EP0960332A1

EP0960332A1 - Method and means for fuel valuation

Info

Publication number: EP0960332A1
Application number: EP98950596A
Authority: EP
Inventors: Ahmad Reza Shirazi
Original assignee: Individual
Current assignee: Individual
Priority date: 1997-10-22
Filing date: 1998-10-22
Publication date: 1999-12-01
Also published as: SE9703871D0; SE9703871L; JP2001513898A; WO1999022573A2; AU9659798A

Description

Method and means for fuel valuation.

The present invention relates to a method for the fuel valuation of fossil fuels and mineral substances contained therein for a specific combustion system.

Coal consists of organic material (maceral) and inorganic material (mineral substances) . The mineral substances in coal are present in different quantities and forms. The quantities of mineral substances in naturally occurring coal can vary from 10 to 50 per cent by weight. This fact is of major significance to the economic value and the technically effective use of coal. The mineral substances present in coal can be subdivided into three different categories on the basis of their origin:

1) mineral substances originating from carboniferous plants; 2) mineral substances formed during the first stage of the carbonization process or introduced into the coal sediment by water and wind; and

3) mineral substances formed during the second stage of the carbonization process, i.e. post-consolidation of the coal.

The mineral substances present in coal can occur in different states of aggregation: as small inclusions in organic material, as individual crystals, and as extensive layers (according to Article #2, "Direct quantitative analysis of mineral matter and different forms of pyritic sulfur in coal by electron probe microanalysis (EPMA) and automated image analysis (AIA)", by Ahmad Reza Shirazi, Lars Eklund* and Oliver Lindqvist, Department of Inorganic Chemistry, Chalmers University of Technology and University of Gothenburg, Sweden, ^Swedish Ceramic Institute, Sweden (received on 4 November 1992; revised on 15 March 1993) . In the course of mining, large quantities of minerals can also find their way into the product from sedimentation layers above or below the seams of coal. The presence of mineral substances in coal gives rise to various problems in conjunction with the burning of coal. Minerals such as pyrites or marcasite with their heavy metal impurities (such as Pb and As) and their high sulphur content cause extensive environmental damage. It is necessary, therefore, to study the groups of minerals found in coal, for example clay, sulphur and carbonates, in an isolated state, which means that most of the aceral content must be removed. A new method for producing samples, "Very Low Temperature Ashing (VLTA)", has been developed for the purposes of obtaining an "in-situ" ash, in accordance with US, A 5,492,832 and SE 9102320-0. This unique method enables mineral substances to be preserved and removed from coal without causing losses or damage to the original phases. Mineral substances present in coal were previously quantified indirectly, i.e. the production of ash was determined, and the result was converted into the original mineral substances by applying one of the three empirical formulae, i.e. Parr, King or ASTM, in accordance with Article #2. A new analytical method has been developed in this study for the direct and quantitative determination of the mineral substances in coal by the use of an electron probe micro-analyser (EPMA) in conjunction with automated image analysis (AIA) . These studies have revealed that the mineral substances in coal are usually present in larger quantities than previously assumed.

Table 1 Total mineral content, as a percentage by weight of the analysed coal samples.

Coal Ash^a Parr King^c ASTM^d EMPA-AIA

1 22.9 25.0 24.7 26.0 26.68±0.05

2 8.7 11.3 4.2 7.2 14.42±0.05

3 13.1 17.3 14.5 18.9 16.85±0.05

According to ASTM D3174 ^b MM = 1.08 (ash) + 0.55 (S_tot)

MM = 1.09 (ash) + 0.5 (S_pyι:ιtβa) + 0.8 (C0₂) - l.l(S0_3ash) + (S0₃coal) + 0.5(C1) MM - 1.13 (ash) + 0.5 (S_pyrιtes) + 0.8(CO₂) - 2.8 (C0₂) (S_ash) +

2.8(S_so4) + 0.3(C1)

The mineral substances in coal should thus not be confused with the ash content, since the mineral substances are present in the raw coal, whereas the ash consists of the decomposition products from these minerals after combustion. The difference between the ash content and the mineral substances content is due to the minerals, i.e. the carbonates and clay minerals, which usually decompose during the ashing process (or during combustion) . These minerals are the minerals most commonly encountered in coal.

This claim resulted in a study to assess the effect of mineral substances in coal on the combustion of coal.

