Maritime University of Szczecin
Akademia Morska w Szczecinie
2012, 32(104) z. 1 pp. 136–140 2012, 32(104) z. 1 s. 136–140
The influence of the ship’s motion on the sea wave
on the capacity of the fluidised-bed boiler
Wojciech Zeńczak
West Pomeranian University of Technology, Faculty of Maritime Technology and Transport Department of Heat Engines and Marine Power Plants
71-065 Szczecin, al. 41 Piastów, e-mail: wojciech.zenczak@zut.edu.pl Key words: ship boiler, fluidised bed, experiment, ship’s motions
Abstract
The article presents the selected results of the experimental research of the influence of the ship’s motion on the sea waves on the value of the heat transfer coefficient between the fluidised bed and the heating surface of the ship’s fluidised-bed boiler immersed therein. Pursuant to the research results the likely boiler capacity change has been estimated caused by this disturbance. The research has been conducted at the original physical boiler model.
Introduction
The application of the fluidised bed technology is one of the methods to increase the intensity of the heat transfer process, both in the waste-heat, as well as oil-fired boilers. Moreover, in the comparison to the boiler with the classic furnace it is possible to burn in the fluidised bed the liquid and solid fuels which are cheaper, have lower calorific value and high sulphur content in the manner that is safer for the environment owing to the limitation in the NOx
and SOx emissions. More and more often in the
power engineering the fluidised bed-based furnaces are also used for burning of the solid biomass.
In the circumstances, when more profound fuel crisis has appeared in the effect of the political situation in the world, the faster depletion of the crude oil reserves and the imperfection of the new power engineering technologies, one should con-sider realistically the return to coal burning on the ships. However, in view of the chief negative as-pect of the coal burning which is a large CO2
emis-sion, in comparison to the other fuels, its use would be possible only providing the clean technologies are applied using amongst others the burning in the fluidised bed.
For these reasons the author has been for some time conducting the experiments on the physical models of the fluidised-bed boilers in terms of the
behaviour of the fluidised bed, as well as the examination of the heat transfer process in the con-ditions involving the disturbances caused by the ship’s motion on sea wave. The description of the research stand, the applied measurement methods and the results of many research works, obtained with the application of the first version of the boiler model have been presented inter alia in the works [1, 2, 3, 4, 5].
The remarks concerning the physical model of the fluidised-bed boiler
In order to increase the scope of the applicability of the boiler model in its second version, the sig-nificant changes have been implemented in relation to its fluidising column. In comparison to the first boiler model with the circular section of the fluidis-ing column, the new model involves the rectangular section corresponding in a larger degree to the actual boiler structures. Moreover, a provision has been made for placing the heating element (heating probe) at the various heights of the column. The heating element has been, similar as in the first version of the stand, the miniature cylinder probe. The heating probe has been attached to the trans-parent cap which might have been located in one of the openings of the column side wall. The remain-ing column openremain-ings have been blanked by the
transparent caps to guarantee the smoothness of the column outer surface. On the new stand, similar, like in the first version, it is possible to examine both the bubbling and the circulating fluidised beds (Fig. 1).
Fig. 1. The diagram of the fluidising column with the fluidised bed material return system
The results of the examinations of the average coefficients of heat transfer in the circulating beds, published in [5, 6], display the significant relation of the coefficient value inter alia to the distance of the probe from the grid. These examinations, how-ever, have not included the cyclic column deflec-tions from the vertical (the swinging / pendulous movements) which are likely to occur in the operat-ing conditions of the fluidised-bed boiler on the ship. The new construction of the column makes it possible amongst others to examine the average coefficient of heat transfer in the circulating bed depending on the probe location height over the grid during the cyclic swinging movements of the column.
The selected examination results
The examinations aiming at the determination of the average coefficient of the heat transfer between the probe surface and the fluidised bed at various heights over the air distributor have been conducted for the bubbling bed of the standstill height
H = 0.085. The probe has been placed at the three
levels z = 0.3 m, z = 1 m and z = 1.5 m in the plane parallel to the distributor. The velocity of the fluid-ising air has been constant and amounted to w = 6.5 m/s. The fluidising column has remained still in the
vertical position. The results of the examinations are presented in the figure 2.
