PREDICTING THE EFFECTS OF FUEL PROPERTIES ON COMBUSTION PERFORMANCE By AP MANN Queensland University of Technology, Brisbane a.mann@qut.edu.au KEYWORDS: Fuel, Boiler, Cogeneration, CFD, Modelling. Abstract AS THE SUGAR industry generates and exports more renewable electricity it will be more common for sugar mill boilers to be fired with bagasse that has been stored for extended periods. In many cases, year round cogeneration requires the use of supplementary, preferably renewable, fuels. Consequently, sugar factory boilers will have to overcome the challenge of reliably producing steam with a wider range of fuels. If a boiler cannot maintain stable combustion of the fuel in suspension then its steam output and efficiency will be adversely affected. Determining how well a fuel or fuel combination will burn is often done by trial and error but this can be time consuming and expensive. Computer modelling is an attractive alternative to the trial and error approach though experimental verification will remain an essential adjunct. In this work the boiler heat transfer modelling program BOILER and the Computational Fluid Dynamics (CFD) code FURNACE were used to predict how changing fuel properties will affect the operation of a typical sugar factory boiler. The calculations indicate that if all the heat liberated during slow oxidation of the combustible fibre in stored bagasse is used to evaporate moisture from the stockpile then the amount of combustible bagasse fibre saved due to improved boiler efficiency will more than offset the combustible fibre lost during storage. The calculations assume that the heat generation per unit mass of combustible fibre during storage is the same as the gross calorific value of the combustible fibre. More detailed study of the slow oxidation reactions is required to determine whether or not this is the case. The FURNACE model simulations predict that bagasse with reduced volatile matter content, a possible consequence of long term storage, will give higher unburnt fuel losses and therefore will significantly reduce boiler efficiency. Introduction More sugar factories are exporting significant amounts of renewable power (Lowry et al., 2001; Palmer et al., 2009; Trayner, 2008) and the sugar industry is 629
supporting research into manufacturing processes for high value by-products that use bagasse as a feedstock (O'Hara et al., 2009). Year round electricity export and the manufacture of by-products make it necessary to store bagasse for extended periods (Trayner, 2008). Bagasse and supplementary renewable fuels such as green waste (Lowry et al., 2001), sawmill residues and camphor laurel (Palmer et al., 2009) will be affected by chemical reactions and microbial activity during extended storage. Therefore sugar factory boilers operating during the off season are likely to have to produce steam from a fuel supply with variable properties. If a boiler cannot maintain stable combustion of the fuel in suspension then its steam output and efficiency will be adversely affected. Determining how well a fuel or fuel combination will burn is often done by trial and error but this can be time consuming and expensive. Computer modelling is an attractive alternative to the trial and error approach though experimental verification will remain an essential adjunct in at least the medium term. In this work the boiler heat transfer modelling program BOILER (Dixon et al., 1998; Plaza et al., 2006) and the CFD code FURNACE (Luo, 1993; Mann, 1996; Mann and Kent, 1994; Woodfield, 2001) have been used to predict how changing fuel properties will affect the operation of a typical sugar factory boiler. Background Many stockpiles exhibit the phenomenon of self heating that in some cases can lead to spontaneous combustion of part or all of a stockpile. Past research indicates that most of the self heating is due to slow wet and dry oxidation reactions (Dixon, 1988; Sexton et al., 2000). The wet oxidation reaction, as the name suggests, occurs in the presence of moisture and is the dominant self heating reaction at stockpile temperatures around 60 C. At higher temperatures, the dry oxidation reaction dominates. Self heating typically causes internal stockpile temperatures to increase to around 60 C within a few days of stockpile construction. If the rate of heat loss from a stockpile (primarily via moisture evaporation) exceeds the