Field of the invention The invention relates to fluidized bed processes and equipment, materials processed in and products obtained from these, and especially to a method for controlling the amount, composition and properties of ash generated in the fluidized bed process.

Background of the invention Fluidized bed reactors are known, among others, in the combustion and gasification of solid or semi-solid organic substances for producing energy or product gas.

Fluidized bed technology has become more common in the past decades because of the technical and economic benefits offered by it and along with the development of technology. In a fluidized bed it is possible, when needed, to burn or gasify simultaneously several solid fuels, such as peat, wood chips, arable crops, and wood-, animal- or plant-based waste or plastic waste. Fluidized bed processing can occur as so called circulated fluidized bed combustion or in a bubbling fluidized bed, these techniques requiring a different type of equipment solution.

Even if fuel can vary in its moisture, ash content, ash composition and other quali- tative properties, the combustion rate is good and the process is relatively free from disturbance, because each solid substance particle to be burned becomes surrounded by combustion gas in the process and is simultaneously heated to a temperature corresponding to pyrolysis.

In gasification there occurs no combustion or it is incomplete. In this case the fluid- ized gas is low in oxygen or oxygen-free (contains e.g. the compounds CO ₂ , H ₂ 0 and N ₂ ).

It is believed that the gasification of solid bio-fuels, waste containing bio-organic components and fossil waste (plastic, rubber, textiles) by fluidized bed technology will become increasingly more common in the near future. Fluidized bed material is often ordinary sand rich in quartz, but it can also be some other inorganic material with suitable particle size and other properties. In CFB reactors a large part of the bed is typically ash, the other bed material also being finer, with a particle size in the region of 0.1 - 03 mm, when again in BFB reactors the typical particle size range is 1 - 3 mm.

The amount, composition and properties of generated ash or gasification residue are above all impacted by the fuel or material to be gasified and the impurities landing into the process along with it, but also the bed material and possible additives fed into the process. Bed material remains in the ash as such, more finely pulverized and possibly chemically dispersed, depending on its properties and the process. Ash or the gasification residue can contain heavy metals, such as cadmium, arsenic, chromium or mercury, alkali metals, such as sodium or potassium, halogens, such as chlorine or fluorine, or non-metals, such as phosphorus, sulphur or selenium. Ashes, especially gasification residue ashes, can also contain carbon.

There are principally two types of ash generated in fluidized combustion, bottom ash and pulverized fuel ash. Bottom ash or slag remains at the bottom of a reactor, such as a boiler, and pulverized flue ash exits along with the flue gas and is recovered from it by filters.

Substances accumulating in bottom ash typically include gravel and sand, clay, scrap metal, glass, sintered or non-sintered bed sand and melted fuel-based ash coming with the fuels. Part of the bottom ash has such a big particle size that fluidized air is not able to keep it in the bed. This is why ash is generally removed during the process and replaced with new bed sand. Coarse bottom ash is removed from the process, but a part of it can also be returned to the boiler after screening.

It is thus in itself known that bed material affects the amount, composition and properties of ash. It gets always pulverized to some extent in the process and mixed with fine-grained ash and, on the other hand, agglomerating or sintering ash takes it along into the bottom ash. It is also known to mix the bed sand with material, which is more easily pulverized and decomposing, which is known to have an advantageous impact on the process and to alleviate or solve problems in it, such as agglomeration of the bed material.

The specification US4448589 relates to the pyrolysis or combustion of carbonous waste in a fluidized bed reactor, in which the usual fluidized bed material is quartz sand. The problem with the processing of waste material is the agglomeration and sintering of bed sand, and it is presented as a solution to the problem that 5 - 35% of calcium, magnesium or barium carbonates or oxides are mixed with the bed material. Depending on the case, a suitable amount of appropriately selected auxiliary material prevents the bed sand from agglomerating. An essential part of the operation of the auxiliary material is that it is pulverized in the bed material consid- erably faster than quartz sand.

The specification FI108942 relates to the gasification of bio fuels that are difficult to gasify, such as different types of agricultural waste and energy bio masses in a fluidized bed reactor. A problem with this is the agglomeration of ash, because the gasification temperature has to be raised higher than the melting and sintering point of ash. The solution is that hard fluidized bed material is mixed with 10 - 30% of porous pulverizing bed material, which binds the generated ash to itself, the combination then being removed from the process. Limestone and dolomite, i.e. essentially magnesium and calcium carbonates are presented as possible porous pulverizing materials. The generated ash is considered to be usable as ferti- lizer for fields and forests.

