PROCESS FOR OPERATING A PARTIAL OXIDATION PROCESS OF A SOLID CARBONACEOUS FEED

The present invention is directed to a process for operating a partial oxidation process of a solid carbonaceous feed to prepare a mixture comprising of CO and H2. Mixtures of CO and H2 are also referred to as synthesis gas.

US-A-3976442 describes a process wherein a solid carbonaceous feed is transported in a CO2 rich gas to a burner of a pressurized gasification reactor operating at about 50 bar. According to the examples of this publication a flow of coal and carbon dioxide at a weight ratio of CO2 to coal of about 1.0 is supplied to the annular passage of the annular burner.

Process control is important in a process wherein solid carbonaceous feeds are partially oxidized. It has been found that the quality of the synthesis gas as obtained may vary, due to e.g. disturbances or variations in the solid carbonaceous stream and the oxygen containing stream being fed to the gasification reactor, the amount of ash in the carbonaceous stream, etc. If for example coal is used as the carbonaceous stream, variations in H2O content of the coal may result in altered process conditions in the gasification reactor, as a result of which the composition of the synthesis gas will also vary. Various methods of controlling a partial oxidation process are known. For example GB-A-837074 describes a process wherein the carbon dioxide in the product gas of a partial oxidation process is measured to control the steam flow.

US-A-2941877 describes a process for controlling the oxygen-to-carbon feed ratio in a partial oxidation reactor. The oxygen-to-carbon feed ratio is controlled by measuring the methane concentration in the product gas using infrared measurement technique. A disadvantage of using methane as the control input is that the signal is not a sharp signal, making control less accurate.

US-A-4851013 describes a process wherein the partial oxidation process is performed in a pressurized gasification reactor provided with an inside wall consisting of conduits. The conduits are cooled by evaporation of water to steam inside the conduits. This results in a steam rate, which is measured and used as input to control the flow of either oxygen or solid carbonaceous feed, to said gasification reactor.

US-A-4801440 describes a process for the simultaneous partial oxidation and desulphurization of a sulphur and silicate-containing solid carbonaceous fuel. In said process a slurry of solid feed and liquid carbon dioxide is fed to a partial oxidation reactor wherein partial oxidation and desulphurization takes place at a temperature of below 2000 ⁰ F (1093 ⁰ C) . The amount of carbon dioxide is between 10 and 30 wt% basis on weight of feed. It is an object of the present invention to provide a process to prepare a synthesis gas having less inert compounds, such as nitrogen, which process is effectively controlled.

This object is achieved by the following process. Process for preparing a mixture comprising of CO and H2 by operating a partial oxidation process of a solid carbonaceous feed comprises at least the steps of:

(a) supplying the solid carbonaceous feed and an oxygen-containing stream to a burner, wherein a CO2 containing transport gas is used to transport the solid carbonaceous feed to the burner; (b) partially oxidising the carbonaceous feed in the burner wherein a gaseous stream at least comprising CO and H2 is being discharged from said burner into a reaction zone, wherein the temperature in the reaction zone is between 1200 to 1800 ⁰ C and wherein said reaction zone is at least partly bounded by a wall or walls comprised of conduits in which conduits steam is prepared by evaporation of water resulting in a flow of steam being discharged from said reaction zone;

(c) monitoring the conditions in the reaction zone by continually or periodically measuring the rate of the steam flow and using said flow rate as input to adjust the O/C ratio in step (a) .

The process according to the invention provides a process wherein a synthesis gas is obtained which contains much less inert compounds as for example nitrogen. Furthermore a process is obtained wherein the O/C ratio can be controlled in a simple and direct manner. Maintaining an optimal O/C ratio has been found very beneficial for achieving the most optimal yield over time of synthesis gas.

The term solid carbonaceous feed may be any carbonaceous feed in solid form. Examples of solid carbonaceous feeds are coal, coke from coal, petroleum coke, soot, biomass and particulate solids derived from oil shale, tar sands and pitch. Coal is particularly preferred, and may be of any type, including lignite, sub-bituminous, bituminous and anthracite. The solid carbonaceous feed is preferably supplied to the reactor

as fine particulates. With fine particulates is meant to include at least pulverized particulates having a particle size distribution so that at least about 90% by weight of the material is less than 90 μm and moisture content is typically between 2 and 12% by weight, and preferably less than about 5% by weight.

