_ The Zero Emissions Strategy Conference
_ The Zero Emissions Strategy Conference
Case Study submitted to the conference by Ian Bywater, Convertech Limited, New Zealand (bywateri@convertech.co.nz)
The following has been reproduced without its key supporting graphics. For a complete version, please go The Convertech Process (Home Page).
Scott Convertech Limited (SCL) is a development company based in Christchurch, New Zealand. The process Convertech is engaged in is gaining recognition as a significant breakthrough in the fields of power generation and the production of organic chemicals. In brief, the Convertech process opens the way to economic, large scale use of vegetation (that is biomass materials such as trees, energy crops, farm and forestry residues) as a renewable, environmentally sound, greenhouse gas free resource. The technology is, in effect, a refinery process for biomass materials.
The Convertech process overcomes the major problems which have until now hindered the competitive and large scale use of biomass. The process removes water efficiently and the components which cause combustion problems when biomass is burnt conventionally, and causes the controlled partial depolymerisation of the lignin, cellulose and hemicellulose by steam hydrolysis.
Biomass is recognised as a greenhouse neutral resource and therefore meets world environmental demands for renewable energy and a basis for sustainable development. Convertech "closes the loop" and "opens the door" to making full use of all the components of biomass, and does so producing no waste.
SOLAR ENERGY STORAGE: BIOMASS IS NATURE’S BATTERY
The solar energy reaching the earth each day is more than the total energy value of crude oil reserves, past, present and future. Biomass in the form of plants and trees captures a large amount of this energy through the photosynthetic process and stores it as chemical bonds between the carbon, hydrogen and oxygen atoms that make up the carbohydrate plant material. In effect, biomass is solar energy stored in a chemical form.
Fossil fuels like oil and coal are derived from biomass material deposited millions of years ago. While the chemical composition of biomass and fossil fuels is different, it is possible for the same range of end products to be produced from either source.
Biomass can be grown, harvested, processed and stored until needed, thus ensuring a predictable and continuous supply of power, chemicals and materials. This gives biomass a major advantage over other forms of renewable energy.
THE HISTORY OF BIOMASS
Wood was the major source of energy before the widespread introduction of coal, oil and natural gas. Even today, biomass remains the major source of energy for most developing countries. In the industrialised world, biomass use is also significant. In the US alone, the biomass fuelled power generation capacity is over 8,400 MWe (this is over three times the total installed fossil fuel generation capacity in New Zealand). Worldwide biomass contributes an estimated 25 million barrels of oil equivalent per day.
Biomass is generally plentiful and in constant supply. In most parts of the world, it would not compete with food production but complement by means of integrated sustainable farming systems where the whole plant is processed instead of a small part. A much greater variety of plants would be used instead of a few monoculture species as is so often the case at present. All of this is achievable without the need for government subsidies given the right technology.
Even contemplating relatively low yields per hectare, there is enough land to substitute for total current world oil production. With improved cultivation and selection methods much higher sustainable yields are possible.
SUMMARY OF THE TECHNOLOGY
Biomass has long been recognised as a potential large source of renewable energy and organic chemicals. Worldwide, vast sums are being spent on research and development to increase the efficiency of its use, in particular for power generation. However, until the development of the Convertech process, the large scale use of biomass has been impeded by three main problems:
- The high moisture content of most green biomass (on average, it contains about 50% water: if it is burnt wet the thermal energy obtainable is reduced by 10% or more compared to dry wood);
- The presence of alkali salts which make it very difficult to burn it economically in high efficiency, high temperature boilers or gas turbines; and
- The difficulty in accessing the valuable chemicals biomass contains, and which have the potential to substitute for petrochemicals.
The first two problems, for example, are the reason why biomass fuelled power plants are restricted to operating at low efficiency levels of 20% or less instead of about 35% achieved in coal fired power stations.
The Convertech process overcomes these problems by integrating three key features:
- washing out of salts (partly through recycling of the condensed water contained in the green biomass which is evaporated at the last stage of the process), thus reducing the ash content;
- steam hydrolysis to achieve a breakdown of the biomass into its chemical constituents (in particular, various sugar fractions, cellulose, lignin and volatiles);
- very energy efficient multiple effect drying with superheated steam.
The main hydrolysis, chemical extraction and drying operations are carried out in a series of five continuously operated modules. Depending upon the nature of the biomass it is shredded or milled and fed into the plant as a stream of small particles.
Steam is the sole medium used to transport the particles through each module, conduct the hydrolysis and chemical extraction operations, and carry out the drying. This contributes to make the process simple, robust, and clean while minimising corrosion from strong acids.
