A Measurements of biomass gasification gas composition % + CGE% % CO % H % CH % C n H m 3 C 2 H % C 2 H % Benzene ppm Toluene ppm CGE 76% 68-79% % Trimethylbenzene Xylene CO THC H 2 Benzene Toluene Ethylene Acetylene Heating value as a function of time at Ø m = 2.8

25 MJ/ Kg biomass, School of Chemical and Process Engineering, University of Leeds, UK kw/m 2 radiant heat Pine wood Air = 25.6 kg/m 2 s Pine wood Air = 31.6 kg/sm 2 HGE = 86% HGE = 90 % Heating value as a function of time at Ø m = 2 Heating value as a function of time at Ø m = 1.6 Towards the end, HV of the gases is increasing showing that more char is present and higher thermal efficiencies can be achieved, In these two tests MLR was quick initially and some char burning zone was achieved within test time.

26 School of Chemical and Process Engineering, University of Leeds, UK 26

27 School of Chemical and Process Engineering, University of Leeds, UK 27 HGE = 55% Ø = ~4.5 HGE = 80 % Ø = ~6 MJ/ Kg biomass HGE = 42 % Ø = ~3.5 HGE = 81 % Ø = ~6

28 School of Chemical and Process Engineering, University of Leeds, UK 28 Different equivalence ratios are obtained at fixed air flow of air due to differences in the elemental composition and physical nature of the biomass: solid, pellet or powder. Flaming combustion zone Air flow 19.2 kg/m 2 s Char burning zone

29 School of Chemical and Process Engineering, University of Leeds, UK 29 Rich burning of pellets have caused inefficiency and low yield. Need to optimise primary gasification zone to achieve maximum yield of the gasification products EI CO / yield g/kg biomass vs time for different biomass

30 School of Chemical and Process Engineering, University of Leeds, UK 30 Conclusions: There exists an optimum, on an energy conversion basis, equivalence ratio for the primary gasification zone of two stage burning. For pine wood it was 2.8. The results for different biomass indicates that the optimum equivalence ratio was different for different biomass. This implies that optimisation of a two stage burner would require the ability to control the air split as well as the overall excess air. 80% energy conversion from solid biomass to gas was demonstrated for pine wood at Ø = 2.8. The most important gases in order of energy content were CO, H 2, acetylene, ethylene, toluene, benzene, xylene and trimethyl-benzene. There was no significant methane. The cone calorimeter is a good experimental tool to characterise the combustion and gasification of biomass.