WO2003016681A1

WO2003016681A1 - Method and plant for use of biomass as supplementary firing in a gasworks

Info

Publication number: WO2003016681A1
Application number: PCT/NO2002/000277
Authority: WO
Inventors: Henrik Kobro; Johan E. Hustad
Original assignee: Statoil ASA
Current assignee: Equinor ASA
Priority date: 2001-08-16
Filing date: 2002-08-09
Publication date: 2003-02-27
Anticipated expiration: 2004-02-16
Also published as: NO314702B1; NO20014002D0; NO20014002L

Abstract

A method of using biomass as supplementary firing in a combined cycle, combined heat and power plant comprising an internal combustion engine fired with fossil fuel and a steam engine, where steam for the steam engine is generated in a waste heat boiler for the flue gas from the internal combustion engine, and where the biomass is gasified in a gasification reactor to produce a biomass gas, where the biomass gas is carried hot from the gasification reactor and into an afterburner in the waste heat boiler, and combusted there. A combined heat and power plant for carrying out the method is also described.

Description

METHOD AND PLANT FOR USE OF BIOMASS AS SUPPLEMENTARY FIRING IN A GASWORKS

The present invention regards a method and a plant for using biomass as supplementary firing in a combined cycle combined heat and power plant. More specifically, the invention regards a combined heat and power plant in which biomass is gasified and fired in a burner in a flue gas outlet or a waste heat boiler in the plant steam generator.

Backgound When producing energy from biomass, achieving satisfactory profitability may be more difficult than when using competitive fossil fuel. However this picture may change with the introduction of taxes or quota trading on fossil CO₂. A system of quota trading will make it possible for a producer of "clean" energy to achieve a better price than for energy produced from fossil fuel.

Conventional power production plants are generally large installations based on steam boilers and steam turbines. These plants may be fired with most types of fuel, normally coal, brown coal or oil. A modern plant such as this has an efficiency of up to 45%

Combined Cycle Plant

A higher efficiency may be achieved by combining several techniques, so-called Combined Cycle plants, in which diesel engines or gas turbines may be used with waste heat boilers and steam turbines to utilize the heat that would otherwise be lost, for producing additional power in a secondary process. These plants make great strict on the fuel, which should ideally be liquid or gaseous.

Natural gas is a highly suitable fuel for Combined Cycle plants. Based on environmental concerns as well as economic criteria of evaluation, gas fired power plants have become the new standard choice in those cases where the developers are free to choose the plant type and where the gas supply is sufficient.

It is natural to combine the gas turbine and steam power processes. This is done through using the flue gas energy from the gas turbine for producing power in a steam process. This is often referred to as a CCGT process. The development of the CCGT processes is largely connected with the development of gas turbine technology. In a combined plant, the gas turbine typically delivers 2/3 of the total power output of the plant. The output range of a large CCGT block is up to 400 MW. CCGT is the system which currently offers the highest efficiency, up to as much as 58%, and this value is expected to rise to 60% within a few years.

CHP (Combined Heat and Power) Production

Combined heat and power plants may be used for concurrent production of both electrical power and heat (co-generation). By also letting a combined cycle plant produce heat, a highly favourable efficiency is achieved with regard to the degree of utilization of the energy in the fuel. The main condition for making this commercially viable is the existence of a buyer/consumer for this heat nearby, such as a process plant requiring addition of heat or a remote heating plant for a built-up area.

This heat production will reduce the production of electricity. The reduction is a function of the level (pressure/temperature) at which the heat is extracted. The lower the temperature of the heat delivery, the higher the overall efficiency of the plant and the larger the proportion of electricity produced. When co-producing electricity and heat in this manner, it is possible in principle to achieve an efficiency of more than 90%. The heat produced may be used for remote heating (90°-120°C) or process steam (150°C or higher).

Gasification

It is often expedient to gasify biofuel and use this biomass gas in subsequent processes. Gasification is a thermochemical process in which biomass is converted to gas via reactions between dry matter and a gasification medium. It takes place through biomass being converted to gaseous fuel by use of air, oxygen or water vapour at high temperatures. As opposed to combustion, gasification occurs through sub- stoichiometric combustion, i.e. with a deficiency of oxygen. The product gas is a mixture of CO and H₂, H₂O and others. Non-combustible ash will also form. This gas is hereinafter referred to as LHV gas, which is an abbreviation of Low-heating- value. The gasification process may roughly be divided into three different phases [Beyer, "Kraft/varme produksjon vedforgassing av biomasse", hovedoppgave 1995 ("Power/heat production by gasification of biomass", thesis 1995)].

