ENERGY FROM BIOMASS IN PULP & PAPER MILLS
Pulp and paper mills generate various quantities of energy-rich biomass as wastes, depending on the technological level, pulp and paper grades and wood quality. These wastes are produced in all stages of the process : wood preparation, pulp and paper manufacturing, chemical recovery, recycled paper processing, waste water treatment. Energy recovery from wastes of different origin has become a generally accepted alternative to their disposal.
The pulp and paper industry expresses an interest in adapting and integrating advanced biomass energy conversion technologies into its mill operations. Industrial adoption of these new technologies has the potential for higher efficiency, lower capital costs, and safer operation than conventional operation that burn fossil fuels for energy. Incineration with energy recovery has the advantage of hygienic disposal, volume reduction, and the recovery of thermal energy by means of steam or super-heated water that can be used for heating and power generation.
Energy recovery from wastes of different origin, e.g. industrial or agricultural, has become moreover a generally accepted alternative to their disposal or incineration. Waste-to-energy is gaining more and more attention as landfill costs and environmental concern increase and, at the same time, space available for land filling waste is diminishing, especially in densely populated areas.
The paper and pulp industry is an energy-intensive but energy-efficient industry. Energy can represent up to 40% of manufacturing costs. Many paper and pulp mills generate more than half of their energy needs from biomass fuels recovered from solid waste and process streams. Energy-rich biomass, derived from black liquor, wood chips, bark, sawdust, rejects and sludge is the result of atmospheric carbon dioxide amassed by trees during growth and transformed into organic carbon substances.
Biomass resources include agricultural residues, wood wastes from forestry and industry, residues from food and paper industries, municipal green wastes, sewage sludge, dedicated energy crops such as short rotation (3-15 years), coppice (eucalyptus, poplar, willow)(See Forest Waste), grasses, sugar crops (sugar cane, beet, sorghum), starch-containing crops (corn, wheat) and oil crops (soy, sunflower, oilseed). Residues and wastes are mostly used for heat and power generation. Sugar, starch and oil crops are primarily used for fuel production.
The pulp and paper industry has adapted and integrated advanced biomass energy conversion technologies.
Black liquor is the most important biomass fuel in a pulp mill. Black liquor contains around 50% from wood substance as dissolved organic fraction. Burning of black liquor in a special recovery boiler generates around 4 tons of steam per ton of pulp. By processing of steam in a back-pressure turbine, a high quantity of electricity is produced.
The following table shows the biomass wastes in the pulp and paper industry :
|TYPE OF BIOMASS||SOURCES OF BIOMASS WASTES|
|Black Liquor||Chemical pulp manufacture|
|Bark and wood residues||Chemical and semi-chemical pulp processes and mechanical pulp manufacture|
|Rejects of screening and cleaning processes||Chemical pulp production; recycled paper processing; paper stock preparing|
|Mechanical-Chemical Sludge||White water treatment and effluent treatment|
|Biological Sludge||Biological effluent treatment|
|De-inking Sludge||De-inking of recycled paper|
|Mixed Sludge||Different Sources|
WOOD WASTES AS A FUEL
The source of wood wastes in a pulp mill is the preparation of pulpwood. The wood wastes generated in a pulp mill are : sawdust from the slasher deck, bark falling from the debarking drum, pins and fines from chip screening, wood residue from wood-yard.
- Wood wastes consist of particles with various dimensions and shapes (from sawdust to log butts)
- Moisture content of wood wastes is high and time depending
- Heating value of wood wastes is highly influenced by their moisture content
- Generation of wood wastes in the pulp mill is time-depending
The heat value of the above samples (bark, sludge) is analyzed with an oxygen bomb calorimeter system to compare the calorific value of the samples to that of coal and peat as a replacement bio-fuel in paper mills.
In order to burn, wood wastes must be screened to remove oversized pieces, and pressed or dried, to reduce their moisture content. Wood-waste is a medium heating value fuel. In order to enhance the boiler efficiency, fossil fuels (coal, oil, natural gas) are co-fired with wood wastes.
Solid residues from wood waste combustion are ash and incompletely combusted carbon.
Other sources of fuel in pulp and paper mills are as follows :
- REJECTED RECYCLED PAPER PROCESSING : The portion of the reject which is suitable for incineration has to be discharged at the highest possible dryness accompanied by the highest possible calorific value. The rejects from the pulping of recycled paper contain large quantities of plastics depending on the recycled paper grade.
- PAPER SLUDGE AS FUEL : The organic fraction in paper sludge is renewable, and therefore it does not contribute to the carbon dioxide emissions. A few mills incinerate paper sludge in their boilers as fuel. The heating value is very low and the high moisture of the sludge affects its ability to burn efficiently, therefore it is important to measure the calorific value of the paper sludge as a fuel.
- RECOVERED PAPER AS FUEL : With the continuing increase of world paper consumption, more and more waste paper of poor unknown quality is entering in the solid wastes circuit. Sources of waste paper fibres are as follows : varied paper quality, boxboard cuttings, mill wrappings, old newspapers, over-issue news, ground wood and container plant cuttings, pulp substitutes, purchased de-inked white and coloured ledger, computer printout, ground wood, coated book and bleached sulfate sheet and cuttings.
Mixed recycled paper represents a valuable source of energy :
- It is easy to separate from the waste stream
- It is relatively homogeneous and mostly free from metals, putrescibles and othernon-combustible materials
- It requires minimum processing to be converted into densified form of energy suitable for direct combustion
- Its heating value is fairly high
- It has a low sulfur content and low nitrogen oxides emissions.
Approximately 1 ton of wastepaper combusted as fuel will produce approx. 9.8 GJ of thermal energy.
In the pulp and paper industry energy-rich biomass is represented by pulping liquors, wood wastes, sludge and rejects.
Black liquor is the most important biomass fuel in a pulp mill and contains around 50% from wood substances. Wood waste is the second biomass fuel important in a pulp mill. Combined wood waste and sludge generated at the mill are sufficient for combustion demand, because of low heating value resulting from their high moisture and ash content. In order to enhance their efficiency, fossil fuels (coal, oil, and natural gas) are co-fired with wood waste.
The analysis of the calorific value of the above mentioned wastes is important to calculate the combustion efficiency of each waste in order to compare its efficiency to fossil fuels like coal and oil to ultimately generate enough energy to be used as bio-fuel.