WASTE TO ENERGY TECHNOLOGY.

Transcription

1 WASTE TO ENERGY TECHNOLOGY. Introduction. The Holistic Waste To Energy Process as portrayed on our flow sheet for the conversion of organic wastes into gaseous and liquid fuels, electrical power, fertiliser and recycled water combines several treatment modalities. Mixed anaerobic bacteria and specific Clostridial bacteria are used within these processes to produce renewable fuels. It can also be seen that two distinct yet interrelated operational envelopes are present. Anaerobic digestion for the successful treatment of wastewater was first reported in France in 1881 and has since become established practise for municipal sewage treatment plants. In England in the latter part of the 19 th Century the streetlights of Exeter were illuminated with sewer gas and the Birmingham sewage works pioneered the use of sewer gas for large-scale power generation. Successive generations of sewage engineers (Cameron, Travis, Imhoff) in Britain, the USA and Germany improved the original design to arrive at the present standard of efficiency. However treatment of household organic waste (HOW) with anaerobic bacteria had not received any attention until quite recently. Whereas the Clostridial fermentation or ABE process has been known since 1916 (Weizmann) and used in Britain until world War II on an industrial scale for mixed alcohol production. The burgeoning petroleum industry provided low-cost feedstocks and hence less expensive production of mixed alcohols. Increasing crude oil prices coupled with environmental and greenhouse concerns has lead to renewed interest in this process. Utilisation of saccharides in extruded HOW by Clostridium acetobutylicum (ATCC 824) for mixed alcohol production has recently been part of a trial in the Netherlands. In the USA, Canada, Europe and New Zealand, considerable R&D has been carried out using lignocellulose as feedstock for alcohol production. The Biomass to Energy Process presented in this model intends to utilise lignocellulose from suburban greenwaste, which is readily available and of predictable quality. The process can also use wheat and rice straw, rice hulls, sugarcane bagasse and many other agricultural residues and indeed purposely grown energy crops. The method, which our company favours utilises steam explosion and autohydrolysis as a pre-treatment stage followed by dilute sulphuric acid and enzyme treatment to release the pentose and hexose sugars. These processes may require a separation of the pentose sugars from the hexose sugars for fermentation by suitable organisms or else fermentation of the mixed sugars may be carried out using specially engineered bacterial species such as Clostridium acetobutylicum. 1

2 The part of the technology under greatest development worldwide is the hydrolysis of the cellulose and hemicellulose for the production of the fermentable sugars. Most of the methods of hydrolysis produce substances, which can adversely affect the fermentation, produce less than optimum yields of sugars or produce by-products for which no current market exists. Additionally, enzymes used in some of these methods are expensive and add to the overall cost of production. In Brazil and the United States, (producing 60% of the world s total ethanol) extensive subsidies are required to support the industry. Our developmental work is along similar lines to many other research and development organisations with the exception that we use a waste to energy process using anaerobic bacteria, which supplies all the energy for our processes. This holistic process enhances the conversion by cost reductions and by-product credits. Recovery of the alcohols from the ferment liquor in our preferred embodiment is by distillation, however solvent extraction and membrane separation could also be used. The alcohols produced from this process are also widely used as industrial solvents, however their use as a fuel additive for octane enhancement and as a fuel extender is predicted to increase. The use of ethanol in motor spirit worldwide is predicted to exceed 23 billion litres by the end of 2001 and with the proposed phaseout of MTBE, production of alcohol is expected to increase further. The market for the individual alcohols as solvents in some cases may predicate their use as solvents rather than motor fuel extenders. COMPARATIVE YIELDS OF BIOMASS RESOURCES. Raw material tonnes/ha Ethanol l/tonne Ethanol l/ha Softwood (Pinus sp) ,800 Hardwoods ,800 Wheat straw ,000 Rice straw The time frame over which these biomass crops are grown vary from 3 or 4 months in the case of wheat or rice straw to several years for trees. From the figures presented above it can be seen that considerable quantities of fuel alcohol can be produced from tree crops and crop residues. As a comparison presented below are figures for traditional alcohol crops. Time frame (months) Sugar cane Maize (corn) ,100 4 Sugar beet ,600 5 Wheat ,120 4 Cassava ,000 8 Grapes ,