The CC>2 containing stream supplied in step (a) may be any suitable CO2 containing stream. Preferably the stream contains at least 80%, preferably at least 95% CO2. Furthermore, the CO2 containing stream is preferably obtained by separating the CO2 from the synthesis gas as prepared and recycling said gas to step (a) .

Preferably, the CO2 containing stream supplied in step (a) is supplied at a velocity of less than 20 m/s, preferably from 5 to 15 m/s, more preferably from 7 to 12 m/s. Further it is preferred that the CO2 and the carbonaceous feed are supplied as a single stream, preferably at a density of from 300 to 600 kg/m ³ , preferably from 350 to 500 kg/m ³ , more preferably from 375 to 475 kg/m ³ .

According to a preferred embodiment of the process according to the present invention, the weight ratio of CO2 to the carbonaceous feed in step (a) is less than 0.5 on a dry basis. Preferably this ratio is in the range from 0.12-0.49, preferably below 0.40, more preferably below 0.30, even more preferably below 0.20 and most preferably between 0.12-0.20 on a dry basis. It has been found that using the relatively low weight ratio of CO2 to the carbonaceous feed in step (a) less oxygen is consumed during the process. Further, less CO2 has to be

removed from the system afterwards than if a more dilute CC>2 phase would have been used.

In step (b) the carbonaceous feed is partially oxidized in the burner. A gaseous stream comprising CO and H2 is discharged from said burner into a reaction zone. The reaction zone is at least partly bounded by a wall or walls comprised of conduits in which conduits steam is prepared by evaporation of water. An example of such a wall is a so-called membrane wall wherein the parallel positioned conduits are interconnected such to form a gas tight wall as described in Gasification, Chris Higman and Maarten van der Burgt, Elsevier Science, Burlington MA, USA, 2003, pages 187-188. A suited and well-known example of a gasification reactor provided with a membrane wall is the Shell Coal Gasification Process as described in the afore mentioned textbook 'Gasification' on pages 118-120. Other publications describing such gasification reactors are for example US-A-4202672 and WO-A-2004005438. Said publications describe so-called side-fired reactors. The invention is however also suited for top fired reactors having a reaction zone provided with walls comprised of conduits in which steam is prepared by evaporating water. In such so-called top fired reactors the synthesis gas and slag both flow in a downwardly direction relative to the burner .

The pressure in the reaction zone may be higher than 10 bar, preferably between 10 and 90 bar, more preferably lower than 70 bar, even more preferably lower than 60 bar. The temperature in the reaction zone is between 1200 to 1800 ⁰ C. The burner and other process conditions for performing a partial oxidation in such burner are for

example described in US-A-4887962, US-A-4523529 or US-A-4510874.

Preferably the synthesis gas obtained in step (b) comprises from 1 to 10 mol% CC>2, preferably from 4.5 to 7.5 mol% CO2 on a dry basis when performing the process according to the present invention.

In step (c) the conditions in the reaction zone are monitored by continually or periodically measuring the steam flow rate and using said flow rate as input to adjust the O/C ratio in step (a) . A preferred method in which the steam flow rate is used will be described below. Said preferred method comprises a first step (i) wherein a relation between synthesis gas flow and the optimal steam production is obtained. This relation can be obtained by model calculations or by experiment in the gasification unit itself. The optimal steam production is defined as the steam flow rate at which the most selective conversion to carbon monoxide and hydrogen is achieved for a certain synthesis gas flow in step (b) . In model calculations use will be made of the quality of the solid carbonaceous feed, for example the carbon content, ash content, water content, the quality of the slag layer which will form under said conditions and feed quality and the resultant heat transfer to the wall comprising of conduits .

In a subsequent step (ii) the relation is embedded in a control algorithm of a computerized control system.

