This invention relates to a method of oxy-fuel combustion, more particularly, steam-moderated oxy-fuel combustion, in which combustion occurs in water vapour and/or steam diluted oxygen atmosphere, unlike nitrogen atmosphere as in the known form of combustion.
The benefits of this invention are lower NOx lower volumetric flows in comparison with conventional combustion, easy retrofitting, less air leakage and hence economic and efficient carbon dioxide sequestration.
This invention relates to the combusting of hydrocarbon fuels for all industrial applications such as coal or natural gas fired power plants, metallurgical industries and so on.

n general, most of the known combustion processes involve air as the oxidizer and hydrocarbons, such as coal, as their fuel source. Consequently, most of the combustion products of these fuels are harmful. Carbon dioxide is recognized as a global warming gas while carbon monoxide, unburnt hydrocarbons, NOx and SOx gases, particulates, are atmospheric pollutants and are harmful to the environment. While the production of carbon dioxide and water vapour are inevitable in the burning of hydrocarbon fuel, the extent of production of other products depends greatly on the type of combustion equipment, method and the fuel used. NOx gases are formed as a result of the combination of high temperatures present in the furnace and abundant supply of nitrogen in the air supplied to the furnace.

Fuel-bound nitrogen may also lead to the formation of NOx. SOx gases are formed if there is sulphur in the fuel and particulates are due to the non-combustible mineral matter (ash) in the fuel.
Apart from the hazardous effects of pollution, there is also the widely-publicized threat of global warming arising out of the accumulation of carbon dioxide gas in the atmosphere. Recent developments in filtration and hot gas cleaning do not alter the basic dynamics of combustion, but are used as post-combustion measures to reduce emission of pollutants into the atmosphere. On the other hand, new combustion technologies such as high temperature air combustion (HiTAC), flameless combustion, and moderate and intense low oxygen dilution (MILD) combustion alter the dynamics of the

combustion profoundly in such a way that the production of pollutants is suppressed at the source itself.
Oxyfuel combustion technology relates to the possibility of carbon dioxide capture at source. In its original form, it involves burning a hydrocarbon fuel in the presence of pure oxygen, that is, without nitrogen being present, unlike the case when air is used as oxidant. This generates high flame temperatures which are necessary, for example, in glass melting applications. This also generates high NOx if nitrogen is present in the fuel or if it leaks into the combustion chamber. A variant of this method, is the replacement of nitrogen in air by the flue gas in carbon sequestration.
The invention proposed herein, however, pertains to a method in which the nitrogen in

air is replaced, not by the flue gas, but by the water vapour contained in it. Normal combustion produces a flue gas which contains mostly nitrogen (-^70% by mass) and a little carbon dioxide (-10%) and is therefore not conducive for carbon dioxide removal. Normal combustion is also carried out at atmospheric pressures and the volumetric flow rate of the flue gas is therefore quite high. In the invention proposed steam and /or water vapour is used to dilute oxygen, to obtain a dried flue gas that is rich in carbon dioxide (greater than 80% by mass) which can then be readily sequestered.
This invention will now be described with reference to the accompanying drawings, the single Figure in which illustrates, by way of example, and not by way of limitation, one of possible embodiments of the

apparatus for carrying out the method proposed herein.
The method of combustion according to this invention operates under steam diluted conditions as shown in the block diagram given in the drawings. Air is taken from atmosphere using blower/compressor 1 and is sent to the air separation unit 2 to get high purity heated oxygen (preferably oxygen concentration being >95% or higher by weight). This is mixed with water vapour and/or steam of temperature above 450 K which can be generated by the heat of the flue gas and is mixed with oxygen such that about 5 to 10% excess O2 is present in the oxidizer than stoichiometric requirement to ensure complete combustion. Hydrocarbon fuel from a fuel source and the hot oxidant from 2 are fed to the combustion chamber 3. The combustion products

exchange thermal energy with a primary energy recovery system (in one form, this may be a steam generating system in which high grade energy will be recovered) as well as with a secondary energy recovery system 4 where water is heated to steam by the flue gases to be sent to the combustion chamber 3 for dilution. The rest of the energy of the flue gas may be used for other purposes. In case flue gas is laden with ash, the particulates can be removed through hot gas filtration/ electrostatic precipitator 5. The combustion products are then sent to the flue gas conditioner 6 for water vapour removal and the dried flue gas which is rich in carbon dioxide can be sent to a carbon dioxide utilization/sequestration system wherein carbon dioxide is removed fully or partly from the flue gas. The separated streams are then disposed of after treatment, if necessary, to meet emission

