COAL PREPARATION

Abstract: Deashing of coal is achieved by oil agglomeration and subsequent separation of a carbon oil phase. The oil is removed by steam stripping either simultaneously and/or after the coal oil agglomerates have been disintegrated. The fine coal product can be used in steam generating plants such as power stations.

Description: Coal Preparation This invention relates to an improved method of preparing mined coal for its end use and in particular to the preparation of mined coal, as a feedstock for power generating stations. Co-pending patent application 55574/80 relates to a process of deashing coal which comprises crushing mined coal into small sized particles, subjecting said mined coal to wetting with a hydrocarbon liquid and forming agglom- erates of carbonaceous material in said coal, separating said carbonaceous agglomerates from non carbonaceous mat¬ erial present in said coal, subjecting said carbonaceous agglomerates to vapour separation treatment in the absence of oxidizing gases to separate the hydrocarbon liquid from said carbonaceous material to produce the deashed coal product and recycling said hydrocarbon liquid for use in wetting said mined coal . The content of the disclosure of application 55574/80 is incorporated herein by reference. . This prior application was primarily concerned with recovery of oil from agglomerated coal pellets in a fluidized bed in which the integrity of the pellet is retained. This addresses the end use of the product as coke oven feed or similar application in which product handle- ability is of importance. In applications within both the coking and steam¬ ing coal industries where charging or firing systems hand¬ ling fine sized material are in use, the disintegration of the agglomerate pellet is necessary at some stage. Further the residence times required and the heat input required were substantial in the disclosures of the prior patent application. It is an object of this invention to provide a method in which low residence times are achieved in the steam stripping operation. To this end the present invention provides a method of separating an agglomerated mixture of coal particles and a liquid hydrocarbon to form finely -gυRE » O.YΓI divided coal and recover the hydrocarbon liquid whi comprises disintegrating said agglomerates and subsequent and/or simultaneously subjecting agglomerates to vapourpha separation in the presence of steam and in the absence oxidizing gases to recover the liquid hydrocarbon from t finely divided coal particles. In a preferred form all of said agglomerates a above 1mm in size, said steam temperature is above 200 the residence time of the coal particles in the ste stripping zone is less than 5 seconds and at least 70% the coal product comprises particles less than 0.3mm a final product oil content less than 2.5%. The exposure of the relatively high specif surface area of the particles after disintegration of t agglomerate pellet during the stripping process in this ca offers the potential for the achievement of greatly enhanc heat and mass transfer rates. Comminution of the agglomerates prior to t vapour phase separation may be carried out in any conven ional comminution device. In a preferred method the agglo erates are subjected to initial attrition to reduce t particle size of the agglomerates and subsequently passi said agglomerates into the path of a high velocity stream steam to further reduce the coal particle size and separate such hydrocarbon liquid into a vapour phase. Application of ' this invention to the use coal-oil agglomerates offers several advantages over t alternative method of steam stripping in a fluidized be Foremost among these is the potentially large reduction solids hold-up in the stripping system and subseque improvement of response times due to the reduction residence time in the steam stripping zone. Much of t complexity of the fluid bed system is removed and contr functions are related to steam flow and inleτ temperatu and pressure alone. Where the steam is introduced as a jet t velocity and the internal shape of the particle entrain may be chosen to be sufficient to disintegrate the agglomer¬ ates. In this embodiment said agglomerates are passed into a high velocity stream of steam to simultaneously separate they hydrocarbon liquid and to form the finely divided coal particles. The system at a commercial scale would still utilize underwater storage (tanks or ponds) of the coal-oil agglomeration stage product and the slurry reclamation and de-watering systems as specified in the prior process of 55574/80. This feed material would be then fed to the front end of a conveying pipe to which superheated steam would also be fed. An initial short section of the conveying pipe would be used to achieve disintegration of the feed and the ramainder to accomplish removal of the oil from the coal surfaces to the gas stream. Disengagement of the solids from the dry vapours would be achieved in a high efficiency cyclone system with the solids discharging to a storage hopper prior to independent delivery of the fuel to the burners. This then could be performed in lean or dense or