optimisation of low load boiler operations.pdf

Mann AP, Scott SJ Proc Aust Soc Sugar Cane Technol Vol 33 2011 ______________________________________________________________________________________

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OPTIMISATION OF LOW LOAD BOILER OPERATIONS

By

AP MANN1, SJ SCOTT2

1Queensland University of Technology, Brisbane 2Harwood Mill and Refinery, Harwood

[email protected] KEYWORDS: Bagasse, Boiler, Efficiency, Combustion.

Abstract

NEW SOUTH WALES Sugar Milling Co-Op is required to operate its Harwood boiler at low load during the non-crushing season to supply refinery steam demand. Boiler loads in the slack season typically fall to around 25% of Maximum Continuous Rating (MCR), resulting in inefficient operation due to the elevated excess air ratios required to maintain suspension burning. Excess bagasse generated during the crushing season is insufficient to supply the full refinery off-season demand, resulting in the purchase of other biomass fuels or coal. As part of Harwood’s EEO (Energy Efficiency Opportunities) initiatives, trials were designed and undertaken to operate the boiler with air entering a reduced portion of the grate, with the specific aim of improving low load boiler efficiency. The successful trials with modified mode of operation gave significantly lower flue gas losses to produce a 2.7% increase in boiler efficiency and a 4.5% reduction in fuel consumption. This work will form the basis for further extended operational trials in 2011 designed to establish permanent, lower cost, low risk, reliable and sustainable low firing operations.

Background

New South Wales Sugar Milling Co-Op is required to operate its Harwood boiler (MCR 160 t/h, 1 720 kPa g steam pressure, 258°C steam temperature) at low load during the non crushing season to supply refinery steam demand.

Boiler loads in the non-crushing season typically fall to around 25% of MCR, resulting in inefficient operation due to the elevated excess air ratios required to maintain suspension burning. Excess bagasse generated during the crushing season is insufficient to supply the full refinery off-season demand, resulting in the purchase of other biomass fuels or coal.

As part of Harwood’s EEO (Energy Efficiency Opportunities) initiatives, trials were designed and undertaken to operate the boiler with air entering a reduced portion of the grate, for the specific aim of improving low load boiler efficiency.

Introduction

Primary combustion air enters the furnace of the Harwood boiler through six ducts. Each duct supplies air to a separate chamber under the grates from where the air passes through the tiles and into the furnace. When operating at reduced capacity it is common to utilise only three or four of the six feeders but in these circumstances primary air is still distributed to all six grates because of a desire to avoid grate overheating. As a consequence air velocities through the grates tend to be low and large ratios of excess air need to be introduced to maintain reasonable fluidisation of the fuel. The chambers under each grate are isolated from each other so it is possible to use the dampers


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in the air inlet ducts to control the combustion air flow into each grate. If combustion can be confined to a smaller part of the furnace then it should not be necessary to supply air to all grates and the total amount of air entering the furnace should reduce, thereby increasing boiler efficiency.

During normal operation of the boiler at low load, fuel enters the furnace through No. 1, No. 3, No. 4 and No. 6 fuel feeders and combustion air enters through all six grate air inlet ducts (the fuel feeders and grate air inlet ducts are number 1 to 6 from left to the right looking from the front of the boiler).

A trial was designed for a modified mode of operation where all the fuel would enter the boiler through the middle two feeders (No. 3 and No. 4) and all the undergrate combustion air would enter through the corresponding grate air inlet ducts (No. 3 and No. 4). At the pre-trial planning meeting the main risks discussed related to closing the dampers in the No. 1, No. 2, No. 5 and No. 6 under grate air inlet ducts and the potential for overheating of the corresponding furnace grates. There was also a concern that the reduced flue gas flow through the convection bank would lead to reduced turbulence and therefore heat transfer. It was decided to proceed with the trials to determine if the boiler could operate continuously in the modified mode.

If these trials were successful, efficiency testing would be carried out to compare boiler efficiency and fuel consumption for the normal and modified modes of boiler operation. It was decided that the boiler fuel during the combustion trials and efficiency testing would be a mixture of woodchip and sawdust.

