GB2465055A

GB2465055A - Manufacturing cement by burning wood and grit aggregate

Info

Publication number: GB2465055A
Application number: GB0917783A
Authority: GB
Inventors: John William Carson
Original assignee: Individual
Current assignee: Individual
Priority date: 2008-11-07
Filing date: 2009-10-12
Publication date: 2010-05-12
Anticipated expiration: 2029-10-12
Also published as: GB0917783D0; GB2465055B; GB0820403D0

Abstract

A process for manufacturing cement comprises the burning of a mixture of wood and grit aggregate in a fluid-bed furnace, wherein the grit aggregate disintegrates on burning. The grit aggregate comprises from 50 to 80wt% silica, 20 to 40wt% alumina, less than lOwt% calcium oxide and less than lOwt% magnesium oxide. When the grit aggregate disintegrates, it may yield oxidic species in a finely divided form. Preferably, the wood is selected from chipped virgin timber, waste wood, treated waste wood or a mixture thereof. A cement product is also claimed, which may be used as a replacement for Portland cement in ready-mix concrete, pre-cast concrete, mortar, grout, tile casting, reinforced concrete, high alumina cements, fast curing cements etc. Another independent claim is directed toward the use of a grit aggregate comprising 50 to 80wt% silica, 20 to 40wt% alumina, less than 10wt% calcium oxide and less than 10wt% magnesium oxide as a pyrolysis aid in a wood burning fluid-bed furnace.

Description

IMPROVED CEMENT

The present invention relates to a process by which a cementitious product can be manufactured without the use of significant quantities of fossil fuels.

The process defined herein is based on the combustion of timber to provide thermal energy for the generation of electrical power.

Wood burning power stations that use fluid-bed furnaces generate an ash as a by-product of the combustion of timber.

The combustion products normally derived from the furnacing of timber are highly calcarious oxide mixtures. Such ashes have limited usefulness.

The process of the present invention allows this ash to be converted into a cement. The cement formed is similar to Portland Cement but has environmental and techno-commercial advantages.

Portland Cement is the most common type of cement in general use around the world.

However it has some disadvantages, for example: Conventional Portland Cement is made from three main components, a calcarious mineral, a silicaceous mineral and fossil fuel. The calcarious mineral is usually a limestone or chalkstone which is mined and crushed via high energy processes. Similarly the silicaceous mineral, which can be a clay or cement stone or a clay limestone, is mined and crushed via high energy processes. These two minerals are then calcined, typically in low thermal efficiency rotary kilns using a fossil fuel such as coal or oil, or a derivative there from. Extremely high calcination temperatures and long kiln residence times are needed for the formation of the calcia-silica chemical entities present in Portland Cement. Therefore the calcination process described above is highly expensive in terms of fossil fuels. The semi-finished product obtained via this fossil fuel-intensive process is a cement clinker. This must be ultra-finely ground to produce the finished cementitious product, and, thus consumes still further energy.

It is an aim of the present invention to provide a cement having improved properties and/or which is prepared by a process which overcomes at least one disadvantage of the prior art.

According to a first aspect of the present invention there is provided a process for manufacturing a cement product, the process comprising burning in a fluid-bed furnace a mixture of wood and a grit aggregate wherein the grit aggregate disintegrates on burning.

The process of the present invention preferably comprises oxidative pyrolysis in a wood burning furnace of a mixture of timber and grit aggregate which disintegrates on burning.

According to a second aspect of the present invention there is provided a cement product formed by the process of the first aspect. This cement product may also be referred to herein as the cementitious entity.

The product produced by the present invention is suitably a cementitious entity comprised principally of finely divided and reacted mixed oxides of calcium, silicon and aluminium.

It is a preferred feature of the present invention that the nature of the grit aggregate used in the fluid-bed furnaces is modified compared to that used in the prior art. The modification provides for the production of finely divided reactive particles which combine with the wood ash to produce the desired cement product.

The grit aggregate used in the present invention disintegrates during burning. Suitably the grit aggregate will disintegrate upon oxidative pyrolysis.

In the process of the present invention the grit aggregate is suitably used as a pyrolysis aid in a fluid bed furnace. This may be chosen or manufactured so as to purposefully disintegrate during furnacing to yield oxidic species in finely divided form.

Examples of appropriate aggregate grits are natural and synthetic minerals that, upon oxidative pyrolysis, can yield silica, alumina, calcia and other finely divided oxides for example fluxes.

Thus preferred grit aggregates for use in the present invention are materials which oxidise under the furnace conditions and form finely divided oxide products. These minerals may be natural or synthetic or a mixture thereof.

These finely divided oxides may unite, during the furnacing process, with finely divided oxidic ashes derived from the timber combusted in the furnace.