In use the steam flow rate as measured in step (c) is compared with the optimal steam production valid for the actual synthesis gas production by the computerized control system. If the measured steam flow is lower than the optimal steam production the O/C ratio will be adjusted to a higher value. If the measured steam

production is higher than the optimal steam production the 0/C ratio will be adjusted to a lower value. With the term lower and higher steam flow rate is meant a condition wherein the absolute difference between the optimal steam flow and the measured steam flow exceeds a certain pre-determined difference value.

Modest deviations between the optimal steam rate and the measured steam rate will be used to control the 0/C ratio as in the present process. With a modest deviation is here preferably meant a deviation of below 25%, wherein this percentage is calculated as 100% times ABS ((optimal steam rate) minus (measured steam rate) )/ (optimal steam rate). Above this range another control measures are triggered. For example a wide deviation from the optimal steam rate may indicate an upset stage, calling, for example, for shutdown procedures .

The 0/C ratio can be adjusted by adjusting the rate of the oxygen-containing stream, the rate of the solid carbonaceous stream or both. Preferably the 0/C ratio is adjusted by adjusting the flow rate of the solid carbonaceous stream, while keeping the oxygen-containing stream constant.

For the present invention the 0/C ratio has the following meaning, wherein 'O' is the weight flow of molecular oxygen, O2, as present in the oxygen containing stream and wherein 'C is the weight flow of the carbonaceous feed excluding the CO2 as present as carrier gas . The person skilled in the art will readily understand how to select the initial 0/C ratio for a specific solid carbonaceous stream to as used in step (a) . The starting 0/C ratio may e.g. be determined using known energy

content data for a specific carbonaceous stream such as the heating value of the feedstock in J/kg. Usually, having determined the desired selected 0/C ratio, the O2 content in the oxygen-containing stream will be determined and the suitable flow rates for the carbonaceous and oxygen containing feed streams will be established to obtain the desired 0/C ratio.

The person skilled in the art will readily understand that the streams supplied in step (a) may have been pre- treated, if desired, before being supplied to the gasification reactor. However it is more difficult to pre-treat a solid feed than to for example purify the synthesis gas as obtained in step (b) . Therefore it is preferred to further process the synthesis gas as obtained in step (b) . As an example, the synthesis gas may be subjected to dry solids removal, wet scrubbing, removal of sulphur compounds, like for example H2S and

COS, a water gas shift reaction, removal of metal carbonyls and removal of HCN. Preferably the synthesis gas is subjected to a hydrocarbon synthesis reactor thereby obtaining a hydrocarbon product, in particular methanol or dimethyl ether. The hydrocarbon synthesis may also be suitably a Fischer-Tropsch synthesis. An example of a possible line- up wherein the synthesis gas is treated and subsequently used as feed for a Fischer-Tropsch synthesis is described in WO-A-2006/070018. The line-up as described in said publication may also be used to prepare a feed for the aforementioned methanol and dimethyl ether synthesis processes. The methanol or dimethyl ether products may serve as feed for further processes to prepare lower olefins, i.e. ethylene, propylene and butylene and gasoline type products.

The invention is therefore further directed to a process wherein additional step (d) is performed:

(d) shift converting the gaseous stream as obtained in step (b) by at least partially converting CO into CO2, thereby obtaining a CO depleted stream.

Also it is preferred that the process further comprises the step of:

(e) subjecting the CO depleted stream as obtained in step (d) to a CO2 recovery system thereby obtaining a CO2 rich stream and a CO2 poor stream.

It is even further preferred that the CO2 poor stream as obtained in step (e) is subjected to a methanol synthesis reaction, thereby obtaining methanol, to a dimethyl ether synthesis reaction to obtain dimethyl ether or to a Fischer-Tropsch reaction to obtain various hydrocabons .

According to an especially preferred embodiment the CO2 rich stream as obtained in step (e) is at least partially used as the CO2 containing stream as supplied in step (a) . Any type of Cθ2-recovery may be employed, but absorption based Cθ2-recovery is preferred, such as physical or chemical washes, because such recovery also removes sulphur-containing components such as H2S from the process path. An example of a suited process is the Rectisol® Process from Lurgi AG.