phases in steam or air. The cyclone overhead vapours are then totally condensed, and the hydrocarbon liquids separat¬ ed and returned to the agglomeration system. Control of the residual oil level of the partic- ulate coal product may be achieved in this system by control of the inlet steam temperature and steam to oil mass ratio both of which strongly influence the kinetics of mass transfer of the oil from the coal surfaces. Further, the product is steam blanketed throughout the stripping and storage systems and no oxidation of the particulate material or spontaneous combustion prior to the burners need be risked. Integration of the stripper as a conveyor into the boiler control systems of power stations should be more readily achieved with this system than the prior fluid bed system. In another aspect the present invention provides a method of preparing mined coal for use as fuel in steam generation comprising crushing mined coal into small siz particles subjecting said mined coal to wetting with hydrocarbon liquid and forming agglomerates of carbonaceo material, separating said carbonaceous material from n carbonaceous material present in said coal and subsequent distintegrating said agglomerates and simultaneously and/ subsequently subjecting the disintegrated agglomerates to vapour phase separation in the presence of steam and in t absence of oxidizing gases to recover said hydrocarb liquid and form finely divided coal particles as ste generating fuel. The method of agglomeration is as described co-pending application 55574/80. A plant for preparing and delivering fuel to steam generator comprising a storage for a slurry crushed, mined coal, apparatus for agglomerating said co with a hydrocarbon liquid, separation means for separati said coal agglomerates from the water phase of said slurr comminution apparatus to disintegrate said agglomerate means to dispense said disintegrated agglomerates into stripper through which steam is passed at vapour pha separating conditions to vaporize said hydrocarbon liqu from said coal particles, separation apparatus to separa said coal particles and recover said hydrocarbon liquid a means to convey said coal particles to. said steam generato In an alternative embodiment said comminution apparatus omitted, and the velocity of steam and the internal shape the particle entrainer which constitutes said stripper selected to disintegrate said agglomerate. An example of one configuration of such a syst at the pilot plant or commercial scale is shown in Figure In this scheme unstripped agglomerates are recovered from storage pond or tank 3 and pumped to a set of dewateri screens 4. Dewatered agglomerates are then fed to a sma hopper/ feeder 5 at the front end of the stripper and was water is pumped out through line 6. Agglomerates fed to t stripping tube 7 are picked up by the conveying steam 12 a pass through an initial short length of pipe constructed internally to disintegrate the agglomerate material as it passes through. The remainder of the tube provides the additional residence time for oil vapourisation. Stripped solids then pass with the steam and hydrocarbon vapours to a cyclone 8 where the solids are disengaged. The overhead vapours are then totally condensed in condenser 9, hydro¬ carbon liquids separated with any coal fines from the water and returned to the agglomeration plant. Solids exit from the cyclone to a surge hopper 10 from which they are then air conveyed by line 13 to the burners 11 of the power generator plant. The following is set out as an example of a preferred form of the invention. A sample of coal was treated to the oil agglomer¬ ation process as set out in pending application 55574/80. The agglomerating oil • used was a light gas oil with a boiling range of 240-340 C. The ash content was reduced from 26% on the feed coal (DCB, dry coal basis), to 13.6% on the agglomerate (DCB). The particle size of the agglomerates is given in Table 1 and the particle size of the coal particles within the agglomerates is shown in Table 2. The oil and water contents of the agglomerates were 12.3% (total agglomerate basis - TAB) and 4.8% respectively. ' - 6 - TABLE 1 AGGLOMERATE SIZE Size % t. mm > 5.6 1.2 4.75 - 5.6 0.5 3.35 - 4.75 1.0 2.36 - 3.35 6.0 1.7 - 2.36 26.6 1.18 - 1.7 37.8 < 1.18 26.9 TABLE 2 COAL PARTICLE SIZE Size % t. mm > 1.7 4.8 0.85 - 1.7 2.2 0.425 - 0.85 4.7 0.212 - 0.425 9.3 0.106 - 0.212 14.0 0.53 - 0.106 16.2 < 0.53 48.8 A continuous steam stripping rig was utilized in these examples. The rig is shown in Figure 2. Saturated steam generated in boiler 21 at 100 psig passes through a pressure reducing valve 22 dropping the pressure into the 0-4 psig range. The steam then passes into a superheater 23 and from the superheater through a jet 24 into an entrainer 25. Agglomerates are also fed from Hopper 27 to the entrainer 25 through a rotary valve 28. Breakdown of the agglomerates occurs under action of the steam jet ' within the entrainer 25 10 and the particles are then transported through a carrier pipe 29 of approximately lm in length within which oil is vapourized from the agglomerate surface. The stripped solids are separated