Procedure

Trials were carried out on the 2 June 2010 with the fuel feeders turned off one by one and the grate air inlet duct dampers shut one by one until only the No. 3 and No. 4 feeders were operating and only the No. 3 and No. 4 grate air inlet duct dampers were open. The air flow into the furnace was then slowly reduced by adjusting the speed of the forced draft fan. The air flow is measured by a venturi downstream of the forced draft fan. This air subsequently passes through the air heater and enters the furnace through the grate and secondary air nozzles. The air used by the spreaders is supplied by a separate fan and is not included in the venturi air flow measurement.

After each change the boiler was allowed several minutes to adjust to the changed conditions before a further change was made. Static pressures in the grate air inlet ducts were measured before and after each change and furnace combustion behaviour was observed through the viewing windows under the spreaders. Boiler instrumentation logs the temperatures in each of the grate chambers and these temperatures were monitored to identify any possible overheating of the grate tiles. The results of these preliminary trials suggested that all parameters of concern were able to be controlled without the risk of damage and without compromise to the responsiveness of the boiler to meet steam demand from the refinery.

On the 3 June 2010 efficiency testing of the boiler when operating in normal and modified modes was carried out. The original plan was to have each efficiency test last for two hours. As part of each efficiency test, gas temperatures and compositions were measured at four locations across the width of the air heater gas exit ducts (two locations per duct) using a Testo 350-XL gas analyser.

A measurement traverse across the air heater gas exit ducts was completed every half hour. Ambient air temperature and relative humidity were measured every half hour at the inlet to the forced draft fan using a TSI VelociCalc Plus meter. Fuel (a mixture of woodchip and sawdust) was sampled from the belt upstream of the ploughs every half hour. Ash from the conveyor downstream of where the ash from the undergrate, submerged ash belt and the fly ash from the flue gas dust collectors combine, was also sampled every half hour. The half-hourly samples of fuel and ash


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samples were bulked to form a combined fuel and a combined ash sample for each test. The combined fuel samples were subsequently sent away for analysis to determine their moisture content, ash content, ultimate analysis and calorific value. The combined ash samples were analysed for loss on ignition which is the commonly used method for determining the combustible matter content. Boiler instrumentation recorded the other information required for the efficiency testing (steam flow, steam temperature, steam pressure and feedwater temperature).

The boiler efficiency was calculated according to the losses method described in BS2885:1974 (British Standards Association, 1974). In the calculations, radiation losses were estimated using the standard chart from the American Boiler Manufacturers’ Association (ABMA) (Stultz and Kitto, 1992) and an unaccounted efficiency loss of 0.5% was assumed. The efficiency calculations require an estimate of the ash flow rates to calculate unburnt solid fuel losses.

It is difficult to measure the ash flows from the boiler so the procedure adopted in this work was to estimate the ash flow rate from the calculated fuel rate and ash content of the fuel from the fuel analysis. An initial boiler efficiency so calculated was used to update the calculated fuel and therefore ash flow rates. The calculations were repeated until the calculated quantities did not change. The loss on ignition analyses were done on samples that included ash from the submerged ash belt and the fly ash screw conveyors.

Results

Combustion trials

The combustion trials carried out on the 2 June 2010 were successful and the boiler was able to produce 45 t/h steam output when operating in the modified mode. The logged grate chamber temperatures increased when the boiler was operating in modified mode but were not high enough to be of major concern to factory staff. At times during the trial there was fuel build up on the unused grates from carry over but this build up would mostly burn away without requiring intervention by operators.

The air flow recorded by the boiler instrumentation reduced from 105 t/h for normal operation to as low as 82 t/h when the boiler was operating in the modified mode. The power consumption of the forced draft fan motor reduced from around 70 kW for normal operation to around 25 kW when operating in the modified mode. The logged oxygen concentration at the exit of the convection bank reduced from approximately 12% (wet basis) for normal operation to approximately 9% (wet basis) when operating in the modified mode.

