CUSTOM DESIGNED FLUID BED CALCINER

FOR NABALCO PTY LTD

D J Brodie

Nabalco Pty Ltd

H W Schmidt

Lurgi Metallurgie GmbH

ABSTRACT

The Nabalco Alumina Refinery at Gove in the Northern Territory of Australia commenced operation in 1972. Four oil fired rotary kilns are used for the calcination of alumina trihydrate to alumina. Continually increasing production rate has led to the requirement for additional calcination capacity. For this reason a project was commenced to install a modern Fluid Bed Calciner at the refinery.

The economic justification of the project required a calciner of sufficient capacity that two rotary kilns could be shut down, providing significant fuel and maintenance savings. A stringent requirement of particle breakage to be no more than 1% higher than that in the rotary kilns was imposed to ensure product quality was not compromised. Further limitations on product alumina temperature and particulate emissions added to the unique design of the calciner.

Lurgi (Australia) Pty Ltd, were successful with their bid to supply a 2700 mtpd Circulating Fluid Bed Calciner for the Nabalco Refinery. The calciner was designed specifically for Nabalco’s requirements, based on the successful design, operation, upgrading and modification of the 35 Lurgi calciners operating on Alumina worldwide.

 

CUSTOM DESIGNED FLUID BED CALCINER

FOR NABALCO PTY LTD

D J Brodie and H W Schmidt

1.0 INTRODUCTION

Nabalco Pty Limited operates an alumina refinery on the Gove Peninsula in the Northern Territory of Australia. The refinery was commissioned in 1972 with a design capacity of 1.0 million tonnes per year of metallurgical grade "floury" alumina. The current production rate is 1.75 million tonnes per year of "sandy" alumina. Due to its participation in the competitive world market, Nabalco is under continual pressure to reduce production cost, increase production output and to maintain and improve certain product quality parameters. These factors, combined with the requirement to increase the calcination capacity, has led to the decision to partially replace existing rotary kiln capacity with modern Circulating Fluid Bed Calcination (CFBC) technology. The expenditure was economically justified by:

  • Increased overall calcination capacity
  • Reduced fuel consumption
  • Steadier refinery operation
  • Reduced maintenance costs

The benefits of using Circulating Fluid Bed Calciner technology have to be off set against an expected increase in particle breakage. Through 27 years of operating experience and recently through the application of Computational Fluid Dynamics, Lurgi have been able to improve the design, resulting in a reduction in particle breakage in their calciners.

Nabalco’s specific requirements for the new calciner are outlined in this paper. The specific design changes made to achieve these requirements are also discussed.

    1. SPECIFIC REQUIREMENTS OF NABALCO CALCINER
    2. Production Capacity

The current production capacity of the refinery is 1.75 million tonnes per year (1997). The requirement to conduct refractory and other mechanical repairs every 12-18 months results in the current yearly calcination capacity of 1.72 million tonnes per year. The difference is sold as alumina trihydrate (referred to as "hydrate") to various customers. Sales of hydrate are less profitable than alumina; therefore increased calcination capacity is required to eliminate the need for hydrate sales.

The strategic plan for Nabalco requires incremental increases in alumina production, requiring additional calcination capacity. The most cost-effective method of increasing calcination capacity was determined to be the installation of a Circulating Fluid Bed Calciner of 2700 tpd nominal capacity.

2.2 Specific Energy Consumption

The parameter that is used to define fuel consumption is Specific Energy Consumption (SEC). This is defined as the amount of heat used per tonne of alumina calcined, the units being GJ/t. Further it is now accepted that the SEC is based on the net calorific value. The SEC of the standard Lurgi CFBC has been reported as 3.050 GJ/t. 1(Schmidt 1996)

The rotary kilns consume on average 100 kg of heavy fuel oil per tonne of alumina calcined. Based on the net calorific value of heavy fuel oil of 39728 kJ/kg, the SEC of the rotary kilns is 4.02 GJ/t. This figure includes the heat input from the sensible heat of the fuel, which is heated to 120C prior to combustion, and from the high pressure steam used to atomise the fuel. The reference temperature is 60C. The major areas of heat loss from Nabalco’s rotary kilns are indicated in Figure 1.