Fluidized bed combustion (FBC) and powdered coal combustion (PCC) were simulated by means of simple models in accordance with Article #3. The mineral substances in coal also give rise to impurities and slag formation in combustion plants, in addition to which large quantities of ash are formed, which also causes environmental problems. It has also been demonstrated that the mineral substances in coal are a. source^" of additional problems: the calorific value of coal (based on the absence of mineral substances) during combustion reduces as the content of mineral substances increases (according to Article #3, "The impact of mineral matter in coal on its combustion, and a new approach to the determination of the calorific value of coal", by Ahmad Reza Shirazi, Olle Bcrtin, Lars Eklund* and Oliver Lindqvist, Department of Inorganic Chemistry, Chalmers University of Technology and University of Gothenburg, Sweden, ^Swedish Ceramic Institute, Sweden (received on 17 August 1993; revised on 18 April 1994) . This study includes a discussion of the determination of the calorific value of coal by means of established methods. Three coal models with different grades of coal and with different mineral substance contents were selected for calculating the theoretical calorific value of coal combustion. Combustion was simulated in FBC (combustion temperature 850°C) and PCC (combustion temperature 1200°C) with an air factor (0₂/C) of 1.2.

These calculations show that the quantity of energy (calorific value) that can be obtained from coal is inversely proportional to the quantity of mineral substances present. This is attributable to the endothermic decomposition reactions of the mineral products and the thermal capacity of such minerals and their products (ash) . The object of this invention is to show that the calorific value of fossil fuels is not a constant "unit", but that it varies significantly with the composition and constituents of the fuels and in comparison with different combustion systems. The invention proposes to calculate the "actual" calorific value of a specific fuel in comparison with a specific combustion system. The results show a considerable combustion energy loss (0.5-9%) .

In order to absorb the sulphur in fossil fuels (present in the form of S0₂) during combustion, it is usual to add limestone or dolomite to the combustion bed (for FBC) or to the combustion gas (flue gas) (for PCC) . Since limestone and dolomite also contain minerals, such as clays, the effect of adding these to the combustion process should be identical with the effect of minerals in coal. A limestone and dolomite model is used to calculate the effect of added limestone or dolomite on the calorific value during combustion.

The study shows that the energy loss attributable to the added limestone/dolomite is also significant (1-5%) .

The effect of the addition naturally increases with the increasing sulphur content in the fuel, since larger quantities of absorbents must be added, which results in an increased energy loss.

As a result, the energy loss associated with the combustion of raw coal (maceral + minerals + limestone/dolomite) or any other fossil fuels can be even greater as a consequence of mistakes when determining the calorific value and when optimizing the boiler.

The principal object of the present invention is thus, in the first instance, to find a method that permits a new way of determining the calorific value in comparison with a specific combustion system, and to optimize combustion when using coal and other fossil fuels and absorbent additives.

The aforementioned objectives are achieved by means of a method in accordance with the present invention, which is characterized essentially in that the calorific value of the actual fossil fuel concerned, such as coal, peat, natural gas and oil, etc., and of a particular combustion method, is calculated and recorded by means of a system, that any energy loss is calculated for mineral substances and added absorption agents and is recorded, i.e. the calorific values are determined for the fuels concerned by thermodynamic calculation, and all the exothermic and endothermic reactions and conversions that take place during the combustion of all the constituent parts of the aforementioned fuels (such as minerals and macerals in the fuel and any additives) are compared, and the thermal capacity of all the substances formed after combustion, are compared, in both solid phases and gaseous phases.

A further object of the invention is to make available means to permit the implementation of a method in accordance with the foregoing for the calculation of the "true" calorific value of fossil fuels for a particular combustion system.

Said object is achieved by means in accordance with the present invention, which is characterized essentially in that it involves the comparison of all the complicated conversions that are made of the original phases in the fuel and the additives, as well as the conversions to which newly formed phases are subjected, in both solid and gaseous phases, depending on the combustion circumstances, such as the combustion temperature, air factor and additive quantity, etc.

The present invention proposes determining the calorific values for the actual fuels in question by thermodynamic calculation, comparing all the exothermic and endothermic reactions and conversions that occur during the combustion of all the constituent components of the aforementioned fuels (such as minerals and macerals in the fuels and the additives) , and comparing the thermal capacity of all the substances formed after combustion, both solid phases and gaseous phases.

The expression "optimized" in the Claims and Description denotes that the necessary air factor for the specific fuel (and/or fuels + additives) and the specific combustion system to permit complete thermodynamic combustion are calculated precisely. Once the minimum necessary air factor has been calculated, the minimum possible air factor is measured in order to avoid energy losses due to incomplete combustion and energy losses due to excessive air, which causes large energy losses through its high thermal capacity. The efficiency of the combustion process in question is increased by the algorithm mapping out and assuming the reactions and events (conversion temperatures, minimum possible air factor, exothermic/endothermic reactions, thermal capacity of formed phases, gases and solid phases, new conversions, exothermic or endothermic, and their thermal capacity of newly formed phases, etc., etc.), which can take place on the basis of the input data, mineral content + maceral and additions of lime and dolomite. It is thus possible to "optimize" and thus to increase the "efficiency" of the system by avoiding the most endothermic events (reactions and thermal capacity) which can occur from the phases contained in the fuel.