from the steam and oil in a cyclone 30. The steam and oil are passed through a water cooled condenser 31 -*--- from which the oil and water can be separated as distinct liquid phases. The solids are passed through ball valve 32. Prior to feeding to the steam stripping unit, the agglomerates were part broken up in a rod mill and screened to a top size of 1.18 mm. 20 Data on processing conditions for four runs carried out on the unit are set out in Table 3. Feed and product size distributions and water and residual oil contents are shown in Table 4. TABLE 3 OPERATING CONDITIONS ■ 9- TABLE 4 FEED AND PRODUCT ANALYTICAL DATA The data show that a considerable degree of break down occurs in the entrainer. Variations to the desig geometry of the entrainer will effect the degree of breakdow as will the velocity of steam at the jet. The examples give are indicative of process performance only ' and should not b taken as limiting the scope of entrain ent device claimed i the patent. Analysis of the data shows that residual oil level of 0.5 to 2.5% (TAB) may be achieved at residence times o less than 1 second. As a comparative example, a sample of the tota agglomerates of the size shown in Table 1, were strippe using the alternative fluidized bed steam stripping techniqu disclosed in pending application 55574/80. Comparative dat are given in Table 5. The data show that comparable oi removal can be achieved using the fast stripping technique i less than 1 second, compared to the 5 minute residence tim required when using the fluidized bed technique. TABLE 5 FAST STRIPPER PERFORMANCE COMPARED TO FLUIDIZED BED As a further example of the present invention stripping model was devised which shows the effectiveness the invention at the higher steam temperatures available power stations and also treats a much lower particle s range based on complete comminution of the agglomerat Development of this model for the kinetics of hydroiar and water removal from the product of a coal-oil agglom ation process is based primarily on consideration of t product in its disintegrated form. Exposure of the f surface area of the finely ground constituent partic provides potential for heat and mass transfer at grea rates than those obtained experimentally in the fluid steam stripping of the primary agglomerate product. Studies of the structure of agglomerated materi with respect to internal voidage and the location of b hydrocarbon and water within the structure has indica that, (i) hydrocarbon 'is present in the agglomerate surface film on coal particles and in interpa icle bridges as shown in Figure 2, • ** (ii) micropores within individual particles are wat filled but that this would account for less t 2wt.% water on dry coal basis, (iii) the bulk ' of the water present occupies a porti of the remaining interparticle voidage not occ pied by hydrocarbon. In translating the relative location of hydr carbon and water in an agglomerate structure to th obtained on 'instantaneous' disintegration of the origin structure, it is reasonable to assume that all hydrocarb remains as an even surface film on individual particle Assignment of the location of the water is to a large exte arbitrary and it has been assumed to exist as free drople on a one to one basis with coal particles at the equivale bulk water composition. That is, each coal particle in representative size distribution is associated v/ith a hydr carbon film, typically 15 wt.%, and a water droplet typi ally 8 wt%. Although this is an unlikely occurrance in a practical sense it reflects the approximate distribution of water within the original agglomerate structure and the order of magnitude of water surface available for heat and mass transfer. Other forms of drop size distribution are also examined in the model. Evaporation of hydrocarbon from the films on coal particles and of the water droplets is accomplished by contacting the disintegrated agglomerate material with sup- erheated steam. The model monitors heat and mass transfer as" a function of time thus determining the rates of hydrocarbon stripping from the coal particles, water evaporation and degree of solids heating. Required mass ratios of steam to hydrocarbon and the initial degree of superheat in the steam are predicted. The physical system represented by the model, with a number of simplifying assumptions, is that of pneumatic conveying of agglomerate material in a steam atmosphere. A number of stages can be identified in the system. (i) induction of agglomerates at ambient conditions into a conveying pipe, (ii) breakdown of this material to its constituent 1 particles, (iϋ) movement of the particles down the length of conveying pipe using superheated steam as a carr¬ ier, (iv) disengagement of solids from-steam and hydrocarbon vapours in a cyclone, (v) total condensation of cycloned vapours to recover hydrocarbon. The model considers (i) and (ii) to be instantan¬ eous and examines stripping as a function of contact time with steam i.e. operations (iii) and (iv) are included. Condensation, is not included in the model. The stripping model was run with the following input conditions. (i) agglomerate feed composition: 15 wt.