This application concerns, above all, the utilization of ash generated in the fluidize bed process.

Ash often contains heavy metals to a slightly larger extent than what is allowed in fertilizer and soil improvement use, especially where plant cultivation is concerned. In several cases, ash has to be heaped or utilized in a purpose of lesser value, and in this case especially the nutrients contained in the pulverized flue ash (among others, K, Na, Ca, Mg, P, S, Fe, Mn, Zn, Cu) remain either totally or partly unutilized, which is a big economic loss.

At the moment, for example, ashes from the combustion of ash and peat, let alone most ashes from refuse incineration, contain so much heavy metals that they are not suitable for plant cultivation, but they are directed, among others, to forest fertilization with high spreading costs or to the use with an even lower processing value, such as the fertilization of garden and park areas or as filling compound in excavation work. The amount of taxes imposed on the heaping of ash has continually increased, and with further transport and processing costs, the total costs become high. Thus it would be of vital importance that ashes containing nutrients indisputably needed in plant cultivation would be used to replace industrial fertilizer and land improvement products, for example, by improving fluidized bed processes. Summary of the invention

The object of the invention is to present a solution for controlling the amount, composition and properties of ash in fluidized bed processes so that the possibilities for the utilization of ash are essentially increased and improved. To achieve this object, the method of the invention for controlling the amount, composition and properties of ash in a fluidized bed process is characterized in what is specified in claim 1 of the enclosed patent claims. The other claims define different embodiments of the method.

The invention introduces as an entity a new bed material, and a way of driving bed and supplementary devices slightly deviating from the conventional, which are dependent, in addition to the characteristics of the bed, also on the fuels used, boiler type, automatics etc.

The solution of the invention is used especially for improving the possibilities for a versatile fertilizer use of ash by impacting the composition and properties of ash so that the requirements set for such a use of ash are met. Both the use of ash as raw material for a final fertilizer or soil conditioner and its use as such or provided with additives for such a purpose are made possible.

Detailed description of the invention

A cornerstone of the invention is that by adding suitable easily pulverizing material to the bed material in a fluidized bed process, it is possible to advantageously impact the amount, composition and properties of ash taken the utilization of ash into consideration.

The material to be added in the solution of the invention essentially contains Ca, Mg or Ca-Mg compounds, especially carbonate, or combinations of these. Impacting the amount, composition and properties of ash occurs through the fragmentation, decomposition, pulverization and fine grinding of material particles, chemical decomposition and interaction between particles.

Fragmentation and decomposition refers here to the disintegration of particles into smaller ones, on the one hand as a consequence of big volume changes caused by changes in temperature and, on the other hand, because of large increase in pressure generated in particles in chemical decomposition. Pulverization means the reduction in particle size as a consequence of so-called attrition, when particles hit each other and the reactor walls or other structures. Particles can consist of bed material, ash or sand coming along with fuel.

Grinding generally refers to the reduction in particle size in ways studied above or otherwise.

Changes in temperature create a non-homogeneous stress field inside the particle, which contributes to fragmentation. Pressure caused by the development of gas inside the particle again promotes the decomposition and grinding of particles.

Bed material preferably contains a large amount of calcium and/or magnesium, but a small amount specially of alkali metals, silicon and aluminium and a particularly small amount of heavy metals. The density and particle size range of the bed material of the invention does not necessarily differ much from conventional bed sand rich in quartz in the feeding phase.

An easily pulverizing substance of the bed material preferably comprises calcium carbonate (calcite) CaC03, calcium magnesium carbonate (dolomite) CaMg(Co ₃ )2 or magnesium carbonite (magnesite) (Mg,Fe)C0 ₃ . An easily pulverizing substance can also comprise waste material or by-product with CaO or Mgo content, such as incompletely calcinated dolomite or calcite of lime burning plants, discharge waste of fire-resistant doloma or magnesite brick or sand rich in magnesite generated in connection with talcum production. Also different combinations can be formed of all these materials to achieve an easily pulverizing substance.

