Background of the invention

The kraft process is currently the dominant chemical pulping process. During pulping large quantities of recoverable energy in the form of black liquor are generated. Worldwide some 2.8 billion GJ (780 TWh) of black liquor was produced in 1990 at kraft pulp mills.

The kraft recovery system has two principal functions:

i) Recovery and recure of the inorganic pulping chemicals.

ii) Recovery of the energy value of the organic material, mainly as process steam and electrical power.

The chemical recovery process contributes significantly to the capital intensity of the kraft process. About 35% of the capital cost of a modern pulp mill is attri¬ butable to the recovery process.

The predominant method today for recovery of chemicals and energy from black liquor is the Tomlinson recovery boiler, a technology which was introduced well over fifty years ago. Although an established technology, there are some wellknown disadvantages with conventional recovery technology.

Most often the recovery boiler with its inherent in¬ flexibility constitutes the main production bottleneck in the pulp mill. Economics of scale dictate large capacity units.

Other disadvantages include the low thermal efficiency and risk of smelt water explosions which in turn consti¬ tute a safety problem.

These and other areas of concern have been the driving force for development of new methods and principles for recovering chemicals and energy from black liquor. One of the more promising routes is gasification of the liquor in entrained or fluidized beds. In some cases these alternative processes can be intalled as incremen¬ tal capacity boosters, providing an opportunity to eliminate the recovery boiler bottleneck.

One of the major driving forces for development of new recovery technology has been to improve thermal effi¬ ciency accompanied with higher power to steam output ratios. The present invention relates to a major improvement in this area, using technology based on gasification and energy recovery in an externally fired gas turbine cycle (EFCC) .

Gasification of black liquor can be performed at various temperatures and pressures, resulting in different forms of the recovered inorganic constituents and different calorific values of the combustible process gas.

The inorganics, mainly alkali compounds, are withdrawn from the gasification system and solubilized to form an

aqueous alkaline liquid, which liquor is used for cooking liquor preparation.

Kraft pulp mills are significant producers of biomass energy, and today most mills are designed to use the biomass fuel available at the kraft mill to meet on site steam and electricity needs via back pressure steam turbine cogeneration system. Electricity demand is often higher than internally generated, in particular for integrated mills, and often electricity is imported from the grit.

Process steam requirements for a modern kraft pulp mill is in the order of 10 GJ per ton of air dried pulp. The internal electricity demand is around 600 kWh/ton of air dried pulp.

The biomass gasification gas turbine cogeneration system of the present invention will meet mill steam demand and has the potential to produce excess electricity for export.

The present invention can be practised using various types of gas generators and gasification principles, exemplified in prior art documents.

In US 4,917,763 and US 4,692,209, gasification of spent cellulose liquor, such as black liquor, is described. The gasification temperature is in the range of 1000- 1300°C, resulting in the evolvement of molten inorganics and a combustible gas. The molten alkaline chemicals are withdrawn from the gas stream in a cooling and quenching stage, where an aqueous solution is sprayed

into the gas stream. The product alkaline solution is cooled to below 200°C.

The combustible gas is used for generating steam or as a synthesis gas.

Another gasification method is described in US 4,808,264, where recovery and energy from black liquor is carried out in three distinct and separate steps, whereas in the first step concentrated black liquor is gasified in a pressurized gasification reactor by flash pyrolysis at 700 to 1300°C, in which the inorganic chemicals of the black liquor are contained in the form of molten suspended droplets.

Energy is recovered from the resulting process gas for generation of steam and/or electric power in a gas turbine/steam turbine cycle. The steam turbine is of back pressure type, preferably selected to fit the needs of process steam for the mill.

In WO 91/15665 is described a method and apparatus for generation of electricity and steam from a pressurized black liquor gasification process. Energy is recovered in a gas turbine/back pressure steam turbine system. Excess steam generated in the mill is recirculated into the gas turbine or the combustor thereof for increasing the generation of electricity. This procedure is known to the industry as a steam injected gas turbine, herein¬ after referred to as STIG.