The plants are designed to be extremely compact and highly automated. They can process any kind of biomass, ranging from forestry residues, through parks and garden waste, saw mills, farm and crop residues (such as hog fuel, saw dust, sugar cane), to dedicated plantations such as short rotation coppicing or energy crops like bamboo.
Convertech plants achieve high economic and thermodynamic efficiency by processing whole plants into a range of products and by cascading energy throughout the whole process to minimise energy losses. All components of the biomass are used or recycled.
THE ROTARY INTERLOCK
The interlock is the rotating valve that is used to transfer in a continuous fashion the fine biomass material between the different pressure zones used in each module. It is this device that makes the efficient cascading of heat and hydrolysis outlined above possible. Convertech has demonstrated that its interlock can be manufactured very
competitively relative to technology currently in use to transfer solids across pressure zones (such as screw plugs, slide gates or ball valves).
Besides its use in the Convertech process, the interlock has wider uses in any process where solids have to be moved between different pressure zones, such as various food, pharmaceutical and cosmetic industry processes, and in coal gasification. Its use enables linear processing of solids in chemical reaction engineering, and eliminates "dead spots" and "channelling" phenomena which plague pressure reaction applications at present.
BIOFUEL
The remainder of the biomass, which has passed through the process, is a dry, de-ashed particulate fuel, called Cellulig. It can be used for power generation in a number of ways:
- The Cellulig can be used as feed stock for conventional steam turbines. As the biofuel is virtually 100% dry and free from slag producing contaminants, maximum generation efficiency can be reached. However, the real potential for the Convertech process lies in using the Cellulig as feed stock to fire higher efficiency prime movers such as aero-derivative gas turbines.
- Most biomass gasification processes require drying of the feed material. While this can be done with flue gas or steam, none of the conventional gasification processes are able to utilise the spent drying medium in the way the Convertech process does. In fact, depending on local legislation, the spent gas or steam has to be cleaned before it can be released to the atmosphere, which adds to the overall cost. Further, for many gasifiers, biomass ash poses a serious problem when vitreous deposites form on heat surfaces. The Convertech process solves all of the above problems in an efficient way.
- Cellulig can be processed into pellets or briquettes and sold as a clean burning solid fuel. Potential markets include domestic and small to medium scale space heating, and clean burning fuel for cooking stoves in Third World countries.
THE REFINERY APPROACH
The Convertech technology is designed to fractionate and refine biomass into a wide range of valuable chemicals, fuels and fibre. It is envisaged that most installations will be multiple product plants:
- The first module is designed to extract any volatile products contained in the biomass. Some of these, such as essential oils, have a substantial market value. The remaining extracts may be used as fuel for the process itself;
- the hydrolysis of the hemicellulose fraction is carried out in the second module. It releases residual alkali salts for their subsequent removal, and transforms the hemicellulose into pentosans (sugars with five carbon atoms) and further into furfural. Furfural is a valuable chemical with a stable global market;
- the two hydrolysis stages (carried out in the second and fourth module) produce low molecular weight (LMW) lignin and cellulose fibre. The LMW lignin can be extracted with, for example, ethanol and has a wide range of potential applications such as the manufacture of epoxy resins, phenolic resins, binders in building material manufacture, initiator for urethane products, lignocarboxylates, and lignosulfonates;
- the de-lignified LMW cellulose fibre that can be produced jointly with LMW lignin downstream of the fifth module has a wide range of applications in the paper and chemical industries (such as polymers like cellulose acetate for the manufacture of biodegradable packaging materials, rayon or cellophane, cellulose ethers for the production of super absorbent materials, vulcanised fibre products for heat and chemicals resistant materials). Other non-polymer materials include a wide range of products used in the adhesives, food, cosmetics and plastics industries such as environmentally benign detergent ingredients, or precursors for polyurethanes, thermoplastics, epoxy resins and polycarbonates;
- besides its uses for heat and power generation the dried output (Cellulig) can be processed via further hydrolysis to produce ethanol, or a substitute natural gas (SNG) for pipeline supply;
- finally, the nutrients removed from the raw biomass in the wash water have a high fertilising value. Depending on specific applications, they will be used for irrigating biomass crop production or extracted by water treatment processes.
Overall, the technology functions as a key that unlocks a wide range of new competitive uses for biomass in four main market segments (agro-industrial products, organic chemicals, transport fuels, and heat and power generation). Chemicals, heat and power generation applications have the greatest long term potential.