Figure 1

Rotary Kiln Heat Losses

The heat of conversion of alumina trihydrate and the evaporation of free moisture in the feed areas can be considered independent of the calcination technology. The radiant and convective heat loss, flue gas heat loss and product alumina heat losses are affected by the calcination technology used. The general design of CFBC’s allows a significant reduction in the radiant and convective heat loss because thicker refractory can be employed.

The general design of CFBC’s also allows a reduction in the heat losses due to flue gas and product alumina. Recovery of heat from the flue gases is limited by the acid dew point, particularly in Nabalco’s case with the use of high sulphur (3.5%) fuel oil. It was necessary to ensure the acid dew point is not reached, while maximum possible heat is extracted from the flue gases. To achieve this with varying fuel sulphur levels, a facility was included in the Nabalco calciner for a small amount of hydrate to bypass the hydrate pre-drying sections, directly to the furnace. This results in less heat being recovered from the waste gases, ensuring that the gas temperature remains above acid dew point at all times.

Efficient recovery of heat from the product alumina is an integral part of the design of the CFBC. The standard design involves a single fluidised bed cooler with a single cooling cyclone where the fluidising air becomes the secondary air feed to the furnace (See Figure 4). To achieve the desired fuel economy and product temperature the design for the Nabalco calciner had to be further developed.

A recent improvement to the standard Lurgi CFBC design is the Hydrate Bypass system.(Schmidt 1996) This system takes hydrate from the first pre-drying stage to the alumina outlet of the furnace. A reduction in SEC can by achieved with the Hydrate Bypass. The Hydrate Bypass system has been installed on a trial basis at an Australian alumina refinery. While positive results have been achieved, the system was not considered proven technology at the time that the contract was awarded. It will now be incorporated into the Nabalco calciner, but will not be operated during the production guarantee test. The guaranteed production rate, particle breakage, SEC and product quality has to be achieved without operation of the Hydrate Bypass system. Therefore, to achieve the SEC specified by Nabalco, the calciner design was modified to increase the recovery of sensible heat from the product.

The Circulating Fluid Bed System requires a split flow of the combustion air into primary and secondary airflows. As it is also necessary to provide dust free primary air direct preheating of the total combustion airflow in several cyclone stages cannot be used to recover the sensible heat of the hot alumina.

To achieve the desired fuel economy and product temperature, the Nabalco CFBC includes an additional cyclone between the two fluid bed coolers for the air and water-cooled sections. Figure 5 in Section 4.0 shows a flowsheet with the improved cooling section. The guaranteed product exit temperature from the water-cooled fluid bed cooler is 60C.

The typical SEC of Circulating Fluid Bed Calciners operating on alumina with heavy fuel oil is in the range of 2.9 – 3.1 GJ/t. (Schmidt 1996) The guaranteed maximum SEC of the Lurgi calciner is 2.75 GJ/t using high sulphur fuel oil. This is to be achieved with the improved cooling system but without the Hydrate Bypass system.

2.3 Refinery Operation

A significant advantage to be afforded by increased calcination capacity at Nabalco will be smoother refinery operation. Bauxite input is generally not reduced when a kiln is taken off line for maintenance. Bauxite input is maintained at a rate equivalent to 1.75 million tonnes per year of alumina. Prior to the shutdown, the calcination rate is increased above the equivalent alumina input rate. This has the effect of "stripping" hydrate from the precipitation circuit. When the kiln is taken off line, the other three rotary kilns operate at maximum capacity. Since this is well below the equivalent bauxite input of 1.75 million tonnes per year, hydrate builds up in the precipitation circuit. When the maintenance is complete, the four rotary kilns are operated at maximum rates to return the precipitation solids to the normal operating level. Operating the precipitation circuit in this way has the following effects:

  • Reduced liquor productivity during periods of lower solids level.
  • Increased scaling during periods of high solids.
  • Adverse impact on product size control due to limitations of the classification system at high solids levels.

2.4 Maintenance Costs

Maintenance shutdowns of the rotary kilns are a significant cost. In addition, "half-life" refurbishment of the kilns will be required soon. The capacity of the new calciner was selected so that two rotary kilns can be decommissioned.