Material hardness is distinctly smaller than that of a conventional bed material, which mainly consists of quartz and feldspars. The hardness of quartz and feldspar is 7 and 6 on the Mohs scale, while the hardness of calcite, dolomite and magnesite is 3, 3.5 and 4, respectively. A smaller hardness contributes to the fast pulverization of materials in the bed process.

The said carbonate materials, with common denominations of calcite, dolomite an magnesite, are advantageously natural and utilizable merely by having undergone through excavation, crushing, milling and possibly concentration processes, pref- erably without drying. They can contain small or moderate amounts of especially iron, which catalyzes pyrolysis, and phosphorus, which improves the applicability of ash in fertilizing. Conventionally, they also contain small amounts of harmful, neutral or less useful components, such as silicon and aluminium. With the right choice of materials and quality verification, the amount of these harmful components can be kept sufficiently low. The amount of heavy metals in carbonate materials is typically very small.

By adding bed material as described above, the amount, composition and proper- ties of ash can be controlled and desired impacts can be specified especially for improving the utilization properties of ash as fertilizer, soil conditioner or as a raw material of these.

The invention also introduces an advantageous calcination method of calcite, dolomite, magnesite or a combination of these, the materials produced by which can find practical application at least as fertilizer, often a short way of transport from the production plant. Otherwise calcination would have to be performed in a lime burning plant.

The bed material of the invention decomposes chemically in the bed in accordance with the following reactions (oxidizing circumstances): CaCO ₃ -> CaO + CO ₂

CaMg(CO ₃ ) ₂ -> CaO+MgO + 2CO ₂

MgCOs -> MgO+CO ₂

At first, decomposition occurs in the surface section of the particle but advances preferably so that the entire particle has decomposed chemically due to an entirely simultaneous pulverization so that new chemically undecomposable surface is subjected to chemical decomposition occurring by the impact of heat.

Thus, depending on the feed material, the result of decomposition is in addition to carbon dioxide gas also burnt lime CaO, burnt lime CaO and magnesia MgO or magnesia MgO as a solid substance. Of these, the decomposition reaction of magnesite occurs at a lower temperature than that of calcite. Magnesite component decomposes first from dolomite, and as the temperature rises, the calcite component decomposes at approximately 900°C.

The supplied bed sand does not need to undergo full chemical decomposition. The particle, which has not had time to decompose entirely, has an interior of original carbonite and only a surface section of respective oxide. In a circulating fluidized bed boiler, the carbonate particle can circulate the bed several times decomposing all the time into smaller particles, until the carbonate has entirely decomposed also chemically. The same can in principle happen in a bubbling fluidized bed boiler through the recovery and refeeding of bottom ash.

The speed of the said reactions can be adjusted by the bed temperature and the partial pressure of carbon dioxide in the bed. For example, flue gas containing a small amount of oxygen and an ample amount of carbon dioxide, nitrogen and water vapour can be recycled back to the bed zone so that the balance of the reaction moves from the side of the reaction products more to the side of the source materials. This concerns also those combustion reactions with carbon dioxide on the side of the reaction products. The back feed of combustion gases is also known in the technology of oxygen combustion for controlling the bed temperature, among other things. In oxygen combustion the combustion air has been partly or entirely replaced with oxygen. This makes possible a more efficient recovery of carbon dioxide, because the nitrogen content of combustion gases decreases significantly. The technique of the invention is possible also in connection with oxygen combustion.

The chemical decomposition of carbonate bed material consumes heat energy, the amount of which corresponds to the heat of evaporation of the approximately corresponding amount of water. Thus the decomposition temperature of carbonate is not an economic obstacle for the use of bed material. For example, evaporating the moisture amount typically found in wood or peat fuel consumes multiple times the said energy, because the moisture content of the fuel is tens of percent and fuel is supplied at least tens of times more compared to the amount of bed sand fed.

The melting points of earth alkali oxides CaO and MgO are over 2000°C so, in practice, they cannot cause sintering in bed conditions.