Efficiency testing

The efficiency test for normal operation of the Harwood Mill boiler at low load was carried out between 9.40 am and 11.40 am on the 3 June 2010 (test 1). The efficiency test for modified operation of the boiler (test 2) commenced at 12.40 pm but the test had to be terminated prematurely at 1.40 pm because there was a problem with the grate tiles above the No. 5 grate chamber. These tiles were stuck in a slightly open position which caused fuel that would normally burn on the grate to fall through the openings into the grate chamber where it would either burn causing overheating of the grate tiles or fall onto the submerged ash belt. Unfortunately the ash collected during test 2 was contaminated by a large quantity of unburnt fuel that had fallen onto the submerged ash belt. This contamination of the ash with unburnt fuel affected the boiler efficiency and rate of fuel consumption calculations.

Table 1 summarises the average steam and feedwater conditions during the efficiency tests along with the air heater exit gas and ambient air measurements and fuel and ash analyses for each of the tests. The measured flue gas oxygen concentrations at the outlet of the air heater are very


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high for both tests. There was almost no carbon monoxide produced during test 2 and the average final flue gas temperature was approximately 14°C lower during test 2 than test 1. This indicates the convection bank and air heater were more effectively recovering heat from the flue gas during modified operation of the boiler. This significantly increases boiler efficiency.

Table 1—Summary of the steam and feedwater conditions, average gas and ambient air conditions and fuel and ash analyses for each of the efficiency

tests on the Harwood boiler at low load.

Test 1 Test 2 Steam flow (t/h) 44.2 44.0 Feedwater temperature (°C) 105.3 104.0 Steam pressure (kPag) 1 780 1 774 Steam temperature (°C) 245.7 245.5 Air heater exit gas measurements Average oxygen concentration (% dry basis) 14.6 14.0 Average carbon monoxide concentration (ppm dry basis) 358 15 Average gas temperature (°C) 193.8 179.7 Ambient air measurements Average temperature (°C) 22.8 20.9 Average relative humidity (%) 47.9 44.9 Fuel analyses Moisture (% as received) 38.5 38.5 Ash (% as received) 2.22 1.29 Carbon (% as received) 31.07 31.13 Hydrogen (% as received) 3.37 3.52 Nitrogen (% as received) 0.08 0.08 Sulphur (% as received) 0.05 0.04 Oxygen (% as received) 24.72 25.41 Gross calorific value – as received (kJ/kg) 12 140 12 143 Gross calorific value (GCV) – dry ash free (kJ/kg) 20 477 20 166 Ash analyses Moisture (% as received) 30.77 44.52 Loss on ignition (% dry basis) 27.55 52.83

It is notable that the loss on ignition of the ash collected during test 2 is very high (Table 1). This was almost certainly caused by fuel falling through the partially open grate tiles and on to the submerged belt. The moisture content of the ash collected during test 2 is higher than that of the ash collected during test 1 which is consistent with this explanation. The partially open grate tiles also made it difficult to reduce the excess air as much as desired during test 2 which is why the average oxygen concentration measured at the exit of the air heater during test 2 was only slightly lower than the average oxygen concentration measured at the exit of the air heater during test 1.

The information in Table 1 was used to calculate the boiler efficiency according to the losses method of BS885:1974 (British Standards Association, 1974). The main losses and boiler efficiency for each of the tests are shown in Table 2. The unburnt solid fuel percentage is much higher in test 2 than in test 1 and this difference is almost certainly due to the high amount of unburnt fuel in the ash collected during test 2 (Table 1).

This large amount of unburnt fuel in the test 2 ash sample was caused primarily by some of the grate tiles being stuck in slightly open positions, allowing woodchips to fall onto the submerged belt. When the grate tiles are all in closed positions, it is not possible for unburnt woodchips to fall onto the submerged belt. To correct for the abnormally high unburnt fuel levels in the ash collected during test 2, an additional set of efficiency calculations were carried out with the loss on ignition adjusted so that the calculated unburnt solid fuel loss for test 2 would be the same as that for test 1. The main losses and boiler efficiency from this additional set of calculations are shown in the test 2 adjusted column of Table 2.


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Table 2—Summary of the efficiency calculations for test 1, test 2 and an adjusted version of test 2 with the same unburnt solid fuel losses as test 1.