3 The traditional ethanol crops have been used for the production of potable spirits (beer, wine etc) and employ simple conversion methods and use yeast as the organism to produce the final alcohol. Distillation has been used to produce whiskey, rum and brandy from simple stills or 95% alcohol from fractionating columns followed by tertiary distillation to anhydrous or fuel grade alcohol. ETHANOL AND OTHER SOLVENTS FROM BIOMASS. Assuming the feedstock for a proposed fuel alcohol plant is suburban greenwaste, which is readily available, is supplied already chipped, is of predictable quality and supply can be assured then the question is which conversion method to use. The conversion requires a number of discrete steps: Additionally, process energy sources: Disruption technologies: Products: Continued treatment: Disruption. Hydrolysis. Separation. Fermentation. Purification. By-products and co-products. Waste heat. Biogas. Electricity. Process by-products. Steam explosion. Ammonia explosion. Solvolysis. Concentrated acid. Dilute acid. Enzyme treatment. Sugars. Lignin. Cellulose. Hemicellulose. Furfurals. Glucose. Fructose. Xylose. Minor sugars. Furfurals. 3

4 Fermentation organisms: Yeasts. Clostridia. Zymomonads. Transgenic bacteria. Fermentation technology: Batch fermentation. Integrated continuous. Immobilised cell reactors. Membrane systems. Purification: Distillation. Pervaporation. Membranes. Solvents. Process energy sources: Waste heat. Biogas. Electricity. Process by-products. By-products and co-products: Biogas (methane) Animal feeds. Algae. Pharmaceuticals. Microbial biomass. Carbon dioxide. Hydrogen. 4

5 DISRUPTION TECHNOLOGIES. Steam explosion: The feedstock has to be chipped or ground before any treatment can be applied. Size reduction to 25mm and separation of stones and grit is required before steam or ammonia explosion technology is applied. Steam explosion has been used for many years as a method of reducing wood to a fibrous mass prior to recombination as Masonite or proprietary fibreboards. Depending on the temperatures and pressures used the result is an autohydrolysis of the hemicellulose to xylose and a slurry composed of cellulose and lignin. The mixture is now ready for enzyme hydrolysis to convert the cellulose to glucose. Furfural is also produced and must be removed as it inhibits the production of alcohol during fermentation. Prior to the enzyme step the xylose may be separated for fermentation with an different organism. Ammonia explosion: The same preparation is required as above except that ammonia is infused into the lignocellulose at various temperatures and pressures and the mass exploded in similar fashion, where the ammonia is recovered for reuse. Solvolysis: Perhaps the method most worthy of note is the ACOS Process. (Acid Catalysed Organosolv Saccharification). In this process acidified acetone is used at elevated temperature and pressure to dissolve the lignin. The acetone-lignin solution is separated from the cellulose and hemicellulose and the acetone recovered. The cellulose and hemicellulose is hydrolysed to glucose and xylose prior to fermentation. The lignin may be used for power or steam generation or further processed to aryl-methyl ethers for addition to motor fuel. Concentrated acid: In this process concentrated sulphuric acid is used to dissolve the cellulose and allow subsequent hydrolysis of the hemicellulose. The acid must be recovered for reuse and the corrosive nature of the acid requires special materials and handling techniques. Dilute acid: In this method dilute sulphuric acid at elevated temperatures and pressures is used followed by enzyme treatment. Also known as the simultaneous saccharification and fermentation method. Overall this method is simplest, however as with all the methods there are drawbacks. The acid must be neutralised and is usually removed as Calcium Sulphate. Enzyme treatment: The most difficult aspect is exposing the cellulose and the hemicellulose to attack by the enzymes; hence all the aforementioned treatment methods use either physical, thermophysical, chemical or thermochemical methods. 5