The quick pulverization and chemical decomposition of carbonate materials are a solution, for example, for the reduction of the heavy metal content of generated ash. For example, dilution is carried out so much that the content of the most critical heavy metal, which in wood and peat based fuels is generally cadmium, mer- cury or arsenic, decreases in the obtained pulverized fuel ash safely below the limit set for the manufacture of fertilizers. The limits are different for ashes suitable for different purposes so the dilution is planned in accordance with the purpose of use of the ash. Example 1

The Cd content of pulverized fuel ash is 2 ppm, when using conventional bed sand. Fuel-based pulverised fuel ash is generated 8 units of weight per unit of time. The set Cd content for the planned use of pulverized fuel ash is under 1 ppm. Conventional bed sand, which does not contain cadmium, is consumed in the amount of 2 mass units per unit of time. Of these two mass units 50% i.e. 1 mass unit is pulverized into the pulverized fuel ash in a unit of time. The second bed sand unit ends up mixed with the bottom ash.

The dilution for achieving the Cd content of 1 ppm in the pulverized fuel ash is in this case 9 units of weight of the calcinated bed material of the invention. Because of the normal variation in the Cd content, the chosen dilution is 10 units of weight. The total amount of pulverized fuel ash obtained in a unit of time is 18 units of weight/unit of time, and the Cd content here is under 1 ppm so that the pulverized fuel ash is applicable to the intended purpose of use. 12.5 units of weight of dolo- mite material is fed to the bed. The loss of volatile substances in the dolomite material of the example is 20% (2.5 units of weight).

Even more dilution can occur than what is needed to reach a heavy metal content lower than the set value. For example, dolomitic or calcareous materials from different sources are pulverized at different speeds even if the particle size range and distribution were the same. Excessive pulverization can be controlled, for example, by feeding flue gas with mild oxygen containing carbon dioxide back to the bed zone of a BFB boiler or by increasing the medium particle size of the bed material to be fed. However, the amount of fluidized air should not be increased too much because it increases the internal consumption of the plant. If a sufficient dilution of ash is not achieved, the bed material can be replaced by a softer one or the material can be fed to the bed in a finer structure so that it pulverizes more quickly. Also the supply of flue gases back to the bed zone of the BFB boiler can be reduced. Further, it is possible to use material with appropriate particle size distribution; for example, material with a relatively high amount of particles in the size range of 0.1 - 0.3 mm and 1 - 2 mm, but very few particles in the size of 0.3 - 1 mm. It is further possible to feed partly cooled flue gases to mix with the obtained ash so that secondary carbonate is obtained. This reduces emission of CO2, increases extraction and decreases the pH value of ash. Ashes diluted from heavy metal are processed and stored by conventional means. For example, they can be watered with an appropriate amount of water to control dusting and to ease the processing. In this case, at least part of the free calcium oxide quenches. The ash can be granulated so that its processing and spreading, for example, onto fields becomes easier.

For its ageing and hardening, ash can also be stored, for example, for 2 - 6 months. In the ageing process, the alkalis and earth alkalis of moistured ash react partly with the carbon dioxide in air, forming carbonates. These reactions are accelerated by mixing, heating and watering. The ageing of free alkalis and earth alkalis decreases the pH value of ash to the range of 8 - 9.5, in which the solubility of most heavy metals is minimal.

Ageing, hardening and granulation of ash reduce dusting and the otherwise too fast solubility of ash components and the increase in the pH of soil during the first season, which holds back the dissolution of some heavy metals and extends the fertilizer impact. Among the ash there can also be unburnt coal as a consequence of incomplete combustion. Carbon residue is advantageous for the improvement of the structure of farm land, but residual carbon weakens the extraction of heat or product gas in the bed process.

When applying the method of the invention, ashes are often generated much more, and that is why the size or capacity of equipment used in the transfer of ash silos and ash has to be increased. The same concerns the entity relating to the storing and supply of bed material. These bring somewhat more investment costs, but they are not an obstacle for the utilization of the invention.

The application of an ash product can be impacted by the selection of bed sand used. An ash fertilizer requiring higher alkalinity is provided with more soluble calcium ions, which are rapidly soluble, thus achieving a fast liming effect and increase in pH in the soil. When a high, fast impacting increase in pH is not required from the ash fertilizer, but rather a long-term liming effect and moderate increase in pH, more magnesium ions in relation to calcium ions are arranged into the ash. A fraction of less than 3 mm returned to bottom ash, fuel-based sand included, can be fed back to the bed, when needed. However, this is not necessarily done, but the bottom ash is recovered back to practical use, for example, fertilization, directly or after screening. When needed, bottom ash can be combined with pulverized fuel ash or ash obtained from an economizer or air preheater or a mixture of these. Combining can also occur during the mixing of ash promoting ageing or in connection with a granulation process.

