METHOD OF USING HIGH TEMPERATURE FLUE GAS AS SUPPLEMENTARY FUEL FOR UPGRADING LOW-RANK COAL Field of the Invention 5 This invention relates to a method of using high temperature flue gas as a supplementary fuel for upgrading low-rank coals. Background of the Invention Any discussion of the prior art throughout the specification should 10 in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field. China is among a few countries where coals play a major role in their energy-consumption. As the international crude oil price soars, paces of looking for the substitution of crude oil quickened. Against the 15 background of expensive crude oil and cheap coals, and further driven by the high profits of coal gas, coal oil and coal methanol, capitals marched their ways into the industries of coal gasification and coal liquefaction. Given the high threshold of said industries such as high technical difficulty and huge capital investment, there exist great risks of resource 20 wasting. In the mean time, coal gasification and coal liquefaction are industries of high water and energy consuming, and high waste discharging and/or emission, yet China is facing a situation of contrary distribution of water and coal resources, hence one should act not only according to his capability but also to the water resources. 25 Drying and low-temperature pyrolysis technologies of low-rank coals, thank to their low technical thresholds, small investment and mature industrial experiences on one hand, and low water/energy consumption, 1 environmentally friendly discharging, as well as their high added values, efficient and green combustion on the other hand, have become a hotspot in the development of comprehensive utilization of low-rank coals in recent years. The technology of using hot gas as a heat carrier to upgrade 5 low-rank coals (especially lignite) through low-temperature pyrolysis started up in the early years of last century, and a lot of technologies established their own industrial demonstration apparatuses in 1980s and have grown up to the industrialized ones in this century. Due to its high volatile content and activity, lignite is highly 10 demanding for the oxygen content of heating gas. During the period of dry-distillation in particular, an oxygen content of less than 1% is required, therefore the hot flue gas from coal burning can hardly meet the needs. LNG or LPG can be used though, when China's national condition of rich in coal and scarce in oil is considered, especially when the truth of 15 China's lignite production area is taken into consideration, it turns out to be a great waste in energy. Furthermore, some flammable composites are contained in the off gas of pyrolysis so they will pollute the environment and waste energy if discharged directly, whereas the heat value thereof is low, normally it is hard to ignite or combust completely for use. 20 It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative. Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise", "comprising", and the 25 like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to". 2 Although the invention will be described with reference to specific examples it will be appreciated by those skilled in the art that the invention may be embodied in many other forms. 5 Summary of the Invention According to a first aspect of the present invention there is provided a method of using high temperature flue gas as a supplementary fuel for upgrading low-rank coal comprising the following steps: 1) high temperature oxygen-containing flue gas produced 10 from combustion of low-rank coal briquette, being used as an oxidizer and a supplementary heat source after dust removing, mixes with low heat value gases produced from the upgrading of low-rank coals so as to combust the low heat value gas completely, wherein the high temperature flue gas and low heat 15 value gas is mixed according to the heat-supply ratio of 1:4; 2) anoxic hot gas with the oxygen content of less than 1% is obtained through mixing and combusting; 3) said anoxic hot gas is circulated and acts as the heat source for upgrading low-rank coals in the process of low temperature 20 dry-distillation; wherein said high temperature oxygen-containing flue gas has its oxygen content controlled at 9 to 10%. The invention relates generally to providing a method of using high 25 temperature flue gas as a supplementary fuel for upgrading low-rank coals, which is capable of lowering the production costs by a big margin. Accordingly, it is provided in this invention a method of using high 2a temperature flue gas as a supplementary fuel for upgrading low-rank coal with the steps as follows: 1) high temperature oxygen-containing flue gas produced from combustion of low-rank coal briquette, being used as an oxidizer and a 5 supplementary heat source after dust removing, mixes with low heat value gases produced from the upgrading of low-rank coals so as to combust the low heat value gas completely, wherein the high temperature flue gas and low heat value gas is mixed according to the heat-supply ratio of 1:4; 2b 2) anoxic hot gas with the oxygen content of less than 1% is obtained through mixing and combusting; 3) Said anoxic hot gas is circulated and acts as the heat source for upgrading low-rank coals in the process of low temperature dry-distillation. 