Table 1: The various phases of the gasification process

When using bark as a fuel for gasification, the chemical equations are as follows: Drying: Moist bark + heat = Dry bark + H₂O (steam) Pyrolysis: Dry bark + heat = C + CO₂ + various hydrocarbons Oxidation: C + H₂O + heat = CO + H₂ C + CO₂ + heat = 2 CO

As shown above, all these reactions are endothermic, i.e. they require the addition of heat. This energy requirement is met thorough combustion, where the rate of combustion is controlled through the addition of air or possibly pure oxygen. A critical factor for the gasification process is to ensure correct adjustment of the air supply, to prevent combustion of too much fuel and ensure the correct gasification temperature.

Air based gasification produces a gas with a calorific value in the range 4-6 MJ/Nm³, while gasification using oxygen or oxygen enriched air gives a gas with a calorific value in the order of 15 MJ/Nm . This gas may form a basis for conversion into hydrogen and for production of chemicals.

Reactor types The actual gasification process takes place in a reactor. There are several types of reactors, each with their own advantages. The following briefly outlines the characteristics of the various types.

Table 2: Some common reactor types

Updraft fixed bed, bubble flow fluidized bed and circulating fluidized bed are the most commonly used reactors for gasification of biomass.

Fixed-bed reactors

This type of rector is best suited for gasification of small quantities of fuel. This means a reactor feed of around 100 kg of biomass per hour, equivalent to 500 kW_th or less. Of these, downdraft reactors are the most suitable for gasification of biofuels, due to better heat transfer, less formation of tar and a cleaner product gas.

Fluidized bed

The principal difference between an FB and a CFB is the velocity. In a CFB, the air entering the reactor has a higher velocity. The FB is also called a stationary fluidized bed, as the bed materials here remain in the reactor. A CFB makes use of smaller particles and a higher gas velocity. This causes some of the bed material in the CFB to be entrained in the gas flow. The material is collected and recirculated by means of a cyclone and a return line. Of the two types, the CFB reactor has the greatest capacity. Both technologies are commercially available and have a long period of service behind them in other installations. Gasification or pyrolysis of biofuels is known in combined heat and power plants. At Varnamo power plant, a demonstration programme was carried out in the period 1996- 2000, in which gas from a gasification plant for biofuel was used as fuel for a combined cycle plant. In addition, IEA Bioenergy: T3: 1998:01 describes a combined cycle combined heat and power plant integrated with a paper-mill.

Studies from other plants using biofuel to fire combined heat and power plants show a thermal efficiency of 40-50% in a combined cycle plant. However such plants have disadvantages both on the economic and practical level.

Gas from a gasification plant contains particles, and must therefore be cleaned before being used as fuel for a gas turbine, hi the existing bio-fired IGCC plants, the main operational safety problem is associated with the actual cooling of the gas and the purification downstream of the reactor [Varnamo Demonstration Plant. The Demonstration programme. Sluttrapport (Final report), 2001].

Summary of the invention

It is therefore an object of the present invention to provide a method of using biofuel as supplementary firing in a combined heat and power plant, which method allows the use of biofuel in a combined heat and power plant, but which avoids the above mentioned disadvantages.

According to a first aspect of the present invention, a method is provided for the use of biomass as supplementary firing in a combined cycle combined heat and power plant comprising an internal combustion engine and a steam turbine, where the steam for the steam turbine is generated in a waste heat boiler for the flue gas from the internal combustion engine, and where the biomass is gasified in a gasification reactor in order to produce a biomass gas, where the biomass gas is sent, in an uncooled state, from the gasification reactor and into an afterburner in the waste heat boiler in order to be combusted there. Preferably, the biomass gas is carried from the gasification reactor to the afterburner in the waste heat boiler in an unconditioned state.

Preferably also, the temperature of the gasification reactor is between 600 and 1000 °C, preferably between 700 and 900 °C.

Preferably, the energy in the biomass constitutes between 1 and 50%, preferably between 2 and 30%, of the total amount of added energy.

Moreover, it is preferable for the biomass to consist of bark or chips.

It is a further object of the present invention to provide a plant for carrying out the method.

According to another aspect of the present invention, a combined heat and power plant of the combined cycle type is thus provided, comprising an internal combustion engine fired with fossil fuel, a steam engine and an afterburner in a waste heat boiler for generation of steam for the steam engine, where the combined heat and power plant further comprises a gasification reactor for gasification of biomass in order to produce a biomass gas, an afterburner for combusting the biomass gas in the waste heat boiler, and also means of carrying the gas uncooled and unconditioned from the gasification reactor to the afterburner.

According to a preferred embodiment, the internal combustion engine is a gas turbine.

Preferably also, the steam engine is a steam turbine.