In addition to the actual bed sand feeding system, also a feeding system of fuels can be used as supply channels for inorganic materials. Further, an extra channel can be used, which is used for supplying additives especially in some bigger plants.

Hard particles can be fed into the bed so that their amount in the bed does not exceed 50 percentages by weight. These can be quartz, common bed sand, olivine, corundum, granulated blast-furnace slag, or any particles with the hardness over the value 4 in the Mohs scale. Many fractions isolated from used fire-resistant materials are especially advantageous, including materials based on magnesium oxide, corundum, calcinated bauxite, chamotte, andalusite and spinel or combinations of these. Hard particles can also be present in carbonate materials as impurities. If calcite proves to be too soft a material in some boilers, according to the invention other carbonates can be used as bed material options, such as dolomite, the Mohs hardness of which is 3.5, when again the respective hardness of calcite is 3.

It has to be noted that the Mohs scale is logarithmic, which means that the Mohs hardness of 3.5 of a material is almost five-fold compared to the Mohs hardness of 3.0, if absolute hardness is considered. The use of dolomite as bed material, either entirely or partly, thus reduces pulverization compared to the use of calcite.

Part of the usable waste and by-product materials can have a distinctly smaller hardness than initial carbonites. Possible are, for example, partly calcinated carbonate materials as waste material of lime burning plants as well as magnesia or doloma brick material obtained from disassembled fire-resistant linings. Imperfectly burnt dolomite or calcite materials contains, in addition to residue carbonates, also free calcium and/or magnesium oxide or hydrated forms of these.

Magnesite sand is mainly magnesium carbonate, with a clearly bigger hardness than the other mentioned waste materials, but its fineness is under 0.2 mm so that its pulverization is comparatively big. The carbonate in magnesite sand is ferriferous breunneritic carbonate. The iron in breunnerite can bind harmful compounds of fuels, such as chlorine, but magnesite sand increases somewhat the heavy metal load, among others, for the part of nickel. This has to be taken into consideration when determining the mixture ratio of the bed sand to be fed. Excessive dilution of fuel-based pulverized fuel ash with bed material may lead to problems in flue gas filters, because they can be clogged. This has to be taken into account when planning the dilution rate and usability of the obtained ash. Most preferably the dilution is made at least 1 .1 - 3-fold (dilution 10% - 300%). Magnesite or impure magnesite sand can be used alone or in addition to other specified materials. The grinding hardness of magnesite is about ten-fold compared to calcite, so adding it to the bed material reduces significantly the pulverization of the bed material. The pulverization speed of the bed material is adjusted with a correct choice of materials, which is impacted, in addition to the type of the material itself, also by the particle size, fuel used, purpose of use of the ash, drive parameters of the process and use of possible other inorganic additives. A second possibility to adjust the pulverization of the bed material is to feed the combustion gases back to the bed zone.

Bottom ash, which for example in the bubbling bed boiler consists almost entirely of bed sand particles, has a clearly poorer nutrient value than pulverized flue ash, but it contains relatively high amounts of phosphorus, if peat or fuel rich in phosphorus has been used in the fuel mixture. Because of this bottom ash is as such poorly suited for fertilizing. Its heavy metal content is usually clearly smaller than that of pulverized fuel ash. In the technique of the invention the bottom ash contains less silica, iron and aluminium compared to common ones, but more calcium and/or magnesium, either oxidized or as carbonite. Bottom ash particles containing carbonite are usually the biggest, and the carbonate remaining non-degradable is inside the particles. The surface parts of the particle have calcinated. The liming value of the bottom ash of the invention increases the fertilizing value of the ash compared to common bottom ash.

The bottom ash does not include significantly more nutrients (K, Na, P, Fe, S) than conventional bottom ash. Because the heavy metal content usually is low, the bottom ash can be combined with pulverized fuel ash, when needed; for example, in the ageing, hardening or granulation phase. It can also be utilized separately as soil conditioner, for example, for the purpose of liming. The particles in pulverized flue ash are very fine, and they do not contain much residue carbonate.

The nutrient value of ashes applicable in fertilizer use can be increased by mixing with them other inorganic or organic fertilizer products or materials, by-products or applicable waste. For example, the nitrogen content can be increased by adding different types of bio sludge in the ash wetting phase, such as sewage sludge, sludge from the forest industry or digestion residue of bio power plants. Phosphorus can be added in a suitable form or potassium, for example, as phlogopite or a derivative of phlogopite.