Fig. 2. The relation of the average coefficient of heat transfer between the fluidised bed and probe surface to the height over the distributor at the constant velocity of the fluidising air
w = 6.5 m/s
The course of the relation indicates the decrease of the average heat transfer coefficient together with the probe distance from the air distributor. This should be explained by the decrease in the bed material concentration in the higher parts of the fluidising column. A similar course of the relation has also been obtained by the authors of the publi-cation [7].
Throughout all the examinations of the heat transfer process in the bubbling bed in the column being in the swinging motion it has been disclosed that the value of the heat transfer coefficient is smaller during the swinging movements than as the column remain still. Moreover, within the investi-gated column motion periods it has turned out that the shorter the motion period the smaller the heat transfer coefficient are. The figure 3 presents the
Fig. 3. The average heat transfer coefficient between the bub-bling bed and the probe surface at the blower constant rpm value (n = 4400 min–1) during the column swinging motion
with various periods and at the immovable vertical position (T = ∞) T = ∞ T = 57 s T = 34 s 166,57 142,38 138,26 120 125 130 135 140 145 150 155 160 165 170 α, W /m2K [W /m 2 K] [W/m2K] z [m] 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 air outlet cyclone a b c d A
fluidising air inlet
auxiliary air
distributor a, b, d – blanking caps for
the openings in column for the alternative positioning of the cap c/w probe c – cap c/w probe
A
probe
graph of the values of the heat transfer coefficient in the bubbling bed at the constant rpm of the blower (n = 4400 min–1) for the motionless column in the vertical position and for the periods of the swings T = 57 s and T = 34 s at the deflection angle from the vertical equal to 30º. The height of the bed in the standstill amounted to H = 0.12 m. With the smaller rpm values of the blower, thus with the smaller average velocity of the fluidising air jet, the same tendency has been observed, namely the decrease of the heat transfer coefficient accompany-ing the shorter swaccompany-ingaccompany-ing periods.
The investigations have been conducted also for the other velocities of the fluidising air jet and for the other bed height in standstill, i.e. H = 0.08 m. Also the column deflection angle has been reduced down to 25º. The figure 4 shows the average heat transfer coefficients in the bubbling bed with the three different rpm values of the blower (n = 4557 min–1, n = 5107 min–1 and n = 5264 min–1) as
the functions of the average angular velocity of the column swings. In this case, in the graph the column swinging period has been replaced by the average angular velocity of the swinging which is inversely proportional to the period value. The condition when the column remains immovable corresponds to the angular velocity of ω = 0. The troublesome determination on the graph of the infi-nite value of the swinging motion period is being avoided in this way.
Fig. 4. The average heat transfer coefficient between the bub-bling bed and the probe surface at the various blower rpm values as the function of the average angular velocity of the column swinging [pendulous] movements (the deflection angle 25º, bed height H = 0.08 m)
Quite another tendency of the changes of the average heat transfer coefficient occurs in the circu-lating bed within the fluidising column which is brought into the swinging motion simulating the ship’s rolling movements. In this case, as the swinging starts the value of the average heat trans-fer coefficient grows between the circulating fluid-ising bed and the probe surface.
In the experiment confirming such nature of the changes of the average heat transfer coefficient, the recording of the measurement results has been commenced already before the bubbling bed has completed the transition into the circulating bed state. Bringing the column into the swinging mo-tion has taken place only after the circulating bed has been formed. Thus, the decrease of the average heat transfer coefficient has been shown during the transition from the bubbling bed state into the cir-culating bed state, caused by the diminishing of the material concentration. In the circulating bed state the fluidising air jet velocity has been kept constant at the level of w = 7.3 m/s. The bed height in stand-still has been H = 0.085 m. The figure 5 shows the time course of the temporary values of the average heat transfer coefficient during the transition from the bubbling bed into the circulating bed state (the air velocity grows from w = 6.14 m/s up to w = 7.3 m/s – immovable column) and its change in the circulating bed upon the column swinging motion commencement (since 12:31:41 at the time axis).