 

2.5 Particle Breakage

2.5.1 General

Of major significance during the decision to expand calcination capacity at Nabalco was the need to safeguard the reputation of Nabalco as a supplier of consistent high quality alumina. The main quality parameters affected by the choice of calcination technology are the amount of fine material and alpha alumina content. The amount of fine material is important for dusting and materials handling considerations. Fine material is defined as the amount of material that passes through a 45 m sieve. The increase in fines between hydrate and calcined alumina is termed particle breakage.

Studies have linked alumina dustiness to the amount of fines in the alumina. ,(Allais 1996, Syltevic 1996) The absolute amount of fine material in the alumina leaving the refinery logically has as effect on dustiness. Given that modern refineries now produce sandy alumina with –45 m consistently less than 10%, the particle breakage on handling of the alumina between the refinery and the smelting cell can have a significant effect on dustiness. (Welch 1993)

The amount of fines in alumina also has an effect on the flowability of the material. A critical value of 7% has been proposed, above which alumina flowability changes significantly. Another factor affecting flowability is the presence of a wide particle size distribution. 3(Allais 1996)

While the absolute value of –45 m content is of vital interest to smelter operators, it is the variability in this and other parameters that is of greatest concern to smelters.

2.5.2 Factors that Effect Particle Breakage

Particle breakage is affected by the hydrate properties and by the processing during calcination. The major hydrate property is the strength of the particle. The strength of Nabalco’s hydrate was inferred from the behaviour of the hydrate in existing calcination equipment.

The particle breakage experienced in the existing rotary kilns at Nabalco has been used as a base line for measuring the performance of the new FBC. During performance testing for the FBC, the particle breakage on both 44 and 20 m sizes will be compared with the existing calciners for a period of one week. While alumina particle size is normally measured by sieving (-45 m) the Malvern Laser Particle Size Analysis (-44 m) will be used for both hydrate and alumina. The particle breakage on the –44 m fraction, as determined by Malvern Laser Analysis, in the existing rotary kilns is shown in Figure 2.

Figure 2

Nabalco Monthly Alumina Size and Particle Breakage

(Malvern Laser Particle Size Analysis)

The way in which the hydrate is treated depends predominantly on the type of calcination technology employed. The effect of thermal shock on the particle is the subject of current research. Particle-particle collisions and particle wall collisions are known to cause attrition. Minimisation of the effect of these collisions is an area of ongoing development at Lurgi. The new Nabalco calciner can be regarded as containing the latest improvements in this area.

Lurgi have guaranteed that particle breakage in the new calciner, operating at the maximum continuous rating of 2700 tpd, will be no more than one percent higher than the breakdown in the rotary kilns, based on –44 m size, and no more than 0.5 percent higher for the –22 m size.

2.5.3 Measures to Reduce Particle Breakage

As reported earlier ,(Schmidt 1980, Schmidt 1992) the velocity in certain areas of the calciner such as venturis and cyclone inlets must be limited. Sharp edges and sharp bends also need to be avoided in areas where the gas solid flow is transported.

It has clearly been found from investigation of operating calciners, that attrition takes place in the areas mentioned above and not in the CFB, where the velocities are relatively low. It should be pointed out that the concentration of the solids in the CFB is in an area which can be defined as a highly expanded fluid bed (Reh 1971) which is significantly less than in a standard fluid bed.(Squires 1994)

In order to reduce the attrition from the influence of high velocities in critical areas, a Computational Fluid Dynamic (CFD) model of the gas/solids flow in segments of the calciner was developed. Development of the model was done in cooperation with CSIRO.

The results of the CFD modelling provided a solution in a three-dimensional flow field in several critical areas such as cyclone inlets, solid feeding point, venturi nozzles and ducts. With the support of these results these areas were optimised to ensure a minimum differential velocity between solids and gas.

3.0 CAPACITY INCREASE WITH HYDRATE BYPASS

The design provides a possibility to increase the capacity of the calciner by use of the so-called Hydrate Bypass. As shown in Figure 5, a certain quantity of the pre-dried hydrate (about 10% - 15%) can be introduced directly to the calciner discharge alumina flow with a temperature of about 960 C. The partial hydrate stream is calcined directly with part of the enthalpy of the hot alumina. This enthalpy does not have to be recovered by preheating of the primary and secondary airflows in the fluid bed cooler. Therefore, the capacity of the cooler (which was designed for operation with no hydrate bypass) allows a higher throughput, which results in an increased plant capacity. Table 1 shows the influence of the Hydrate Bypass on operation of the calciner.

