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Using fluidized bed technology to treat bagasse-based black liquor could be a lucrative option for small pulp and paper mills
by NK Sharma, KS Bhargava and SK Sharma
A good recovery for FBR technology
No pulp and paper mill using a chemical pulping process could
be described as environmentally friendly without some form of chemical recovery system. This is true whatever type of fiber is being pulped, whether it is wood-based or agro-based. In bagasse mills, there are three main systems which are can recover chemicals from the resulting black liquor - the roaster and smelter system, the conventional recovery boiler and the fluidized bed reactor.
In the roaster and smelter system, black liquor is burnt in a roaster and, with the help of auxiliary fuel, the resulting burnt ash is incinerated in a smelter and dissolved in weak wash to give green liquor. This is then converted into white liquor using lime in a recausticizer.
Although this method is simple in operation and needs low investment costs, it does have its drawbacks. It continually needs auxiliary fuel and it has a very low percentage of chemical recovery. Deposition on boiler tubes is also a major problem if the fuel gases are used for generating steam.
The second method is the conventional recovery boiler. Here black liquor (solid content >60%) is sprayed inside the furnace and burned at high temperatures, typically around 1,000C. The heat that is generated is used to produce steam and the molten sodium compounds are dissolved in weak washwater to obtain green liquor, again for further treatment in a causticizer plant.
 The FBR soda recovery process is ideal for small to medium sized pulp and paper mills
The major advantages of this system are its high thermal efficiency and chemical recovery percentage. Unfortunately, it has a high capital investment, complicated operation and there is a possibility of a smelt water explosion.
The third system is the fluidized bed reactor (FBR). In this system, black liquor is concentrated to just 22-25% in a multiple effect evaporator, but it is concentrated further (to 45% solids) with the help of flue gases in a venturi scrubber and cyclone system. With agro-based raw materials, silica content in the black liquor is high and at high solids concentrations it is very viscous. But as the solids content is low in the multiple effect evaporator the chances of silica scale deposition in the evaporator tubes is cut and the need for cleaning is reduced.
There are a number of advantages of this system including:
- low capital investment
- safe operation as there is no smelt
- applicability to all cellulosic raw materials except raw materials having high chloride and potassium contents, although the chloride content in almost all cellulosic raw material can be brought down through wet cleaning
- it is not essential to install a recausticizer plant
- it is very easy to run the plant with little manpower.
For one and a half years, one pulp and paper mill in India, Shreyans Papers, has used an Enders-Agro FBR for treating 75 tons/day of black liquor solids produced from bagasse (60%) and wheat straw (40% after wet cleaning) raw material.
The mill is situated in the green belt of Punjab, where wheat straw, sarkanda and bagasse are available in abundance as cellulosic raw materials for making paper. Rice huck is also available in sufficient quantity as an alternate fuel to coal to generate steam and power.
Before the installation of the FBR, the main hurdle facing Shreyans Papers was to reduce the biological oxygen demand (BOD) levels in its effluent. The mill had already installed two activated sludge treatment stages.
Shreyans added one extra washer to its three-stage countercurrent brown stock washing system to bring the level of total dissolved solids in the weak black liquor to 8%. Free residual active alkali in weak black liquor was also increased to 6.0 g/l to give the black liquor better fluidity at high concentrations and to avoid the precipitation of lignin.
The mill claims that the multiple (six-stage) effect evaporator plant has not experienced any problems with the black liquor at 24% solids, although occasional water/caustic boiling and tube cleaning using high pressure water (600 bar) are necessary to keep the evaporator tubes clean. There have been no major problems in concentrating the black liquor from 22% to 45% solids in the venturi scrubber system either.
During the startup of the FBR, the temperature is raised at a rate of 45C per hour by firing diesel in the air pre-heater. This helps protect against any adverse effect on the refractory lining of the FBR. After a temperature of 480C is reached, sodium carbonate pellets are introduced into the bed to raise its depth by 204 mm/hr at a uniform temperature.
| Figure 1 - Savings Achieved by Running FBR Soda Recovery Plant at Black Liquor Solids of 75 tons/day |
| Expenses |
| Fixed cost |
Rs 885,000 |
| Diesel consumption per month |
Rs 102,000 10,000 l @ Rs 10.20/l |
| Charcoal consumption per month |
Rs 45,000 10,000 kg.@ Rs 4.50/kg |
| Power consumption |
Rs 1.04 million 400,000 units @ Rs 2.6/unit |
| Steam consumption 4,006 tons |
Rs 1.522 million (3,306 tons in evaporator, 400 tons in FBR) @Rs 380/ton |
| Others |
Rs 34,000 |
| Total |
Rs 3.628 million |
| Income |
| Sodium carbonate pellets production |
Rs 3.78 million per month 540 tons @ Rs 7,000/ton |
| Saving in power, urea and DAP |
Rs 365,000 in wastewater treatment |
|
| Total |
Rs 4.145 million |
| Net savings |
| Savings per month by installing |
Rs 517,000 |
| non-conventional FBR Recovery Plant |
| (Rs40=$1) |
When the bed height reaches 1,830 mm its temperature is raised to 540C, charcoal is then injected into the FBR and the temperature allowed to rise by 1.5C/min. When the temperature of the lower and upper bed reaches 670C, the black liquor is sprayed through the feed gun at a droplet size of 2.4-4 mm.
Once the bed stabilizes, both diesel and charcoal supplies are cut off and the feed rate of black liquor into the FBR is adjusted so that the temperature in the upper and lower beds remain steady at approximately 670C.
Problem solved
In the beginning, Shreyans had a lot of trouble getting the bed to defluidize. The main reason for this was that the chloride content in black liquor, as NaCl, was more than 0.5%, due to the use of wheat straw without wet cleaning.
The mill introduced a wet cleaning system for wheat straw including a screw press. With this arrangement more than 40% wheat straw along with bagasse can be used as agro-cellulosic raw materials, while maintaining the chloride content in the black liquor below 0.5% NaCl.
There is no need for a causticizer plant as the sodium carbonate pellets produced by the process are sold to a nearby manufacturer of sodium silicate helping the operation to make an overall saving (Figure 1). The process has also prevented contamination of its wastewater with weak black liquor, which used to contain a high BOD loading and had a very high percentage of sodium compounds.
Shreyan's experience demonstrates that small and medium sized agro-based integrated pulp and paper mills could successfully convert black liquor from agricultural residues into sodium carbonate pellets. The mill reports that the FBR soda recovery plant is low capital cost, occupies less space, has safe and easy operation and a low manpower requirement when compared to conventional soda recovery plant. It is not as energy efficient as a conventional plant but it is suited for handling high viscosity, high silica content black liquor produced from agricultural residues.
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