Common for US 4,808,264 and WO 91/15665 is that they both are based on energy recovery using a direct fired gas turbine combined cycle, including a back pressure

steam turbine. The direct fired cycle presents certain difficulties in connection with gasification of cellulose waste liquors, which will be further explored below.

All direct fired cycles require fuel enhancement to reduce ash components in the process gas to enable safe operation of the gas turbine. Combustion products passing through the gas turbine present limitations not only on the maximum temperature at entry to the gas turbine, but also on the peak temperature to which the mineral matter has been exposed during combustion. A proportionally large fraction of alkali, increasing with increasing temperature, is vaporized during pyrolysis and gasification and forms a submicron aerosol upon condensation as the gas cools down, which alkali deposits on the turbine blades. Alkali compounds, comprising sodium and potassium, are particularly aggressive at high gas turbine inlet temperatures. Various forms of hot gas clean up systems including ceramic filters are currently under development.

An alternative to the hot gas clean up system of WO 91/15665 is the approach in US 4,808,264, where the combustion products are cooled down to temperatures below about 200°C, which facilitates the removal of harmful inorganic components in the gas. A drawback with this system is however the lower overall efficiency and electricity generation potential.

The objective of the present invention is to provide a safe, efficient and less capital intensive process for production of electric power and process steam from

gasification of black liquor, based on energy recovery in an externally fired gas turbine cycle with indirect heating of the gas turbine motive fluid by counter- current heat exchange with hot gas generated during gasification. Before entering the main heat exchange zone, a substantial portion of the alkali inorganics formed during the gasification are removed from the gas, which removed alkali is further processed to cooking liquors.

In a specific embodiment of the present invention, the relatively clean process gas from the gasifier is trans¬ ferred to an externally fired gas turbine system, in which gas turbine motive fluid steam or water is in¬ jected to increase motive fluid mass flow and provide better conditions for heat transfer in the heat ex¬ changer.

Although conventional gas turbine cycles have inherent thermodynamic advantages, simple cycle gas turbine systems suffer from some wellknown disadvantages as well, such as the large parasitic load of cooling air on the system to decrease the turbine inlet temperature. This effect is minimized when practising the present invention, as will be further explained below.

Furthermore, in light of the strong scale economics of gas and steam turbine cycles and other factors described herein, conventional direct fired gas turbine and steam turbine cycles, based on heavy duty industrial turbines, may not be the best candidates for applications in the relatively modest scales in conjunction with black liquor gasification.

The exhaust from the gas turbine contains a large quantity of sensible heat, and if discharged to atmosphere, large quantities of potentially useful energy are wasted. However, this exhaust heat can be exploited in various ways, for example to produce steam in a heat recovery steam generator (HRSG) , which can be used for process needs direct or in a cogeneration figuration, or to produce more power in a condensing steam turbine.

The principal use of the gas turbine exhaust in the present invention is as oxidant in the gasifier and/or for final oxidation of the process gas before entering into the heat exchanger.

Yet another method to exploit the heat content of turbine exhaust in the present invention is to raise superheated steam which is recirculated and injected in the compressed gas stream, thereby increasing motive fluid mass flow, see e.g US patent No 3,978,661. Steam injection in biomass gasifier gas turbine cogeneration systems for forest product industry applications is for example described in PU/CEES Working Paper No 113 by Dr Eric Larson, Princeton, February 1990.

It is to be understood that a major drawback of direct fired gas turbine cycles as exemplified in prior art documents is the high sensitivity to fuel gas quality.

Indirect fired or externally fired gas turbine cycles are considerably less sensitive and can accept fuels of approximately the same quality as steam generators.

Indirect cycles, currently under development for coal gasification applications, can accommodate a wide variety of conventional equipment. Advanced combustors and high temperature heat exchangers are commercially available or under development.