In a start-up phase of the presently claimed process it may be desirable to use nitrogen as the transport gas. This because carbon dioxide may not be readily available at start-up conditions and will be available, as a by- product of the present process, after the process has started up. Suitably when the amount of carbon dioxide is recovered from the gaseous stream prepared in step (b) or

from the effluent of a possible downstream water gas shift reaction is sufficient the amount of nitrogen can be reduced to zero. Nitrogen is suitably prepared in a so-called air separation unit which unit also prepares the oxygen-containing stream used in step (a) . The invention is thus also related to a method to start the process according to a specific embodiment of the invention wherein the carbon dioxide as obtained in step (e) is used in step (a) . In this method nitrogen is used as transport gas in step (a) until the amount of carbon dioxide as obtained in step (e) is sufficient to replace the nitrogen.

Figure 1 shows a process scheme suited for performing the process of the present invention. In this scheme the lower and worked open part of a gasification reactor (1) is shown. Such a reactor may be suitably a reactor as disclosed in WO-A-2004/005438. Figure 1 shows a pressurized storage vessel (15) containing the solid carbonaceous feed provided with a supply conduit (16) to supply fresh feed.

The mixture comprising of CO and H2 is referred to as stream (18). Also shown are supply means (4) to supply the solid carbonaceous feed and supply means (6) to supply an oxygen-containing stream to one or more of burners (3). Typically, the pressure inside the storage vessel (15) exceeds the pressure inside the reaction zone (2), in order to facilitate injection of the powder coal into the reactor.

The reactor (1) has two pairs of diametrical opposed burners (3) of which 3 burners are shown in Figure 1. More of such pairs may be present. A CO2 containing transport gas is supplied via stream (5) and mixed with the carbonaceous feed. The mixture of transport gas and

solid carbonaceous feed is transported via (4) to the burner (3) . In the burner (3) the solid carbonaceous feed is partially oxidised resulting in that a gaseous stream at least comprising CO and H2 is being discharged from said burner (3) into a reaction zone (2).

The reaction zone (2) is at least partly bounded by a wall (20) comprised of vertical positioned conduits (19) in which conduits steam is prepared by evaporation of water resulting in a flow of steam being discharged from said reaction zone (2) via conduit (10). Fresh water is fed to the wall (20) via supply conduit (9) . Also shown is a common distributor (23) for water as supplied via (9) and a common header (25) for steam.

The steam flow rate in conduit (10) is monitored via measuring device (11), which provides a signal to computerized control unit (12). In said control unit (12) the steam rate is compared to the optimal steam production valid for the actual synthesis gas production (18) . When the measured steam flow as measured by device (11) is lower than the optimal steam production the O/C ratio will be adjusted to a higher value by adjusting the valves (8) and (7) via control lines (13) and (14) respectively. Preferably only valve (7) is controlled by unit (12) . When the measured steam flow as measured by device (11) is higher than the optimal steam production the O/C ratio will be similarly adjusted to a lower value .

Figure 1 also shows a water slag bath (22) for collecting slag, which will flow downwards along the wall (20) . The slag bath (22) is provided with water supply means (24) . Slag and water will be discharged via stream (17) . Further a ring (21) is shown through which quench

gas is added to cool the upwardly moving hot synthesis gas (18) . Example 1

The following Table I compares the use of carbon dioxide and nitrogen as transport gasses. The synthesis gas capacity (CO and H2) was 72600 NM^/hr, but any other capacity will do as well. The middle column gives the composition of the synthesis gas after being subjected to a wet scrubber using carbon dioxide as transport gas. The right hand column gives a reference where N2 was used as transport gas .

Table I: composition (in wt.%)

As can be seen, the nitrogen content in the synthesis gas is decreased by more than a factor of ten utilizing the invention relative to the reference. The CO ₂ content has increased a little relative to the reference, but this is considered to be of minor importance relative to the advantage of lowering the nitrogen content.

Example 2

The following Table II illustrates the influence of the weight ratio of CO2 to the solid coal feed. As can be seen from Table II, the oxygen consumption per kg oxygen in example Tl, T2 and T3 are significantly lower than the oxygen consumption in T4.

Table II: influence of weight ratio of CO2 to the carbonaceous feed