It is possible to "auto-optimize" the system by feeding the combustion temperature and the air factor interval with which the system is able to operate into the program, and the program/algorithm is then able to calculate and draw a 3-dimensional curve with the x-axis as the calorific value, the y-axis as the combustion temperature interval, and the z-axis as the air factor interval. This curve provides a visual illustration of the energy loss cases that take place during the intervals that are fed in. It is possible to auto-optimize the combustion process by taking account of the specific fuel and the specific curve. The invention is based on new findings made by the inventor, which were achieved through research at the Chalmers University of Technology in Gothenburg and are presented below.

The aim of this invention is to demonstrate that the calorific value of fossil fuels is not a constant "unit", but that it varies significantly with the composition and constituents of the fuel and in comparison with different combustion systems. The invention proposes to calculate the "actual" calorific value of a specific fuel in comparison with a specific combustion system. The conclusions presented above have given rise to a commercially developed method and software known as "Fuel

Valuation™".

The method for the fuel valuation of fossil fuels and mineral substances contained therein for a specific combustion system in accordance with the invention proceeds in such a way that the calorific value is calculated for the actual fossil fuel in question, such as coal, peat, natural gas or oil, etc., and for a particular combustion method, and is recorded. Any energy loss for mineral substances and added absorption substances is also calculated and recorded.

In this way, the values obtained are compared in order to reduce the energy loss, and to optimize the combustion on the basis of the recorded values that are obtained.

The composition and quantity of the ash and combustion gases are also calculated after combustion and are recorded.

Different fuels are evaluated in respect of their mineral composition and the ability of the absorption medium to absorb S0₂ and the effect that the addition of these has on combustion from the point of view of energy conversion. The means for implementing a method for the fuel valuation of fossil fuels and the mineral substances contained therein for a specific combustion system consist in accordance with the invention of a computer program with an algorithm for the calculation of the true and exact calorific value of the fuel in question. In order to optimize the financial gain from using coal or other fossil fuels for energy production, commercial software (Fuel Valuation™) has been developed for the calculation of the "true" calorific value of fossil fuels for any combustion system.

This new method for determining the calorific value has given rise to commercially developed software, known as "Fuel Valuation™", which includes a unique algorithm for calculating the "true" and more exact calorific value of any fossil fuels for a specific combustion system. More efficient energy conversion of the fuel in this combustion system (and improved boiler efficiency) can thus be achieved if one has better insight into the nature of the fuel and the reactions that occur during combustion.

It is possible by means of "Fuel Valuation™" to calculate the calorific value of any fossil fuels, such as coal, peat, natural gas or oil, etc., for all combustion methods. The software also calculates the energy loss attributable to the mineral substances and the added absorption medium. The composition and quantity of the ash and the combustion gas (flue gas) are calculated after combustion (before and after the addition of the absorption medium) . The computer program enables the user to "demineralize" the fuel, with simultaneous optimization of the combustion parameters (and the boiler efficiency) , such as the minimum required air factor, the most appropriate combustion temperature (for example 700-1600°C), and the minimum quantity of the special absorption medium required in order to reduce the energy loss to a minimum with the greatest possible absorption of S0₂.

Fuel Valuation™ can replace costly and tirtie- consuming analytical methods, such as the adiabatic bomb calorimeter and the isoperibolic bomb calorimeter, which are used for determination of the calorific value, and TGA (Thermogravimetry Analysis) , which is normally used for determining the calcination capacity of the special absorption medium.

Determination of calorific value The calorific value is an indicator of the chemically stored energy in coal and is a very important ^"parameter when assessing the value of coal as a fuel. The calorific value of coal can be expressed in two forms. One of these is the gross calorific value (gross combustion heat) , i.e. "the heat that is generated by the combustion of a unit of quantity, at a constant volume, in an oxygen bomb calorimeter under special conditions, so that all the water in the products remains present in liquid form". The other is the net calorific value (net combustion heat), i.e. "the heat that is generated by the combustion of a unit of quantity of fuel, at a constant atmospheric pressure, under conditions such that all the water in the products remains present in the form of vapour"⁴. The net calorific value is calculated according to the ASTM method from the gross calorific value by "a deduction of 1030 Btu/lb (572 cal/g) of water which comes from the unit of quantity of fuel, including both the water that was present originally in the form of moisture and the water formed by combustion". The calorific value of coal is determined by methods, as described in ISO 1928, ASTM D2015 and ASTM D3286, for example. According to all these methods, the gross calorific value is measured by means of the bomb calorimeter method, and the net calorific value is calculated. ASTM D2015 describes the use of an adiabatic bomb calorimeter, and an isoperibolic (isothermal jacket) bomb calorimeter is used in accordance with ASTM D3286. According to each of these methods, a weighed quantity (usually 1 g) of coal (maceral and minerals) is burned in a closed vessel in the presence of oxygen under pressure, and the quantity of heat released is measured. The factors that give rise to mistakes in conjunction with such an analysis include the difficulty in obtaining a representative sample of such a heterogeneous material as coal, a lack of reproducibility between the samples and the analysts, and the incomplete combustion of the sample. Because the coal sample contains both minerals and maceral, the results obtained from such an analysis ^"contain errors.