Stack gas recirculation to use all the cycle air for combustion can be attractive in indirect cycles, minimizing NO emissions and lowering capital cost.

As will be subsequently explained herein, use of an indirect fired gas turbine cycle in combination with steam injection or compressed air hu idification by water injection is an attractive alternative embodiment of the present invention.

The practice of the present invention will be described by reference to the appended description, example and figure as applied to the recovery from black liquor. It should, however, be recognized that the invention is applicable to the recovery of other cellulose waste liquors, such as for example spent sulfite or soda pulping liquors.

General description of the invention

In the subject process, a cellulose waste liquor containing hydrocarbonaceous material and inorganic alkali compounds is reacted with an oxygen containing gas in a free flow gas generator to produce a com¬ bustible gas. The gas generator operates at a reaction zone temperature of between 600-1500 C and at a pressure of 1-100 bar.

During gasification, molten inorganic particles compri¬ sing sodium and potassium compounds are formed, which molten particles are entrained in the gasflow as a combustion residue.

A large portion of the molten inorganics are separated from the hot gas stream in one ore more separate gas diversion and residue separation zones. The first gas diversion and residue separation zone may be directly connected to the gas generator.

The hot raw gas stream comprising the molten inorganics undergoes a change in direction in the first separation zone, and is transferred to a second separation zone consisting of a staggered array of refractory tubes, acting as an impact separator.

In the first separation zone the molten particles and slag pass through an outlet in the first gas diversion zone and drop by gravity into a pool, comprising an alkaline liquid.

Optionally a small stream of bleed gas of the hot gas stream from the reactor is passed through the outlet promoting the separation of slag from the diverted main gas stream.

The discharge of molten particles and slag in the first separation zone could preferably be arranged as a quench system including a cooled dip tube for directing the particle flow into the pool of alkaline liquid.

The second separation zone is located in the hot gas stream, acting as an impact separator for particles and

slag entrained in the gas leaving the first separation zone. Particle inertia tends to counteract the gas fluid drag forces which act to sweep the particles past separator tubes arranged in the gas stream.

Impact separators are quite effective for removing particles above a certain diameter, ranging from 5-50 microns. The second separator thus acts as a slag screen. At the slag screen exit, the hot gas is directed to a slag collection zone before being decelerated to and passed through a ceramic filter.

Ceramic filters, utilized for hot gas filtration of gas streams, will be exposed to a variety of conditions in addition to heat. Ceramic barrier filter devices currently under development and commercial use include candles, cross flow, tubes, bags and granual beds. Any of those alone or in combination may be used as final hot gas clean-up before entering the main heat exchanger, when practising the present invention.

The separated smelt particles and slag comprising alkali components are withdrawn from the separation zones and further processed to form cooking liquors.

The hot clean gas, leaving the last separation zone, is directed to a combustion zone, where the gas is com¬ busted in the presence of an oxygen containing gas, preferably part of the gas turbine exhaust.

The heating value of the process gas leaving the gas generator is dependent on the type and amount of oxidant used in the gas generator. The use of air as oxidant results in that a large portion of the product gas

consists of nitrogen, thus resulting in a gas with a rather low calorific value.

An important feature of the present invention is that sulfur compounds, introduced into the gasifier by the cellulose waste liquor or other sulfur containing streams entering the gasifier, is reduced to sulfides and furthermore that a large portion of these sulfides are bound into alkali.

Thus, formation of hydrogen sulfide and sulfur dioxide is to be minimized during gasification. The conditions in the gasifier have to be carefully selected to prevent excessive hydrogen sulfide formation.

Binding of sulfur into the alkali smelt is promoted by high temperature in the gasifier and low gasification pressure. Addition of various forms of metal oxides, such as titandioxide, mangandioxide or calcium com¬ ponents to the reaction zone, may be employed to minimize formation of alkali carbonates and hydrogen sulfide formation during gasification.

The oxidant to fuel ratio in the gasifier has further¬ more to be adjusted to minimize formation of sulfates and sulfur dioxide.