norms. In the case of partial flue gas recirculation to mix with steam, the flue gas is treated in NOx or SO2 removal systems 7.
As a variation of the method proposed herein, a part of the flue gas can be recycled after 4 to be mixed along with steam and oxygen. This mixture, which contains significant amount of carbon dioxide, is sent into the combustion chamber.
EXAMPLE
The implementation of the invention in a typical pulverized coal power plant adapted for carbon sequestration will be as follows: Steam is mixed with high-purity oxygen (>95% oxygen by mass) such that the oxygen to water vapour proportion is of about 25-30% to 70-75% by mass. Water,

drawn from a source, is preheated by the boiler exit flue gas in the secondary energy recovery unit to steam and is mixed with oxygen which is also similarly preheated. The oxidant mixture is fed to the combustion chamber along with the fuel/pulverized coal. Steam is generated in the conventional way using steam generators such as economizer, super heaters, reheaters and evaporators located within the combustion chamber. The flue gas is sent to the secondary energy recovery system, then through a particulate removal system and a moisture removal system (flue gas condenser), and finally for carbon dioxide sequestration/disposal.
Further variants of the method proposed herein are possible. Lower levels of steam can be injected for dilution and it can be

replaced with flue gas after proper energy recovery in desired proportion.
Notable points of difference between steam-moderated oxyfuel combustion of this invention and the conventional oxyfuel combustion are in NOx formation, peak temperatures and materials of construction required. In the former, the peak temperatures can be reduced by injecting water vapour, which has a high specific heat, into the oxygen stream. This reduction in the flame temperature leads to reduced NOx formation. Since the flame temperature is lower, the cost of materials of construction can also be reduced. The advantage of pure steam moderation as opposed to flue gas moderation is a simpler lay out of the plant as one particulate removal system can be reduced. Another significant advantage is that the elimination

of the recycling of the flue gases also reduces the leakage rate of air into the system and enhances the concentration of carbon dioxide in the flue gas after moisture removal. This enhances carbon dioxide sequestration efficiency.
Finally, due to the high specific heat capacity of steam, the mass and volumetric flow rates in steam-moderated oxy-coal combustion are lower than with normal air combustion. There is therefore a possibility of enhancement of power generation capacity in case of retrofit of existing coal-fired units.
Thus, it can be said that this invention provides a stable environment for hydrocarbon combustion which yields a lower NOx, higher concentration of carbon

dioxide in the flue gases for a more efficient carbon sequestration.

We Claim:
1. A method of oxy-fuel combustion
comprising the steps of obtaining high purity
oxygen from atmospheric air mixing the
oxygen with water vapour and/or steam in a
predetermined proportion to form an oxidant
mixture; feeding a hydrocarbon fuel and the
said mixture into a combustion chamber;
deriving useful energy, through energy
recovery systems, from the combustion
gases, the carbon dioxide being separated
from the exiting flue gases and sent for
utilization/sequestration.
2. A method as claimed in Claim 1 wherein
the fuel is partly or wholly in solid, liquid or
gaseous form.

3.A method as claimed in Claim 1 or Claim 2 wherein the recovery systems consists of a steam generation system.
4.A method as claimed in Claim 1 or ClainjG wherein the recovery systems consist of a steam turbine system.
5. A method as claimed in any one of the preceding Claims wherein the steam is preheated to a minimum temperature of 450 degree K and is mixed with the oxygen such that total oxidizer temperature does not fall below 383 K and the oxygen content is in the range of 20 to 30% by mass, this mixture being sent to the combustion chamber as oxidant
6. A method as claimed in any one of the preceding Claims wherein a part of the flue

gases exiting from the combustion chamber is recirculated into the combustion chamber
7. A method as claimed in any one of the preceding Claims wherein high purity oxygen is obtained by an air separator receiving atmospheric air from a blower.
8. A method as claimed in any one of the preceding Claims wherein the water vapour is partially mixed with the flue gas from the combustion chamber.
9. A method as claimed in any one of the preceding Claims wherein the particulates are removed from the combustion chamber.
10. A method as claimed in any one of the
preceding Claims 7 to 9 wherein the oxygen
from the air separator is heated and then
mixed with water vapour and/or steam

before sending the same into the combustion chamber.
11. A method as claimed in any one of the preceding Claims wherein the NOx gases from the recirculated flue gas are removed partly or fully.
12. A method as claimed in any one of he preceding Claims wherein the SOx gases from the recirculated flue gas are removed partly or fully
13. A method as claimed in any one of he preceding Claims wherein the H2S gas from the recirculated flue gas is removed partly or fully.
14. A method as claimed in any one of the preceding Claims wherein the combustion is

carried out under pressure in the range of 3 to 50 bar.
15. A method as claimed in any one of the
preceding Claims wherein the carbon
dioxide utilization/sequestration is not done
fully or partly and the flue gas is vented to
atmosphere after treatment, if necessary, to
meet emission regulations.
16. A method of oxy-fuel combustion
substantially as herein described with
reference to, and as illustrated by, the
accompanying drawings.