5 Preferably, said high temperature oxygen-containing flue gas has its temperature controlled at 700-900'C at the outlet. Preferably, said high temperature oxygen-containing flue gas has its oxygen content controlled at 9- 10%. This invention is advantageous in the following aspects: 10 1, In situ resource utilization at the lignite production areas, instead of utilizing high heat value gaseous fuels such as LNG and LPG, saves great production costs and lowers demand for high heat value gases; 2, It is ignited as a supplementary fuel to subject to complete combustion with coal gas of low heat value produced from gaseous carrier and 15 low-temperature distillation of lignite, and the anoxic hot gas produced after combustion is used as a heat source for lignite's low-temperature distillation, so as to realize the reutilization of resources and avoid wasting and pollution; 3, The problem of too high oxygen content of high temperature hot flue gas when used as a heat carrier gas has been solved. 20 Description of Drawings Fig. 1 is a structural schematic diagram of the apparatus used in this invention. 1- air blower 25 2- briquette furnace 3-cyclone separator 4- air blower 3 1280570CN-AU 5- gas combustion furnace 6- combustor 7- air blower 8- induced draft fan 5 Embodiment As shown in Fig. 1, a briquette is combusted in a briquette furnace 2 to produce high-temperature oxygen-containing flue gas which, after dust-removing in the cyclone separator 3, enters the combustor 6 to mix with low heat value gas 10 and is ignited. A great amount of anoxic gas is produced through a complete combustion in the gas combustion furnace 5. The high-temperature flue gas and low heat value gas are mixed according to the heat supply ratio of 1:4. With the aid of induced draft fan 8, the produced anoxic hot gas enters the dry-distillation reactor to supply heat for the low-temperature dry distillation. Air blowers 1, 4 15 and 7 coordinately control the oxygen content of the hot gas and combustion state of low heat value gas. Embodiment 1 The lignite briquette, made of the lignite from Xilinhot of Inner Mongolia 20 with the net calorific value of some 2512kcal/kg, is used as the fuel of briquette furnace, wherein the heat value of low heat value gas is some 360kcal/kg and temperature thereof is 88'C. See Table 1 for its components. The amount for combustion is 1000ton/day. Table 2 shows the specific parameters in the case of the oxygen content of the anoxic gas from the outlet of gas combustion furnace 5 25 being 0.75%, and other. Table 3 shows the costs comparison between using briquette and LNG. Table 1: Components of pyrolysis gas 4 1280570CN-AU gases N 2 02 CO 2 CO CH 4 NCHC* H 2 volume ratio 50.7% 0 24.6% 2.4% 1.2% 1.3% 0.006% gases H 2 S SO 2
NH
3 NO Ar H 2 0 volume ratio 0.13% 0.67% 0.025% 0.013% 0.86% 18.1% *NCHC- The general term of noncondensable hydrocarbons except CH 4 Table 2: specific parameters in the case of oxygen content of the anoxic gas from the outlet of gas combustion furnace 5 being 0.75% load of lignite furnace MW 10.2 net calorific value of lignite kJ/kg 14000 lignite consumption kg/h 2619.3 mass fraction of additives in briquette ni 0.25 mass fraction of lignite in briquette n2 0.75 net calorific value of briquette kJ/kg 10499 consumption of briquette kg/h 2951 flue gas volume of briquette combustion Nm 3 /h 27756.3 volume fraction of 02 in the anoxic hot flue gas from outlet of briquette 0.101 furnace 2 temperature of the flue gas at the outlet of briquette furnace O 790 heat value of low heat value pyrolysis gas Kcal/Nm 3 361.3 rated volume flow of low heat value gas Nm 3 /h 31287.000 volume flow of flue gas after the two gases being mixed and combusted Nm 3 /h 57979.2 volume fraction of 02 in the high temperature oxygen-containing % 0.75 flue gas from outlet of gas combustion furnace 5 total mass flow of flue gas after mixing and combustion kg/h 79110.3 adiabatic combustion temperature as E 1160 mixing and combusting 5 1280570CN-AU Table 3: Costs comparison between using briquette of low-rank coal and LNG Lignite briquette consumption costs price of raw lignite Yuan/ton 75 consumption rate of kg/h 2951 lignite briquette price of lignite Yuan/ton 200 briquette consumption cost Yuan/h 590.2 consumption cost per Ten-thousand Yuan/year 425 year (7200h) Consumption cost of LNG price of LNG Yuan/ton 3400 consumption rate of kg/h 660 LNG consumption cost Yuan/h 2244 consumption cost per Ten-thousand Yuan/year 1616 year (7200h) 5 To conclude, the above contents only represent the preferred embodiment and will not limit the scope of this invention. Therefore, any amendments, substitution or improvements within the spirit and principle of this invention will fall into the scope covered by this invention. 6 1280570CN-AU