Parameter

Units

No Bypass

Spare Blower

Offline

No Bypass

Spare Blower

Online

With Bypass

Spare Blower

Online

Capacity

tpd

2,700

3,000

3,180

Specific energy consumption

GJ/t

2.9

2.86

2.75

Stack gas temperature

C

170

170

170

Alumina discharge temperature

C

60

62

59

LOI at 300 to 1,000 C

%

0.79

0.79

0.89

a -Alumina

%

5

5

5

BET

m2/g

75

75

75

Table 1

Operating Parameters with Hydrate Bypass and Additional Blower

Table 1 shows that with the increase in capacity achieved by using the Hydrate Bypass, the LOI also increases slightly.

The Loss on Ignition (LOI) can be controlled to the required quality by adjusting the following control parameters:

- Hydrate Bypass Mass Flow

- Calcination Temperature

Figure 3 shows the influence of these two control parameters on the product quality parameters, Loss on Ignition (LOI) and Specific Surface Area (SSA, also referred to as BET). The results are from a test on a 2100 tpd production unit in Australia, where the Hydrate Bypass was installed.

Figure 3

Effect of Hydrate Bypass Flow and Calcination Temperature on Quality

4.0 IMPROVED ALUMINA COOLING

As mentioned earlier, the evaluation for the Gove calciner was done without using the Hydrate Bypass operation. The efficient recovery of the sensible heat of the calcined alumina by preheating the primary and secondary combustion airflows has become an important issue. The flowsheet of the standard CFBC one cooling cyclone concept is shown in Figure 4.

Figure 4

CFBC Flowsheet with Single Cooling Cyclone

Several alternatives were evaluated to achieve an optimal cooling concept while at the same time making use of the advantages of the existing CFB system. The result is a cooling system with two cooling cyclones. The flowsheet for this system is shown in Figure 5.

Figure 5

Flowsheet of CFBC with Improved Cooling System

and Hydrate Bypass

The influence of the second cooling cyclone is demonstrated in Table 2, in which the specific energy consumption figures for the one and two cooling cyclone concepts are compared. The influence of the Hydrate Bypass flow is also indicated in Table 2.

Furnace

Temperature

( C)

LOI

(%)

Hydrate

Bypass

Flow (%)

Specific

Energy

Consumption

(GJ/t)

No 2nd cooling cyclone

960

0.8

9.5

2.93

No 2nd cooling cyclone

960

0.7

0

3.05

With 2nd cooling cylone

960

0.8

9.5

2.75

With 2nd cooling cyclone

960

0.7

0

2.94

Table 2

Effect of Two Cooling Cyclones and Hydrate Bypass on SEC

It is obvious that the most efficient operation will be achieved when Hydrate Bypass operation is combined with the effect of the second cooling cyclone.

 

5.0 CURRENT STATUS

The calciner at the Gove refinery is presently under construction. The commissioning of the new plant is scheduled for August 1999.

6.0 CONCLUSION

The calciner design has been specially modified to meet Nabalco’s requirements. The nominal capacity, low specific energy consumption, product temperature and low particle breakage will be achieved because of these design changes.

The most significant single modification is the addition of the second cooling cyclone to meet the Specific Energy Consumption and product temperature targets. The guaranteed specific energy consumption of 2.75 GJ/t and product temperature of 60C will be achieved at 2700 tpd production rate without the Hydrate Bypass system operating. The hydrate bypass system and an additional blower will be used to provide the maximum capacity of 3180 tpd.

The results of CFD modelling and Lurgi’s operating experience on existing calciners has been applied to the Nabalco design to achieve the stringent particle breakage requirement of less than 1.0% greater than that in the existing rotary kilns.

With a daily production rate of 2700 tpd nominal capacity and 3180 tpd with Hydrate Bypass operation, the calciner will be the largest single alumina producing unit in the world.

Figure 6

2700 tpd CFBC with Two Cooling Cyclones