Another weak point associated with the determination of the calorific value of coal relates to the temperature achieved during combustion. Since 1 g of coal, regardless of its grade, is usually burnt in the calorimeter, the combustion temperature must vary with the grade of coal used. The extent of this variation in temperature and its effect on the calorimetric determination have not been clarified. Because the sample contains both minerals and maceral, the combustion temperature is also dependent on the content of mineral substances, which is why variation occurs in the nature of the mineral substance reactions. Accordingly, the calorific value obtained for the coal sample (based on the absence of mineral substances) will vary both with the grades of coal and the quantity and form of the mineral substances.

Of the other advantages associated with the use of Fuel Valuation™, mention should be made here of the problem of the fuel consumer being obliged to pay for the minerals and the disposal of the residual ash. Fuel Valuation™ enables the user to evaluate different fuels with regard to their mineral composition and absorption medium (the ability to absorb S0₂, and the effect of the addition of these substances on combustion) from the point of view of energy conversion.

The program permits the user to "demineralize" the fuel and thus to calculate the energy gain for different combustion systems for an actual case of demineralization of the fuel before combustion.

The invention is not restricted to the above description, but may be varied within the scope of the Patent Claims without departing from the idea of invention.

Claims

P a t e n t C l a i m s

1. Method for the determination of calorific values for fossil fuels and mineral substances contained therein for a specific combustion system, characterized In that the calorific value of the actual fossil fuel concerned, such as coal, peat, natural gas and oil, etc., and for a particular combustion method, is calculated and recorded by means of a system, and in that any energy loss is calculated for mineral substances and added absorption agents and is recorded, i.e. the calorific values are determined for the fuels concerned by thermodynamic calculation, and all the exothermic and endothermic reactions and conversions that take place during the combustion of all the constituent parts of said fuels (such as minerals and macerals in the fuel and additives) are compared, and the thermal capacity of all the substances formed after combustion are compared, in both solid phases and gaseous phases.

2. Method as claimed in Patent Claim 1, characterized In that the composition and quantity of the ash and combustion gases (flue gas) are calculated after combustion and are recorded.

3. Method for determining the calorific value of fossil fuels, characterized In that the actual calorific value of a specific fuel is calculated in comparison with a specific combustion system, so that the values that are obtained show that the calorific value of fossil fuels varies significantly with the composition and constituents of the fuel and with different combustion systems and combustion parameters .

4. Method as claimed in one or other of Patent Claims 1-3, characterized In that combustion parameters such as the minimum required air factor, the most appropriate combustion temperature and the minimum quantity of absorbents (such as limestone and dolomite) required for a minimum energy loss and the highest absorption of S0₂, are calculated precisely, and in that the efficiency of various combustion systems is increased significantly in this way, i.e. the efficiency of the combustion process concerned is increased by the algorithm mapping out and assuming the reactions and events (conversion temperatures, minimum possible air factor, exothermic/endothermic reactions, thermal capacity of formed phases, gases and solid phases, new conversions, exothermic or endothermic, and their thermal capacity of newly formed phases, etc., etc.), which can take place on the basis of the input data, mineral content + maceral and additions of lime and dolomite.

5. Method as claimed in one or other of the foregoing Patent Claims, characterized In that the fuel is demineralized and the energy gain for different combustion systems is calculated for an actual case of demineralization of a specific fuel before specific combustion.

6. Method as claimed in one or other of the foregoing Patent Claims, characterized In that the fuels are evaluated in respect of their mineral composition and the ability of the absorption medium to absorb S0₂ and the effect that the^' addition of these has on combustion from the point of view of energy conversion.

7. Method as claimed in one or other of the foregoing Patent Claims, characterized In that specific combustion systems are "auto-optimized" against specific fuels.

8. Method as claimed in one or other of Patent Claims 1-6, characterized In that specific fuels are "auto- optimized" against specific combustion systems.

9. Means for implementing a method for the fuel valuation of fossil fuels and the mineral substances contained therein for a specific combustion system in accordance with one or other of the aforementioned Patent Claims, characterized In that it involves the comparison of all the complicated conversions that are made of the original phases in the fuel and the additives, as well as the conversions to which newly formed phases are subjected, in both solid and gaseous phases, depending on the combustion circumstances, such as the combustion temperature, air factor and additive quantity, etc.