% gas oil 8 wt.% water on a dry, oil free coal basis, (ii) steam to gas oil ratios of 2 and 3 kg steam/kg oil, (iii) steam inlet temperatures of 650°C and 450°C. F inlet temperatures were taken as 15 C. Particular size after disintegration ranged fro to 100 microns. An intiial run was performed such that to vaporization of both water and gas oil was achieved. total time required for stripping was 1.67 sees, for steam/oil ratio of 3 kg/kg and inlet steam temperature 650°C. Steam and solids at the end of this time were 13θ°C. The results are summarized in Table 6. TABLE 6 SUMMARY OF RESULTS FOR 15wt.% GAS OIL, 8wt.% WATER FEED AGGLOMERATES. - ^ -- ■ sAC s c:..π // V.'IrO These results indicate two points. Firstly, that removal hydrocarbon oils from the surfaces of coal particles can achieved in fractions of a second where end use of parti ulate coal is acceptable. Secondly, that, dependent on t way in which the water present in the structure is dispers on disintegration of the agglomerate, the potential exis to reduce the steam ratios and temperatures through remov of the hydrocarbon oil before large scale vaporization water has occurred. Some of the advantages of the system of th invention over the current method of fluid bed strippi are, (i) in the case of fluid bed stripping residence tim of 3 - 4 minutes requires hold-up of large amoun of material in the bed. Here the hold-up equivalent to solids content of the lean pha stripper tube, (ii) virtually instantaneous shut-off of the stripp can be achieved by control of the steam flow onl (iii) separation and recovery problems are minimise (iv) residual oil levels can be controlled via t steam inlet temperature. Subsequent usage of the de-oiled particulate co is independent of the stripping system and lean or den phase conveying to burners may be applied.

Claims: THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. A method of separating an agglomerated mixture of coal particles and a liquid hydrocarbon to form finely divided coal and recover the hydrocarbon liquid which comprises disintegrating said agglomerates and subsequently and/or simultaneously subjecting the agglomerates to vapour- phase separation in the presence of steam and in the absence of oxidizing gases to recover the liquid hydrocarbon from the finely divided coal particles. 2. A method as claimed in claim 1 in which the agglomerates are subjected to initial attrition to reduce the particle size of the agglomerates and subsequently passing said agglomerates into the path of a high velocity stream of steam to further reduce the coal particle size and to separate said hydrocarbon liquid into a vapour phase. 3. A method as claimed in claim 1 in which said agglomerates are passed into a high velocity stream of steam to simultaneously separate the hydrocarbon liquid to form the finely divided coal particles and to separate the hydrocarbon liquid. 4. A method as claimed in any one of claims one to three wherein all of said agglomerates are above 1mm in size, said steam temperature is above 200°C, the residence time of the coal particles in the steam stripping zone is less than 5 seconds and at least 70% of the coal product comprises particles less than 0.3mm and final product oil content less than 2.5%. 5. A method of preparing mined coal for use as fuel in steam generation comprising crushing mined coal into small sized particles subjecting said mined coal to wetting with a hydrocarbon liquid and forming agglomerates of carbonaceous material, separating said carbonaceous material from non carbonaceous material present in said coal and subsequently disintegrating said agglomerates and simulta eously and/or subsequently subjecting the disintegrat agglomerates to a vapour phase separation in the presence steam and in the absence of oxidizing gases to recover sa hydrocarbon liquid and form finely divided coal particles steam generating fuel. 6. A method as claimed in claim 5, wherein t disintegration and vapour phase separation are carried o as defined in any one of claims 2 to 4. 7. A plant for preparing and delivering fuel to steam generator comprising a storage for a slurry crushed, mined coal, apparatus for agglomerating said co with a hydrocarbon liquid, separation means for separati said coal agglomerates from the water phase of said slurr comminution apparatus to disintegrate said agglomerate means to dispense said disintegrated agglomerates into stripper through which steam is passed at vapo.ur pha separating conditions to vaporize said hydrocarbon, liqu from said coal particles, separator apparatus to separa said coal particles and recover said hydrocarbon liquid a means to convey said coal particles to said steam generato 8. A plant as claimed in claim 7 wherein sa comminution apparatus is omitted and the velocity of ste and the internal shape of the particle entrainer whi constitutes said stripper is selected to disintegrate sa agglomerates.

Inventors: JONES C, MAINWARING D
Applicants: BRITISH PETROLEUM CO, BROKEN HILL PTY CO LTD, JONES C, MAINWARING D, BP AUSTRALIA LTD
Application year: 1981
Main classification: C10L9
IPC codes: B03B9, B03D3, C10L5, C10L9