Test 1 Test 2 Test 2 adjusted Dry gas loss (%) 18.10 15.31 15.62 Fuel moisture loss (%) 8.78 8.72 8.72 Combustion moisture (%) 6.92 7.18 7.18 Air moisture (%) 0.25 0.18 0.18 Carbon monoxide (%) 0.35 0.01 0.01 Unburnt solid fuel (%) 2.35 4.01 2.35 Radiation (%) 1.55 1.55 1.55 Unaccounted (%) 0.50 0.50 0.50 Boiler efficiency (% GCV basis) 61.19 62.54 63.89

When the unburnt solid fuel losses for test 2 were made the same as those for test 1, the calculated boiler efficiency increased to 63.89%, which was 2.7% higher than the calculated boiler efficiency for test 1. The main reason for the higher boiler efficiency is the lower dry gas loss.

During test 2 less flue gas was leaving the air heater and this flue gas was at a lower temperature. Note that the combustion moisture loss for test 2 is slightly higher than for test 1. This can be explained by the hydrogen content of the combined fuel sample from test 2 being slightly higher than the hydrogen content of the combined fuel sample from test 1.

Table 3 shows the calculated fuel, air and flue gas flows for test 1, test 2 and the adjusted version of test 2. The calculated fuel flow for the adjusted version of test 2 is approximately 4.5% less than for test 1 which is a significant fuel saving.

The corresponding predicted reductions in air and flue gas flow rates are over 10%. Note from Table 2 that the carbon monoxide losses for test 2 are negligible which indicates good combustion. It is quite likely that if the problem with the partially open grate tiles did not occur, the unburnt fuel losses for test 2 would have been less than those for test 1 and the reduction in fuel, air and flue gas flows would have been greater than those shown in Table 3.

Table 3—Calculated fuel, air and flue gas flows for test 1, test 2 and an

adjusted version of test 2 with the same unburnt solid fuel losses as test 1.

Test 1 Test 2 Test 2 adjusted Fuel flow (t/h) 14.60 14.24 13.94 Air flow (t/h) 169.65 151.11 150.81 Flue gas flow (t/h) 183.81 164.97 164.45 Reduction in fuel flow relative to test 1 (%) 2.42 4.48 Reduction in air flow relative to test 1 (%) 10.93 11.11

The calculated air flows in Table 3 are significantly higher than the air flows measured by the venturi in the forced draft fan outlet duct (106.2 t/h for test 1 and 85.2 t/h for test 2). The air entering the furnace from the spreader air fan would only account for a small portion of this difference. It should be noted that when the excess air levels are high, the calculated air flow is strongly affected by the measured oxygen concentrations.

If air heater leaks were worse near the measurement locations then the measured oxygen concentrations would not be representative of the average oxygen concentration of the gas leaving the air heater and the calculated air flows would be too high.

Furthermore, the accuracy of the venturi flow measurement is not known – the venturi reading too low could also partly explain the calculated air flows being higher than the measured air flows.


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Conclusions

This work has shown that, provided the problem with the grate tiles staying partially open can be addressed, the modified mode of operation using No. 3 and No. 4 feeders and air inlet ducts, along with reduced air flow from the forced draft fan, significantly increases boiler efficiency.

The modified mode of operation significantly reduces flue gas losses to produce a 2.7% increase in boiler efficiency and a 4.5% reduction in fuel consumption. Further extended operational trials are planned for 2011 to establish permanent, lower cost, low risk, reliable and sustainable low firing operations.

Acknowledgements

The assistance of Harwood Mill staff with the conduct of the combustion trials and efficiency tests is gratefully acknowledged. New South Wales Sugar Milling Co-Op is acknowledged for giving permission to publish this paper.

REFERENCES British Standards Association (1974) Code for acceptance tests on stationary steam generators of

the power station type. BS2885:1974. Stultz SC, Kitto JB (eds) (1992) ‘Steam – Its Generation and Use’. 40th edn (Babcock and Wilcox

Company: Barberton, Ohio, USA).

optimisation of low load boiler operations.pdf

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