6 The cost of enzymes currently is very high although a vast amount of work is being carried out on new technology to lower the cost. R&D UNDERTAKEN SO FAR BY OUR ORGANISATION. Concentrated acid hydrolysis: A batch of coarse pinewood sawdust was treated batchwise with concentrated sulphuric acid at 100 degrees Celsius for 20 minutes and the resulting mixture neutralised with lime. The sugar solution was fermented with a combination of yeast and Clostridial bacterial culture. The resulting wood wine yielded an interesting mixture of butanol and ethanol. Dilute acid hydrolysis: Batches of pinewood sawdust, bagasse and paper treated with a 2.5% solution of sulphuric acid at 120 degrees Celsius and fermented with a mixed strain of yeast produced ethanol. Enzyme hydrolysis: Paper Making Sludge (PMS) was treated dilute acid to remove the calcium carbonate, filtered and hydrolysed using proprietary cellulase enzymes and fermented with yeast. Ethanol was produced. These early tests were carried out in buckets and pressure cooker sized batches to ascertain where problems of handling would be encountered. Once these have been addressed then more qualitative tests are carried out. Additionally, the Clostridial bacteria used in our current work are the naturally occurring species C. acetobutylicum, which whilst producing butanol and other solvents has a low yield. A high-yielding mutant strain of C. acetobutylicum can be engineering in the laboratory comparatively easily. THE ENERGY BALANCE. Alcohol Production Energetics. Typical corn ethanol plant in the USA. Process stage: Energy input Energy recovery. Cooking etc 4,143 kj/kg Distillation 6,978 Dehydration 5,815 Energy input req d 17,937 kj/kg Heat exchangers 2,000 Anaerobic digestion 10,467 Energy recovery 12,467 kj/kg Total energy expenditure 5,500 kj/kg The energy content of ethanol is 27,000 kj/kg therefore the energy ratio of input versus output is nearly 5:1. 6

7 The anaerobic digestion in the case above utilises the stillage or slops from the distillation process, addressing a pressing environmental problem and delivering high-energy returns. From our flow sheet it can be seen that a dedicated waste to energy and fertiliser facility is designed to be part of the overall biorefinery. The energy produced from the waste to energy facility (biogas and electrical power) flows into the alcohol plant supplying all the energy required for the biomass to alcohol production. Preliminary calculations based on 2,500 tonne per week organic waste input and 2,000 tonne per week greenwaste input indicate that approximately 30% of the biogas is consumed by the alcohol facility. This is based on alcohol energetics of 5.5 MJ/kg, 500,000 kg of alcohol per week and a biogas production of 9,000 GJ/week. (A total of 2.75 GJ is required for the alcohol production). The waste inputs (the resources) with exception of the chicken manure are consigned to landfill disposal and as such command a disposal fee. The current landfill gate fee is around $55 per tonne, however concerns that landfill gas emissions are having a negative effect on the world s climate has prompted a cessation of landfill dumping in the near term. The avoided landfill fee is then paid to the processor of the waste material. From the attached Excel spreadsheet it can seen that the operation of the waste facility is potentially viable. It must be noted that preliminary costings for a plant of that capacity is between $15 and $18 million, which gives a simple ROI of around 3- years. The greenwaste to alcohol plant has been tentatively costed at around $20 million and on the basis of producing 30 million litres per annum for a sale price of $0.5 per litre has a very attractive ROI of around 4-years. Conclusions. The anaerobic digestion technology embodied in the Holistic Waste To Energy Process is well known worldwide and many large digesters turn municipal waste into biogas, power and compost. In Australia landfill dumping is an institution and anaerobic digestion a relatively unknown technology. The Southern Metropolitan Regional Council (SMRC) is supportive of anaerobic digestion as Stephen Lucks carried out preliminary digestion studies for the SMRC some years ago as a Research Fellow at Curtin University Centre for Renewable Energy. The conversion of lignocellulose to alcohol requires further studies and funding to bring it to a commercial reality in Australia. We would like to secure R&D funding to build a pilot-scale alcohol plant with the prime focus on the disruption and conversion to fermentable sugar technology. 7

8 Attachments. Holistic Waste to Energy Technology Process Flow Sheet. Waste to energy technology description.doc 8