Ashes containing high amounts of free calcium oxide can be provided with a component lowering the pH value, for example, bottom ash with a lower calcination rate from the same process or primary dolomite, calcite or magnesite.

Example 2 Dolomite was changed as bed material to a small BFB boiler of 6 MW, in which wood chips and peat in the proportion of 50/50 were burned. The chemical composition of dolomite as annealed was: CaO 76%, MgO 16%, AI203 1 %, Si02 4%, Fe203 2%. The phase composition of unannealed dolomite was: dolomite 60%, calcite 30%, quartz 3%, mica 6%. The granular size range of dolomite was 0 - 3 mm. The composition of ash, using a common bed material was: CaO 34%, K2O 9%, MgO 9%, AI2O3 3%, Fe2O3 7%, SO3 6%, SiO2 21 %, Na2O 5%, P2O5 3%. The amount of cadmium in common ash was 3.7 ppm so the need for dilution to reach the field ash fertilizer class was 60%, as the highest allowed amount of cadmium for ash used in field fertiliz- ing is 1 .5 mg/kg. For the part of other heavy metals, the need for dilution was less than 60%.

The amount of fluidized air was the same as with common bed material, but the need for fluidized air decreased as the bed material started to calcinate, pulverize and fine grind as well as detach from the bed into flue gases. After a run of about 5 hours with the power of 3 MW, the adding of bed material was started and it was observed that the addition need was approximately 100 kg/h. Pulverized flue ash is generated 120 kg/h with common bed sand. The dilution was thus approximately 45%. The obtained pulverized flue ash, in which the Cd content was approximately 2.0 ppm, is applicable in green area development and landscaping in ac- cordance with the regulation in force.

The change in the ash composition corresponded approximately to the theoretical change caused by the addition of dolomite. During the test it was noted that the combustion was efficient and the target power was achieved with a slightly smaller amount of fuel than average. The residual oxygen in flue gas was slightly below the normal level.

The test was continued for approximately 8 days. After the test ended it was noted that only slight deposit had formed at some places into the flue gas channel and onto the surfaces of the heat recovery pipelines, the deposit being removable by conventional means. As an advantage it was noted that in several places there was deposit even on normally bright metal surfaces and on oxidized metal surfaces, which phenomenon protects metal surfaces from erosion. During the test, sand and gravel had landed in the bottom ash, but the bottom ash had no sintering or agglomerates.

Example 3

The test above was repeated, but in the particle size distribution of the dolomite there were more fine fractions represented. In this case, a dilution of over 66% was achieved in the ash, but 10% of hard recycled corundum particles were added to the bed at the same time as the supply of dolomite into the bed was slightly reduced. After this the dilution decreased to 62%, which means that the ash obtained is applicable in field fertilizing.

After the test it was noted that the deposits of fine fractions on the metal surfaces were smaller than in the previous test, even though the test lasted for 12 hours. An increase of 0.3% in the aluminium oxide content compared to common ash was found in the ash analysis, as well as an increase in the contents of CaO, MgO and other component parts of dolomite, corresponding to the amount of added dolomite.

Example 4 Bed sand of a BFB boiler of 20 MW was replaced by dolomite material, the properties of which have been described in the test above. During the test, wood chips, peat and food plastic waste in the proportion of 40/40/20 were burned in the boiler with the power of 70%. Normally, the share of waste of fuel is only 10%, because the HCI content of flue gases rises above the allowed level with a higher amount of waste incineration. A fixed FT-IR analyser and other devices were used for analysing the exhaust gas. At the beginning of the test there were difficulties to make the bed float, but the matter was solved by using a larger amount of fluidized air. Later the need for fluidized air was reduced close to the usual amount. Dolomite was added to the bed while the test lasted. During the test it was noted that the HCI content of flue gas was reduced in the exiting flue gas so that, in this regard, plastic waste could have been supplied as much as 40% of the fuel, which however was not possible because of other reasons. Heavy metal contents of the ash decreased much below the limit set for field ash fertilizer. The use of plastic waste in a power plant in an amount of 20% achieves considerable savings in fuel costs.

The invention can vary within the limits defined by the enclosed patent claims.