According to the present invention these calcarious wood ashes may be thermo-chemically beneficiated by reaction with finely divided silicaceous dusts emanating from the aggregate grit used in the fluid bed furnace.

The process of the present invention provides for the production of chemically reacted and/or blended finely divided calcarious and silicaceous dusts which constitute a cementitious product, similar to Portland Cement, but produced via an advantageous process.

Careful selection of appropriate pyrolysis aiding grit aggregates allows the cementitious entity provided by the present invention to be produced.

Typically, the cement product of the present invention comprises the oxides of calcium, silicon and aluminium, and chemical combinations of these and other oxides, in finely divided form.

As such the product of the present invention resembles Portland Cement, but has advantageous properties, as does the process by which it is made.

Advantages of the process and product of the present invention include the following: The process of the present invention will be seen, specifically, not to need the high fossil fuel input, nor the high energy input generally, of conventional Portland Cement manufacture.

Moreover the product of the present invention is a by-product of a fossil free route to electric power generation.

The product of the present invention is provided as an ultra-fine powder, without the need for energy intensive grinding.

Far less mineral mining and comminution is involved in the manufacture of the product of the present invention.

The furnace type used in the manufacture of the product of the present invention is inherently more thermally efficient than that employed in conventional Portland Cement manufacture.

The carbon dioxide emission associated directly with the process of the present invention is far lower than that produced by conventional Portland Cement manufacture.

The fluidised bed furnacing of wood and timber materials to produce thermal energy for electricity production will in the future be used for large scale power generation.

Preferred aspects of the present invention will now be further defined.

The process of the present invention preferably involves the comminution of wood and/or timber into a chip-sized feed. Suitably this is then fed continuously into a fluid bed, or similar furnace. Such fluid bed furnaces are well known in industrial calcination processes.

Most of the more efficient fluid-bed furnaces employ a mineral sand or grit i.e. a ceramic aggregate in their hearths, which acts as a process aid, that is a pyrolysis improver.

Currently, the choice of the grit to be used in such fluid-bed furnaces is somewhat arbitrary, which is to say that any of a wide range of grits could be employed.

In the present invention it is proposed to use specific fluid bed grits, selected on the basis of their physico-chemical character.

Although it is known to use grit aggregates to promote pyrolysis in a wood burning fluid bed furnace, the resultant ash-containing mixture has not previously been utilised. The grits previously used in the prior art have been slow to thermally decompose and do not disintegrate on burning as quickly as the grit aggregates used in the present invention.

Without wishing to be bound by any theory, it is believed that burning in the furnace causes oxidation of metallic ions in the grit which causes their ionic radii to decrease. This in turn it is believed disrupts the crystal structure which causes the grit to disintegrate. The material produced is typically in the form of a fine powder.

Preferably in the process of the present invention a specific fluid-bed grit is employed which pyrolytically disintegrates, in a controlled fashion, to yield oxidic entities in a finely divided particulate format that contribute elements to the desired cementitious entity being produced.

The specific fluid-bed grit employed may be a natural aggregate, or a blend of natural aggregates or a synthetic aggregate, a blend of synthetic aggregates or a blend of synthetic and natural aggregates.

In some preferred embodiments the grit aggregate comprises a synthetic grit aggregate.

Preferably the grit aggregate used in the present invention comprises at least 4Owt% silica, preferably at least 5Owt%. It may comprise up to 9Owt% silica, preferably up to BOwt%.

Preferably the grit aggregate used in the present invention comprises at least lOwt% alumina, preferably at least 2Owt%. It may comprise up to 5Owt% alumina, preferably up to 5Owt%.

Preferably the grit aggregate used in the present invention comprises less than 2Owt% calcia, preferably less than 1 Owt%.

Preferably the grit aggregate used in the present invention comprises less than 20wt% magnesia, preferably less than lOwt%.

In one aspect the present invention provides a process for manufacturing a cement product, the process comprising burning in a fluid-bed furnace a mixture of wood and a grit aggregate, wherein the grit aggregate comprises from 50 to 80wt% silica, 20 to 40wt% aluminia, less than lOwt% calcia and less than lOwt% magnesia. The invention further provides the product obtained by this process.

In some embodiments, the grit aggregate may be fed to the furnace by a continuous feed mechanism.

The only modification to the conventional fluid-bed furnace, to enable it to be so used in the present invention, is a continuous feed mechanism for the fluid bed grit. Many conventional fluid-bed furnaces are already fitted with such a device, and most others could easily be retro-fitted so as to convert these to continuous fluid-bed grit-fed units.

Alternatively, the speciality grit aggregate may be added in conjunction with the wood chip feed. If this system is used it has been found useful to compound the grit in a slip or slurry of an alum mo silicate clay that adheres to the wood chip feed, and to spray the wood chip feed with this slurry.