Taking these restrictions into consideration, a preferred operating range of the gasifier is at temperatures above about 800°C and a reactor pressure between 1 and 5 bar.

The oxidant air supply to the reactor should be kept below 95% of the stochiometrical value for complete

oxidation of the incoming streams to the reactor and is normally in the range of 30-80 %. The oxidant is pre¬ ferably parts of or all of the gas turbine exhaust.

Final oxidation of the gas is performed in a gas com¬ bustor after the inorganics have been separated and withdrawn from the gas, and in this step no restriction of oxidant supply is encountered.

After final oxidation the hot gas is directed to an indirect fired gas turbine cycle system.

A distinguishing feature of the present invention is the use of a heat exchanger, which transfers a large portion of the gas combustor exhaust energy to preheat gas turbine motive fluid.

The efficiency of the indirect exhaust fired cycle is directly related to the size and performance of the heat exchanger. The heat exchanger in the hot zone must operate at very high temperatures beyond the practical range for today's metal technology.

Practical ceramic heat exchangers are under development and are expected to be commercial in the near future. These ceramic heat exchangers are highly resistant to erosion and corrosive attacks. Ash deposits on the tube surfaces are amenable to control by the use of conven¬ tional boiler soot blowers.

After the temperature has fallen to below about 800°C, more conventional types of heat exchange equipment may be used for heat transfer to gas turbine motive fluid.

The indirect cycle of the present invention may be combined with a steam turbine system in numerous ways.

The range of operating conditions in the indirect gas turbine cycle of the present invention is not as limited as those for the typical steam generation systems of conventional combined cycle power plants. For example, energy available from the combustion system for evapora¬ tion and superheating can be used to circumvent pinch point restrictions normally encountered in the waste heat steam recovery systems of combined cycle power plants.

To improve the power generation potential of the present invention, the mass flow through the turbine can be increased by injecting water or steam into the gaseous stream entering the gas turbine.

In a preferred embodiment, the compressed air stream used as motive fluid in the gas turbine is cooled after compression by adding water to the air stream in a humidification tower, in which all or part of the in¬ jected water evaporates. The dewpoint decides maximum water addition. In the following heat exchanger, the humid compressed air is heated by heat exchange with process gas combustor exhaust.

Maximum heat is recovered from the exhaust gas when the temperature of the air at the inlet of the heat exchangers is equal to the dewpoint temperature. The evaporative regeneration can be performed in one or more steps with humidification towers before the heat exchanger.

By injection of water in the compressed air stream in this way, at least two objectives are reached. The resulting increased mass flow through the gas turbine increases power output and heat transfer conditions in the heat exchanger are improved.

Another specific advantage of the process of the present invention is that it can utilize low level heat from the discharged flue gases, a compressor intercooler or from the gasification process, or utilize low level heat from elsewhere in the mill to preheat water used for evaporation cooling of the compressed air and/or fuel gas, and hence improve overall efficiency.

Yet another method to increase power output when practi¬ sing the present invention is to inject steam into the gaseous stream entering the gas turbine or injection of steam into the gas turbine before expansion.

A disadvantage with water or steam injected cycles is that water added to the system is lost, if no method to recapture the vapor from the exhaust gas is used. For gas turbine systems with evaporative regeneration, the water consumption for humidifcation is in the order of 0.1-0.8 kg water per kWh power and about twice as much for power efficient steam injection systems. In both cases the water has to be processed to boiler feed water quality.

The gas turbine cycle in the present invention can be integrated with a facility for production of de- mineralized water to be used for injection. Such a demineralization plant could be based on various prin¬ ciples known from the sea water desalination industry.

Demineralization plants based on distillation processes are most preferred for use in the present invention, since they can use heat from the exhaust stream direct or use surplus steam or low level heat from elsewhere in the mill.

The invention will be further explained by the following example and appended figure disclosing a preferred embodiment of the present invention, practising steam injection.