CURRENT STATUS OF THE TECHNOLOGY
The Convertech process has been progressively developed since 1989. While a substantial amount of R&D on a wide range of applications is ongoing, the R&D effort regarding the core of the technology is now substantially completed. All components of the technology have been proven at the batch pilot plant stage and through computer simulation. The system and components are well protected world-wide by a strong body of patents. Convertech has now entered the commercial scale demonstration stage, prior to full commercialisation, with a particular focus on:
- Completing and operating the first module to demonstrate the dynamics and chemical processing features. This module has the size required for a 2 MWe commercial scale installation.
- Planning of the first full size ( five module) commercial demonstration plant.
All Convertech plants will be similar in structure and have the same components (only the size of the heat exchangers vary according to plant processing capacity). This modularity of the design enables a commercial scale, five module demonstration plant to be readily contemplated soon after demonstrating the first module, and makes replication in any locality straightforward.
Until now the development effort has been 95% private sector funded with total expenditure on R&D and demonstration of NZ$ 7 million. Financial support for the demonstration stage is currently being provided by the Australian Energy Research and Development Corporation (ERDC).
SUMMARY
Overall, power generation and production of chemicals from biomass through the Convertech process offers the prospect of:
- achieving long term sustainable supply of energy and materials based on a competitive, renewable and storable form of solar energy;
- producing power, chemicals and materials close to consumer markets thus saving expensive investments in power transmission lines, and transport costs of commodities;
- increasing energy efficiency by combining heat and power production when and where needed and in a competitive way using a renewable resource;
The Convertech approach to biomass production leads also to a number of other environmental advantages, for example, erosion control, reforestation, soil desalination. It also enables growth in power demand to be matched with small increments in generation capacity, thus reducing risks and capital costs; and fostering local development in terms of employment, and secure market diversification for local industry, farmers and foresters, through the progressive development of the downstream applications in the building material and chemical industry sectors.
Chemicals From Convertech Cellulose, Hemi-Cellulose And Lignin
Processing of lignocellulose can provide a spectrum of products that include fuels, animal feeds, materials for particle board products and chemicals. This discussion pertains only to the chemical opportunities, not the initial processing of the lignocellulose. It presupposes that routes from the raw Convertech output to the following feedstocks are available: cellulose, hemicellulose, and lignin (Figure 1).
Cellulose
The cellulose that is expected from this steam treatment process is not identical to the alpha cellulose that is obtained by conventional pulping and bleaching process. It is expected to have a low molecular weight (LMW), with the potential advantages and disadvantages that are implied by this condition.
An overview of potential opportunities is shown in Figure 2, with brief descriptions in text. The products are classified into two families: polymers and non-polymers. Even the non-polymers are usually employed to make or modify polymers.
>Convertech cellulose could be reacted with acetic anhydride to produce new grades of cellulose acetate that are expected to be less strong but more easily degradable than are conventional grades of cellulose acetate. The degradability and low cost of Convertech cellulose acetate could lead to large markets.
Instead of producing regenerated cellulose products, such as rayon and cellophane, via the xanthate route which is damaging to the environment, Convertech cellulose could be used to make these products. The route developed by Neste Oy in Finland more than 10 years ago is typical of the desired approach. Low MW cellulose is reacted with urea to generate a cellulose carbamate intermediate. This intermediate is spun into an acidic bath from which ammonia and carbon dioxide are recovered. The ammonia and carbon dioxide are used to make recycle urea. The process is environmentally benign and low in cost.
Reaction of Convertech cellulose with ethylene oxide and/or propylene oxide can provide polymers that are popular for superabsorbent materials, water treatment, etc. Hydroxypropyl cellulose is a typical product in this family.
Figure 1 - Lignocellulose Processing for Chemicals by Convertech Process
(See full linked version)
Figure 2 - Chemicals from Convertech Cellulose
(See full linked version)
Treatment of cellulose fibers with zinc chloride or other strong acids results in an entangled fibrillar polymer that is very strong and resistant to heat and chemicals. This vulcanized fiber material is out of fashion now, despite its excellent mechanical and electrical properties. When made from Convertech cellulose the shorter length of these fibers make it probable that the product will be different from the cotton-based original vulcanized fiber material.
Dextrins are oligomers (LMW polymers); however, they act more like non-polymers. The production of dextrins from starch is a large and profitable business. They are widely used by formulators of adhesives, food and cosmetics. Convertech cellulose could serve as the starting point for analogous products. The process probably would use a combination of acid hydrolysis with enzyme hydrolysis, as is done with starch.