rate of self heating, the stockpile temperature will gradually decrease until it reaches the ambient air temperature. If the rate of heat loss from a stockpile equals the rate of self heating, the stockpile temperature will remain relatively constant at around 60 C. If the rate of heat loss from a stockpile is less than the rate of self heating, the stockpile temperature will increase. Spontaneous combustion is likely to occur once the stockpile temperature exceeds 90 C. When the rate of self heating is balanced by the rate of heat loss at an elevated temperature, bagasse drying can occur without risking spontaneous combustion (Sexton et al., 2001). If it is assumed that the stockpile is heated solely by the wet and dry oxidation reactions and that evaporation is the only way the stockpile loses heat, then the energy for evaporation of moisture from a stockpile must come from the oxidation of (and therefore loss of) combustible fibre in the stockpile. The loss of combustible fibre can be calculated from the amount of moisture evaporated, the latent heat of evaporation of water at the stockpile temperature and the heat liberated 630
by the self heating oxidation reactions (assumed here to be the gross calorific value of combustible fibre in the bagasse). Perfect storage conditions are also assumed. That is, the possibility of moisture migrating into the stockpile from the environment is not considered. Procedure The modelled average properties of bagasse used in the construction of a 50 000 t stockpile are shown in Table 1. The loss of combustible fibre, total stockpile mass loss and therefore bagasse composition were calculated for different amounts of water evaporated from the stockpile. The boiler heat transfer modelling program BOILER was used to predict how the different bagasse compositions affect boiler efficiency and bagasse consumption. These BOILER model simulations were based on the Pleystowe No. 2 boiler which has been studied extensively (Dixon, 1984; Dixon et al., 1998; Luo et al., 1993; Woodfield, 2001; Woodfield et al., 1997; Woodfield et al., 1998). The steam and flue gas conditions for the Pleystowe No. 2 boiler used in this modelling work are summarised in Table 2. Table 1 Modelled bagasse properties at the start of stockpile construction. Moisture content (% as received) 50 Ash content (% as received) 2.5 Combustible fibre content (% as received) 47.5 Volatile matter yield (% dry ash free) 85 Element % dry ash free Carbon 50.51 Ultimate analysis Hydrogen 6.31 Oxygen 43.06 Nitrogen 0.12 Sulphur 0 Gross calorific value (kj/kg dry ash free) 19 667 Table 2 Modelled steam and flue gas conditions for the Pleystowe No. 2 boiler. Steam flow (t/h) 150 Steam temperature ( C) 304 Steam pressure (kpag) 1517 Flue gas oxygen content (% wet basis) 4 Final flue gas temperature 1 ( C) 200 1 For bagasse with the properties shown in Table 1. Parts of bagasse stockpiles have shown evidence of charring (Dixon, 1988) which indicates some loss of volatile matter. The volatile matter content of bagasse is usually high. The value of 85% (dry ash free basis) shown in Table 1 is typical. The heating value of char is higher than the heating value of most of the gases released during bagasse pyrolysis (Lamb, 1979) so the loss of volatile matter alone does not imply any reduction in bagasse heating value. However, in a furnace, volatile matter is released quickly and combustion of volatile matter occurs rapidly. It is likely that volatile matter combustion supplies most of the heat needed for bagasse drying in the near spreader region of a furnace. 631
To quantify the effect of volatile matter content, the CFD code FURNACE was used to compare the combustion performance of the Pleystowe No. 2 boiler when fired with bagasse with different volatile matter contents. The FURNACE code predicts the gas velocity and particle trajectory distributions along with the gas and particle phase combustion that occurs in a boiler. The three dimensional CFD representation of the Pleystowe No. 2 boiler used 1 334 416 cells to represent the flow of gas through the furnace, superheater, convection bank and air heater. Modelling of furnace radiation was carried out on a coarser grid with 2 880 cells. The motion of bagasse / ash particles through the boiler was approximated by tracking 17 280 representative particle trajectories. Results Figure 1 shows the calculated