Example

A kraft market pulp mill produces 1070 ton/day bleached pulp, generating a black liquor flow of 1662 ton/day as dry solids. The mill's internal steam requirement amounts to 112 ton 5 bar steam and 36 ton 13 bar steam. This steam is supplied from bark an hog fuel firing

Electricity consumption in the mill is 600 kWh/ton of pulp or 642 MWh/day.

The black liquor is fed to a gasification system inte¬ grated with an externally fired gas turbine plant. The black liquor has the following data at the gasifier entrance:

Dry solids content 78%

Temperature 170°C

Higher heating value 18.8 MJ/kg DS

Flow rate 19.24 kg DS/s

The gasifier is operated at a pressure of 1,3 bar and a reaction zone temperature of 900°C. Air is extracted

from the gas turbine exhaust stream and used as oxidant in the gasifier.

The smelt formed in the gasifier is separated from the hot process gas in a separation zone arranged in the bottom of the gasifier. Additional slag and particles are removed from the hot gas stream in a slag screen arranged at the gasifier exit.

The smelt from the gasifier have the following composi¬ tion (potassium calculated as sodium) :

The smelt comprising the alkali sulfides are withdrawn from the gasifier and used for preparation of cooking liquors.

The hot process gas is oxidized in a gas combustor in the presence of gas turbine exhaust gas. The oxidized hot gas stream is directed to the shell side of a heat exchanger, exchanging heat to a compressed air stream used as motive fluid in the gas turbine.

Clean filtered air enters the compressor, where it is pressurized to approximately 1.2 MPa and a temperature of 361°C. This air stream is humidified by injection of steam and directed to the tube side of the ceramic heat exchanger, where temperature is raised to 882 C. This

high temperature, pressurized air stream enters the gas turbine, where it expands and generates power.

The humidified air exits the turbine at a temperature of 3 37777 CC ttoo bbeeccoommee tthhee ccoommbbuussttion air supplied to the gasifier and gas combustor.

The cooled shell side gas stream and excess turbine exhaust stream is directed to a waste heat steam genera¬ tor generating steam powering a condensing steam turbine plant.

The gas turbine have the following main design criteria:

Efficiencies

Compressor adiabatic efficiency 0.89 Turbine adiabatic efficiency 0.91

Generator efficiency 0.99

^bient_air_conditigns_at_com2ressor_inlet

Temperature 15 C

Pressure 1.033 atm

Relative humidity 60%

Pressure 12.8 atm

Temperature 882°C

^M i £§Ii§S-:2y§

Combustion and mechanical efficiencies are assumed to be 1.0.

Steam generator exit gas temperature 125°C.

Auxuiliary power is assumed to be negligible.

No provision is made for additional gas turbine cooling.

? ϊ-=ϊ!_2H:EHt

Net power output gas turbine 4θ MW

Net power output steam turbine 51 MW

Additional embodiments

The modern kraft mill often has hog and/or bark fired boilers or gasifiers integrated. Other mills have natural gas available for various purposes, such as lime kiln fuel.

The present invention can be practised in combination with combustion or gasification of other gaseous and liquid hydrocarbon fuels available at the mill. As an example, additional natural gas or biogas can be fired in a preburner in the compressed air stream increasing gas turbine inlet temperature and power output.

The same objective can be reached by blending the combustible gas from the gasifier with another hydrocarbonaceous fuel in the combustor ahead of the heat exchangers.

Yet another method to increase power output in the present invention is to inject high pressure steam in various locations in the gas turbine motive fluid and/or into the gas turbine.

Obviously, various modifications of the invention as herein set forth may become apparent to those skilled in the art without departing from the spirit and scope thereof. Thus, for example, humidification may be employed in one or more steps with subsequent preheat, and water or steam may be injected at different loca¬ tions in the cycle to increase motive fluid mass flow. Therefore, only such limitations should be made as are indicated in the appended claims.