Hydrolysis of cellulose to glucose has been studied intensively for the last 20 years. It is technically feasible but has not been demonstrated to be economically viable. There could be political situations in which glucose from Convertech cellulose could be desirable. It is perferrable to stop the hydrolysis before it is complete or to use the glucose as it is formed. These strategies avoid direct competition with starch-derived glucose.
Alkyl Glucosides from Cellulose
Reactions of alcohols with Convertech cellulose could produce alkyl glucosides that have a variety of uses, depending on the alcohol that is chosen. Note that these reactions are analogous to hydrolysis but produce higher value products than glucose. These processes can be controlled to give alkyl polyglucosides (APGs) which are detergent ingredients, use of which is growing rapidly because they are effective and environmentally benign. Other variations on this chemistry provide desirable ingredients for urethane poymers.
Simultaneous Saccharification and Fermentation
Technology exists to combine the production of glucose from cellulose with the conversion of glucose to ethanol. This method saves the steps involved in isolating glucose and speeds up the saccharification step. This technology could be applied to Convertech cellulose which has the advantage of being a feedstock lower in MW than conventional cellulose.
This concept also could be applied to other fermentations, such as production of lactic acid and citric acid.
Hydroxymethyl Furfural and Levulinic Acid From Cellulose
Treatment of glucose with mineral acid results initially in production of hydroxymethyl furfural. This reactive compound can be isolated and used for production of both thermoplastic and thermoset resins.
If this intermediate is not isolated, the mineral acid treatment leads to levulinic acid and formic acid as the products. Levulinic acid has been sold as a specialty chemical for use in solvents, special epoxy resins and polycarbonates, and as a food acidulent.
These reactions of glucose probably could be effected by use of Convertech cellulose, instead of glucose as the starting material. Hydroxymethyl furfural and levulinic acid are potentially promising chemicals, in part because so much development work has already been done.
Thermolysis of cellulose with acidic catalyses can provide substantial yields of levoglucosan. Because this chemical is glucose with one molecule of water removed, it is very reactive. It forms branched-chain oligomers that resemble dextrins. It can form alkyl glucosides. Urethanes have been made from this polyol.
HEMICELLULOSE
The composition of Convertech hemicellulose will depend on the biomass that is treated by this process. Grasses, straws and woods have very similar celluloses, but different hemicelluloses. In this presentation of opportunities, we focus on straws and hardwoods.
Digestion of hemicellulose that includes substantial amounts of pentosans results in furfural. This produce has been made mostly from surgarcane bagasse and maize stalks ("corn stover").
The technology exists also for production of furfural from hardwood hydrolyzates. Convertech hemicellulose should be a good source of this highly reactive chemical.
Furfural is used mostly for production of furfuryl alcohol. Furfuryl alcohol is used to make foundry core resins and coatings that are highly resistant to chemicals and heat.
Furfural was a source of tetrahydrofuran which is a powerful solvent and a source of nylon monomers. Petrochemicals have captured most or all of this market. There could be circumstances in which low-price furfural could recapture some of this market.
Solvent uses for furfural include petroleum refining. Literally hundreds of reactions of potential commercial value have been described in the scientific, technical and patent literature. However, pricing of furfural by the dominant producer has discouraged use of the produce in otherwise promising areas.
LIGNIN
The lignin that results from Convertech processing is not similar to that which results from kraft pulping or sulfite pulping. It does resemble "steam explosion lignin". That is, it is likely to be soluble in ethanol or acetone and does not contain any sulfur moieties. It is low in molecular weight and more reactive than conventional lignins.
If available at low price, Convertech lignin may be desirable to use as an extender-type ingredient in many compositions. For example, epoxy resins are frequently extended with coal tar products ("coal tar epoxies"). Convertech lignin may compete quite well in such a market. Reactive extenders can be used in some phenolic resins. Urethanes can use extenders to optimize properties and cut costs. These compositions might be used in adhesives, molding compounds, and coatings.
Initiators for Urethane Products
The reaction of ethylene oxide and/or propylene oxide with steam explosion-type lignins generates polyethers than can be reacted with isocyanate monomers to produce urethanes. They can be used in coatings and adhesives. Convertech lignin should also be suitable for this application.
The sodium salt of Convertech lignin should be capable of reaction with carbon dioxide to yield hydroxy carboxylate salts. Such products may have surface activity properties so that they could be used as emulsifiers or as sizing agents for paper products. These products may also be used in production of polyethers for urethanes as described above.
Reaction of Convertech lignin with sulfur trioxide could result in lignosulfonates that could compete for the large market now dominated by conventional lignosulfonates. The Convertech products would differ from conventional products in that they would have lower molecular weights and probably a different degree of hydrophilicity.
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