effect of evaporation on the composition of a bagasse stockpile assuming that slow oxidation of combustible fibre supplies all the energy used for evaporation. The loss of combustible fibre due to slow oxidation is less than the loss of moisture due to evaporation so the proportion of combustible fibre in the stockpile increases. If enough evaporation occurs to reduce the stockpile moisture content to 40%, the combustible fibre content of the bagasse increases from 47.5% to 56.9% and its ash content increases from 2.5% to 3.1%. moisture combustible fibre ash 60 bagasse composition (% wet basis) 50 40 30 20 10 0 0 5 10 15 20 25 30 35 40 evaporation (% initial water mass in stockpile) Fig. 1 Calculated effect of evaporation on the composition of bagasse in a stockpile. Calculations assume slow oxidation of combustible fibre supplies all the energy used for evaporation. 632
The calculated effect of evaporation on the mass of combustible fibre in a stockpile and the total stockpile mass is shown in Figure 2. If sufficient moisture is evaporated from a stockpile to reduce the average final moisture content to 40% (36.2% of the initial water mass) then the amount of combustible fibre in the stockpile will reduce by 4.6% and the total stockpile mass will reduce by 20.3%. Note the calculations used to produce Figure 1 and Figure 2 do not take into account any water leaching out of the stockpile or any water entering the stockpile either by rain or condensation of atmospheric water vapour. combustible fibre mass loss moisture content total stockpile mass loss 25 55 mass loss (% initial mass) 20 15 10 5 50 45 40 moisture content (% wet basis) 0 0 5 10 15 20 25 30 35 40 evaporation (% initial water mass in stockpile) 35 Fig. 2 Calculated effect of evaporation on the loss of combustible fibre and loss of total stockpile mass. Calculations assume slow oxidation of combustible fibre supplies all the energy used for evaporation. Table 3 shows the calculated quantities of water, ash and combustible fibre in a 50 000 t bagasse stockpile with the bagasse properties at the start of stockpile construction (Table 1) and for stockpile conditions where slow oxidation of combustible fibre has reduced the final moisture content of the stockpile via evaporation to 48%, 46%, 44%, 42% and 40%. If the moisture content of the stockpiled bagasse can be reduced to 40%, 9 054 t of water will be evaporated and 1 082 t of combustible fibre will be lost. However, the reduced bagasse moisture content will significantly improve boiler performance. The BOILER program was 633
used to simulate boiler performance with the different bagasse compositions. These performance predictions, also in Table 3, show the boiler efficiency (GCV basis) increases by more than 5 percentage points (from 64.2% to 69.8%) as the bagasse moisture content reduces from 50% to 40%. Consequently boiler bagasse consumption reduces by approximately 23% and boiler combustible fibre consumption reduces by approximately 7.7%. The original quantity of 50% moisture content bagasse in the 50 000 t stockpile is sufficient to produce 113 774 t of steam from the Pleystowe No. 2 boiler. With the 40% moisture content bagasse the same quantity of steam can be produced using 1 835 t less combustible fibre. Taking into account the combustible fibre loss during storage there should be a net combustible fibre saving of 753 t which corresponds to 1 325 t of the 40% moisture content bagasse. There is a net saving of combustible fibre and bagasse because in a stockpile the energy in the combustible fibre is used to evaporate moisture at the temperature of the stockpile while the boiler evaporates the moisture and heats the water vapour to the final flue gas temperature of the boiler (around 200 C for the Pleystowe No. 2 boiler). Table 3 Predicted effects of different amounts of evaporation from a 50 000 t bagasse stockpile on boiler performance and bagasse consumption. Moisture content (% as received) 50 48 46 44 42 40 Ash content (% as received) 2.50 2.63 2.75 2.88 3.01 3.14 Combustible fibre content (% as received) 47.50 49.37 51.25 53.12 54.99 56.86 Moisture content (t) 25000 22839 20877 19088 17450 15946 Ash content (t) 1250 1250 1250 1250 1250 1250 Combustible fibre content (t) 23750 23492 23257 23044 22848 22668 Evaporation during storage (t) 2161 4123 5912 7550 9054 Combustible fibre loss during storage (t) 