Preferably, the gasification reactor is a fixed bed, fluidized bed, cyclonic or vortex reactor, preferably a fixed bed or fluidized bed reactor, most preferably a circulating fluidized bed reactor. According to a preferred embodiment, the waste heat boiler is constructed as a suspended structure in order to give the shortest possible distance between the gasification reactor and the afterburner in the waste heat boiler.

Brief description of the figures

Figure 1 shows a schematic diagram of a plant according to the present invention; Figure 2 is a graphical representation of the efficiency as a function of the temperature of the generated gas; and

Figure 3 shows a schematic diagram of a preferred construction for the waste heat boiler.

Detailed description of the invention

The method according to the present invention uses gasified biomass as supplementary firing in a combined cycle gasworks. The biomass gas from the reactor (also called the LHN gas (Low Heat Value)) is passed directly into an afterburner in an uncooled state, which afterburner is located in a waste heat boiler for the internal combustion engine, in this case a gas turbine. The excess air in the flue gas from the gas turbine is used for combusting the gas from the gasification reactor. This increases the temperature and mass flow of flue gas in the steam boiler, increasing the output of the steam turbine. Figure 1 shows the flow of the process through the plant.

The present method and plant differ from other known and planned plants in that the LHN gas is passed to the afterburner in an unpurified and uncooled state. This eliminates the need for expensive equipment and a complex process that would otherwise be required to cool and purify the generated biomass gas.

In principle, the plant shown in the form of a schematic diagram in Figure 1 is based on a conventional combined cycle combined heat and power plant for fossil fuel such as e.g. oil or gas. This comprises an air intake 1 to a compressor 2. From the compressor 2, the compressed air is passed through a line 3 to a burner 4 with a fuel intake 5. The combusted, hot gas from the burner 4 is passed on through a line 6 to a gas turbine 7 in which it expands. The expanded gas from the gas turbine 7, which may have a temperature of around 400 to 650 °C, is then passed through a line 8 to waste heat boiler 9, in which the gas is heat exchanged against feed water from a water intake 10 in order to generate steam that is passed to a steam outlet 11. The expanded, cooled gas is carried out of the waste heat boiler 9 via an exhaust 12.

The steam in the steam outlet 11 is passed to a steam turbine 13 in which it expands, and passes out through an outlet 14. In the plant shown, the compressor 2, the gas turbine 1, the steam turbine 13 and a generator 15 are mounted on a common shaft 16. This configuration with one shaft is preferred in many plants of this type, as it results in a smaller generator loss than that which would be the case with another configuration.

So far, this plant is the same as a conventional combined cycle plant. The novelty of the present plant is that it comprises a gasification reactor 21 to which bark or another suitable fuel is fed through a fuel inlet 20. As described above, heat is generated in the gasification reactor 21 through a partial combustion in the reactor 21, and the gasification reaction preferably takes place at a temperature of between 600 and 1000 °C, more preferably between 700 and 900 °C, in order to produce a biomass gas which is passed to an afterburner 23 in the waste heat boiler 9 via a line 22.

According to the present invention, where the generated biomass gas is to be combusted at atmospheric or near atmospheric pressure, e.g. a slight gauge pressure of 1.3 bar, the costs associated with building and operating the gasification plant are relatively low compared with plants operated under pressure.

The gas generated in the gasification reactor 21, or the biomass gas, is passed to the waste heat boiler without being cooled or conditioned, in which waste heat boiler the excess air in the combustion gas from the gas turbine is used for combustion of the generated biomass gas. This burner is insensitive or only slightly sensitive to particles and other constituents that are undesirable in gas turbines or other internal combustion engines. Example - Integration of a gasification reactor (CFB plant) into a combined cycle (CC) plant

The CFB plant has been estimated at a total height of 50 metres. The product gas exits this plant from a cyclone that separates the produced LHV gas from ash and bed materials, at the top of the reactor. In order for the transport of hot LHN gas to be as short as possible, it has been suggested that the design of the steam generator be changed so as to construct it as a so-called "suspended" structure. This involves it being held up by tension rods anchored to a solid constructional support over the actual steam generator. This solution replaces an "upright" design in which the boiler is supported.

This preferred design has been shown in Figure 3, where the waste heat boiler 9 is suspended from a constructional support 30 by means of tension rods 31. A gas turbine 7 has been connected to the waste heat boiler by means of flexible bellows 32 for transporting the hot waste gas from the gas turbine to the waste heat boiler. The LHN gas is fed to the waste heat boiler 9 through gas inlet 22, while cooled waste gas is extracted via a flexible bellow 33 and cleaned of ash and other solid particles in a filtering device 34. The ash and other solid particles are removed from the filtering device 34 through ash outlet 35 prior to the waste gas being let out through a flue 36. Water is fed to the waste heat boiler through a water intake 10, and steam is taken out through line 11 and passed on to the steam turbine. Ash is removed from the waste heat boiler through an ash outlet 37.