The present invention therefore provides a simple and economic improvement to a conventional fluid bed furnace when used to burn wood or timber so as to create thermal energy for the generation of electrical power. The co-product of such an improvement is the cement product of the present invention, that is a cementitious composition in finely divided format, having considerable commercial value, rather than a high calcarious ash of little value.

The present invention specifically involves fluid-bed and similar furnaces, rather than rotary kilning or conventional coal burning power station furnaces.

It has been discovered through our on-going experimentation into the physico-chemical calcination reactions involved in fluid bed furnaces, that highly reactive oxidic entities in finely divided form may be produced.

The process involved in the present invention may be described as Sacrificial Aggregate Furnacing for the production of Reactive Oxidic Nuclei. We refer to this as the SAFFRON process.

Fluid-bed furnacing causes the disintegration of the grits used as pyrolysis aids. It therefore follows that oxidic particulates derived from the combustion aid grits become inter-mixed with ash particles produced from the pyrolysis fuel -wood in the case of the present invention.

It is believed from our research that both physical and chemical bonding takes place between the grit-derived aggregate and the wood-derived reactive oxide entities, at the high flame temperature involved in the furnacing operation.

It is believed that the result is a range of physico-chemically bonded sub micron mixed oxide compounds. These compounds are derived from the SAFFRON process which we believe to be akin to the known technique of flame pyrolysis.

In some embodiments the present invention may be used to provide sub-micron dusts containing various fluxed mixed oxide compounds with varying ratios of calcia to alumina to silica.

Such calcium alumino silicate powders have physico-chemical characteristics that are similar to and in some cases superior to Portland Cement.

In some preferred embodiments the surface area and hydraulic reactivity of these flame pyrolysed calcium alumino silicate powders are such that they can exceed both the beneficial short term setting and longer term hardening characteristics of Portland Cement.

Preferably the surface areas and hydraulic reactivities are such in the products of the present invention, that no further grinding is required. The calcium alumino silicate powders, as produced, constitute a beneficial replacement for Portland Cement.

Thus in preferred embodiments the cement product of the second aspect of the present invention is obtained by the process of the first aspect without the need for an additional grinding step. Suitably all that needs to be done is for the calcium alumino silicate powders, so produced, to be bag-filtered or electro-statically precipitated from the furnace flue gases and the product allowed to cool before packaging for distribution. Both bag filtration and electrostatic precipitation are well known technologies in the power generation industry.

The feed stocks for the process as defined in the present invention will now be defined further.

Preferably there are only two feed-stocks needed in the process of the present invention.

These are wood chips and a silica and flux containing grit aggregate.

The wood chips may be derived either from chipped virgin timber or from used and recovered, that is waste wood, or from treated waste wood, or any combination.

It will be noted that wood and timber constitute the conventional feed for a wood-burning power station. Thus the wood chip feed stock for the present invention may be the normal co-product of wood burning power stations.

In the case of either or both virgin or used timber feed-stocks the result of combustion product of wood is a dust having with the following key ingredients: 60% to 90% GaO 1%to 10% MgO 1% to 10% K20 and other fluxing oxides.

Suitably the second feed is to be a specifically manufactured, blended or chosen mineral or ceramic aggregate grit that pyrolytically degenerates into a fine dust during wood kilning.

A suitable siliceous grit dust may be derived from either natural mineral sources or from used and recovered ceramic or pottery industry wastes, that is from synthetic minerals.

A further suitable grit may be obtained from a mixture of estuarine sills and by-products from the chemical industry comprising iron salts.

Preferably the aggregate grit used in the present invention comprises a mineral which produces silica and/or alumina on burning.

In either case the result of calcination of the chosen mineral, via pyrolytic attrition would preferably be a dust having the following range of key ingredients: 10% to 90% Si02 10% to 80% A1203 1% to 40% CaO 1% to 20% fluxing oxides In the present invention it is proposed to process both wood feeds and aggregate grits which preferably comprise siliceous aggregate grits concurrently via the use of the latter as a sacrificial combustion aid during the furnacing of the former for the generation of thermal energy for conversion into electrical power.

The concurrent calcination during the burning process of the present invention preferably yields a mixed and chemically reacted dust containing elements derived from both the wood feed and the grit aggregate.

The proportionality of oxidic entities derived from these two sources may be controlled by the choice of a suitable grit aggregate which disintegrates in a controlled manner. This may be regarded as a pyrolytic attrition process.

Typically, an aggregate grit is chosen which contributes between 5 and 50% by weight of its oxides to the mixed oxidic product. This is to say that in each tonne of calcium alum mo silicate cementitious product produced, there will be between 5% and 50% of the oxides derived from the grit disintegrated.