The observations have shown that the column swinging motion, even at the large air velocity, characteristic for the circulating fluidising bed, cause the accumulation (in the effect of throwing) of the larger amount of grains on the column oppo-site walls. In the effect, those grains while slipping down the column increase the wall-adjacent zone and also the material concentration in the core of the jet in the investigated bed section flushing the probe, thus, contributing to the increase of the value of the heat transfer coefficient.
The increase of the value of the average tempo-rary heat transfer coefficient in the circulating bed upon the commencement of the swinging motion has also been disclosed in the first model with the circular section column [4]. In the investigations with both models there have also been disclosed the characteristic cyclic changes in the value of the heat transfer coefficient corresponding to the period of the swinging motion.
The evaluation of the influence of the chan-ges in heat transfer on the boiler capacity
The changes in the value of the heat transfer coefficient between the fluidised bed and the probe
110 115 120 125 130 135 140 145 150 0 0,01 0,02 0,03 0,04 0,05 0,06 0,07 ω ,1/s α, W/m2K n=4557 1/min n=5107 1/min n=5264 1/min [1/s] 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 [W/m2K] ◊ n = 4557 min–1 □ n = 5107 min–1 ∆ n = 5264 min–1 150 145 140 135 130 125 120 115 110
surface observed in the laboratory conditions will in the actual boilers contribute to the changes in the boiler output. A quick, simplified evaluation can be conducted as shown below.
The heat jet Q transferred to the heat absorbing surface of the boiler could be determined from the relation:
t kF
Q (1)
where:
F – the boiler surface;
∆t – the average temperature difference be-tween the fluidised bed and the working medium (steam-water mixture).
The heat transmission coefficient k for the clean surface is expressed by the formula:
2 1 1 1 1 s k (2) where:
s – the thickness of the partition wall;
λ – partition material heat conduction
coeffi-cient;
α1 – heat transfer coefficient between the
flui-dised bed and the heat exchanger surface;
α2 – heat transfer coefficient between the
ex-changer surface and the working medium. It can be assumed that the heat transmission coefficient k is equal approximately to the heat transfer coefficient α1 from the bed to the surface,
because in the equation (2) the segments 1/2 and
s/ as very minor may be omitted in the technical calculations, ie:
1
k (3)
In the effect, the assumed heat jet will be pro-portional to 1, i.e.:
Q ~ 1 (4)
Thus, the decrease of the heat transfer coeffi-cient 1 in the bubbling bed (eg by 15% from the
example in Fig. 3) due to the occurrence of the column swinging motion (ship’s rolling motion) in the similar degree will influence the heat jet value and boiler capacity.
Similarly, the growth of the heat transfer coeffi-cient 1 observed in the circulating bed upon the
occurrence of the swinging motion (eg by 8% from the example in Fig. 5) shall cause the boiler capac-ity increase.
Conclusions
The investigations conducted show that if a fluidised-bed boiler with the bubbling bed is in-stalled on the ship, then a noticeable decrease in the boiler capacity should be taken into consideration in the circumstances when significant ship’s motion on the wave appears. In case of the fluidised-bed boilers with the circulating bed, whose installation is possible rather on big ships on account of their significant size, in such circumstances a favourable phenomenon of a small increase in the capacity occurs. Thus, it should be concluded that for the economic operation of a ship where fluidised-bed boiler is applied a consideration is to be given to the possible changes in its capacity while sailing on the moderate sea.
Fig. 5. The course of the changes of the temporary average heat transfer coefficient during the transition from the bubbling bed (12:28:48) to the circulating bed (12:31:41) and upon bringing the column into the swinging motion with the period of 45 s (12:32)
125 130 135 140 145 150 12:27:22 12:28:48 12:30:14 12:31:41 12:33:07 12:34:34 12:36:00 12:37:26 α, W/m2K
The bed gets into circulating state (w = 6.14 to w = 7.3 m/s) –
immovable column
The circulating bed – swing-ing motions with period of
45 s, w = 7.3 m/s B ub bl in g be d Time [W/m2K]
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The study financed from the means for the education within 2009–2012 as own research project No. NN 509 404536.