258 493 706 902 1082 Steam flow (t/h) 150 150 150 150 150 150 Boiler final flue gas temperature ( C) 200 197 195 193 191 189 Boiler efficiency (% GCV 1 basis) 64.2 65.51 66.71 67.82 68.86 69.83 Boiler bagasse consumption (t/h) 65.92 62.23 58.92 55.95 53.25 50.81 Boiler combustible fibre consumption (t/h) 31.31 30.72 30.19 29.72 29.28 28.89 Combustible fibre saving during operation (t) 445 848 1208 1539 1835 Net combustible fibre saving (t) 187 355 502 637 753 Net bagasse saving (t) 379 694 944 1158 1325 1 Gross calorific value Note that the loss of combustible fibre calculations assume the heat generation per unit mass of combustible fibre during storage is the same as the gross calorific value of the combustible fibre. An earlier review (Lamb, 1979) describes how different stages of bagasse combustion liberate different amounts of heat. Little is known about the slow oxidation reactions that occur during bagasse storage but it is likely that the heat generated per unit mass of combustible fibre during slow 634
oxidation is less than the gross calorific value of the combustible fibre. If so, the actual loss of combustible fibre during storage would therefore be higher than the calculated quantities shown in Figure 1, Figure 2 and Table 3. In a furnace, volatile matter combustion is likely to supply most of the energy required for bagasse drying. The FURNACE code was used to predict how reducing the volatile matter content of bagasse affects fuel burnout. Figure 3 shows that as the volatile matter content reduces the predicted unburnt fuel loss increases significantly. Char has a higher calorific value than bagasse so the effect on the unburnt fuel boiler efficiency loss (also shown in Figure 3) is even greater. Figure 4 shows side elevation views of the predicted gas temperature distributions in the Pleystowe No. 2 boiler with typical (85% dry ash free) and reduced (65% dry ash free) volatile matter contents. With the reduced volatile matter content the flame is closer to the rear furnace wall and gas temperatures in the bagasse drying zone near the spreaders are lower. With lower volatile matter content a greater proportion of the combustible fibre is consumed in the slower char burnout phase of combustion. This, along with the reduced rate of bagasse drying, will slow the rate of bagasse combustion and therefore increase unburnt fuel losses. unburnt fuel loss unburnt fuel boiler efficiency loss 10 9 10 9 unburnt fuel loss (% dry ash free) 8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1 unburnt fuel boiler efficiency loss (% GCV basis) 0 0 30 40 50 60 70 80 90 volatile matter content (% dry ash free) Fig. 3 Predicted effect of volatile matter content (% dry ash free) on unburnt fuel loss and unburnt fuel boiler efficiency loss. 635
Fig. 4 Side elevation view of the predicted gas temperature ( C) distributions in the furnace of the Pleystowe No. 2 boiler at 150 t/h steam output with typical bagasse volatile matter content (85%) and reduced volatile matter content (65%). Arrows show approximate height of spreaders. Conclusions The calculations in this work show that if the heat liberated during slow oxidation of the combustible fibre in stored bagasse is all used to evaporate moisture from the stockpile the amount of bagasse combustible fibre saved due to improved boiler efficiency will more than offset the combustible fibre lost during storage. The calculations assume that the heat generation per unit mass of combustible fibre during storage is the same as the gross calorific value of the combustible fibre. More detailed study of the slow oxidation reactions is required to determine whether or not this is the case. The FURNACE model simulations predict that bagasse with reduced volatile matter content, a possible consequence of long term storage, will give higher unburnt fuel losses and therefore will significantly reduce boiler efficiency. Acknowledgements The past and ongoing support of Australian sugar factories, the Sugar Research and Development Corporation (SRDC) and the Australian Research Council (ARC) for sugar factory boiler and bagasse stockpile research is gratefully acknowledged. REFERENCES Dixon TF (1984) Preliminary measurements in the flame region of a bagasse fired boiler. Proceedings of the Australian Society of Sugar Cane Technologists 6, 165 171. 636
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