The present invention will now be further defined by way of the following non-limiting example: A fluidised bed furnace was continuously charged with a granite based alumino silicate grit as a sacrificial aggregate. The analysis of this aggregate grit was: 65% Si02 20% A1203 15% fluxing and other oxides.

The furnace was fed with a mixed arising of virgin and waste woods.

The wood feed to the furnace would normally be expected to produce an ash with the following constitution: 80% GaO 10% MgO 10% K20 and other fluxing oxides.

The furnace was operated such that 35% by weight of the mixed combustion dusts produced were derived from the disintegration of the fluid bed sacrificial aggregate grit.

The resulting calcium alumino silicate mixed and reacted dust produced, analysed as follows: 50% GaO 20% Si02 10%A1202 20% magnesia and fluxing oxides.

The product obtained by bag filtration of the flue gases exiting the furnace was a cementitious powder similar to Portland Cement. Its physico-chemical properties may be summarised as follows.

Particle size 99.9% passing 0.1 mm sieve Average particle diameter 0.5micron Di calcium silicate content at particulate surface, greater than 15% Tn calcium silicate content at particulate surface, greater than 15% Initial set, similar to Portland Cement Final hardness, similar to Portland Cement.

The calcium alumino silicate cement produced has been shown to be a suitable replacement for Portland Cement in the following exemplified applications: Ready-mix concrete Pre-cast concrete Mortar Grout Tile casting Reinforced concrete Additionally our on-going research is developing specific formulas for use in: High alumina cements Fast curing cements Special cements Specifically, we have developed cementitious compositions, based on the SAFFRON process described herein, wherein the following elemental additions have advantageous effects.

Lithium Boron Carbon Sodium Phosphorus Sulphur Each of the above elements may be provided via the process of the first aspect of the present invention as described herein.

The above elements are only examples of the additional elements that can be provided.

Several of the above oxidic additions have been shown to operate as fluxes in the process of the present invention, that is they flux the formation of mixed oxide compounds. Such additions thus confer beneficial properties on the cementitious properties of the products of the current invention.

Our research is continuing on the range of sacrificial aggregate grits that can be used in the process of the present invention.

It is a further feature of the present invention that various oxide generating entities i.e. oxide precursors, may be sprayed onto wood chip feed, in the form of an aggregate slip and this slip dried onto the feed, using power station waste heat. In this way still further oxidic entities can be added to the product of the current invention.

A very wide range of both natural and synthetic low cost grit aggregates can be used in the process of the present invention and by this process that many novel cementitious products can be provided in wood fired power stations.

Our research is also continuing on air elutriation and other particle separation processes that can be used in the process of the present invention.

In a further aspect of the present invention the products obtained by the first aspect may be blended with other oxidic materials to produce advanced cementitious entities and concrete products.

Our research has shown that concretes with advantageous properties can be manufactured from the cement products of the present invention.

Taken together we believe the above advancements in the operation of wood fired power stations, so as to produce speciality cements with low fossil fuel involvement, constitutes both a technically advantageous and economically justifiable venture, with positive environmental credentials.

Claims

Claims 1. A process for manufacturing a cement product, the process comprising burning in a fluid-bed furnace a mixture of wood and a grit aggregate wherein the grit aggregate disintegrates on burning.
2. A process according to claim 1 wherein the grit aggregate disintegrates during furnacing to yield oxidic species is finely divided from.
3. A process according to claim 1 or claim 2 wherein the grit aggregate comprises at least 5Owt% silica.
4. A process according to any preceding claim wherein the grit aggregate comprises at least 2Owt% alumina.
5. A process according to any preceding claim wherein the wood is selected from chipped virgin timber, waste wood, treated waste wood or a mixture thereof.
6. A process for manufacturing a cement product, the process comprising burning in a fluid-bed furnace a mixture of wood and a grit-aggregate, wherein the grit aggregate comprises from 50 to BOwt% silica, 20 to 4Owt% aluminia, less than lOwt% calcia and less than 1 Owt% alumina.
7. A cement product formed by a process as claimed in any preceding claim.
8. A cement product according to claim 7 which further comprises one or more elements selected from lithium, boron, carbon, sodium, phosphorous and sulphur.
9. The use of a cement product as claimed in claim 7 or claim 8 as a replacement for Portland cement in one or more of ready-mix concrete, pre-cast concrete, mortar, grout, tile casting, reinforced concrete, high alumina cements, fast curing cements and special cements.
10. The use of a grit aggregate comprising 50 to BOwt% silica, 20 to 4Owt% alumina, less than lOwt% calcia and less than lOwt% magnesia as a pyrolysis aid in a wood burning fluid bed furnace.
11. A process for manufacturing a cement product substantially as hereinbetore described.
12. A cement product substantially as hereinbefore described.