The print-out from the simulation was used as a basis for estimating the dimensions of the steam generator. This gave the surface area of the various heating surfaces in the generator. It was then assumed that the heat transmission takes place across circular pipes with a diameter of 40 mm, and that these pipes cover half of the internal cross section of the flue duct. This means that the total volume is 2.5 times larger than that which is taken up by the pipes.

Table 3: Volumes in the steam generator.

Based on these estimated volumes, a possible design has been proposed for the steam generator in the figure below. The internal area of the flue duct is 8* 12=96 m². The gas from the reactor enters the steam generator at an elevation of 30 metres.

Exiting the reactor will be LHV gas at 900°C. This high temperature makes it necessary for the pipe carrying the gas to have a high heat tolerance. It is anticipated that such a pipe will be expensive, and therefore it is important for this connection between the reactor and the waste heat boiler to be as short as possible. A design has here been proposed, which takes these requirements into account. The pipe must have two steel jackets, where the inner jacket is covered by a layer of ceramic material. Air will flow between the two layers of steel, and then be passed into the reactor as preheated air.

The pipe must be able to transport around 8 Nm /s of gas. At 900°C, this is equivalent to :

1193R^"

SNm* Is = 32.6m³ Is

213K

I.e., if the LHN gas has an inlet velocity of 30m/s, the inner diameter of the pipe must be approximately 67cm.

The afterburner The afterburner must meet certain important requirements, and must as an example tolerate a temperature of 900°C in the fired gas. This presents a number of challenges with regard to the practical optimisation of the process. The afterburner must meet strict requirements for temperature resistance due to the high temperature to which it will be subjected. It must tolerate an external oxidising atmosphere and an internal reducing atmosphere. It must be designed with a view to making the NO_x emissions meet the environmental requirements of the pollution authorities, and it must provide spontaneous ignition of the gas upon contact with oxygen.

Simulation

The setting for the simulations was a CCGT. The General Electric gas turbine GE 9351FA was selected, where the steam turbine, the gas turbine and the generator are mounted on the same shaft. This is a realistic basis, as this turbine is currently commercially available, and its NO_x emissions are low. The starting point was a planned power plant that is to include two such "blocks"; however the effect of supplementary firing of LHV gas is here only investigated for one of these blocks. Simulations were carried out both for a case involving only electricity production and for a case involving extraction of low pressure steam to the paper mill. In addition, the effect of lowering the temperature of the LHV gas was investigated.

The simulations were carried out with the software GT-Pro. Runs were first carried out without combustion of LHV gas, so as to enable the results to be evaluated against a reference. Comparison of the simulation results allowed the effect of the gasification to be assessed. The added output of the steam turbine was measured against the fuel fired in the gasification reactor.

Simulation data

The following parameters were selected for the CC plant: • 3 steam levels with reheat loop

• J- amb ^— ->

• 1 cooling water ^— O U

• Pcond = 0.025 bar

• No pre-compression of natural gas, as it comes pressurised in a pipe. • "Singe-shaft" configuration. Empirical values from gasification plants of approximately 40 MW_th were used for gasification of the biomass. On an annual basis, this is equivalent to 320 GWh of energy.

The mass flow rate of bark and chips entering the reactor with a moisture content of 30% is 3.54 kg/s. The fuel has a net calorific value of 17.35 MJ/kg (LHV_ds).

Table 4: Fuel composition

In order to find the fired effect of the bark, the effective calorific value must first be calculated. The effective calorific value is defined as [Ertesvάg, 2000]:

LHV_eff = LHV_ds (LHV_ds - h "fS ' - w

w - fuel moisture content: 30% h_fg- heat of vapourization of water, 25°C: 2442.3 kJ/kg LHV_eff- effective calorific value of the fuel: 11.41 MJ/kg

I.e. the fired effect to the gasification reactor is:

3.54kg Is ΛlAYMJIkg = 40.4MW

Even minute variations in the composition of the fuel or the LHN gas will cause their calorific values to vary, and as such there is a certain inaccuracy to these calculations. As a result, the fired effect is rounded up to 41 MW. During gasification, the reactor is fed a maximum of 4.27 Nm³/s of air at a pressure of 1.3 bar, and the fuel is converted to a gas with a low calorific value; 3.74 MJ/Nm³. This volume of air corresponds to a stoichiometric air coefficient of 0.36, thus giving incomplete combustion of the fuel (gasification). The product gas has the following composition by volume:

Table 5: Composition of LHV gas (vol %)

The mass flow rate of gas into the afterburner was calculated to be 9.0 kg/s.

To start with, it is assumed that the product gas may pass untreated from the reactor to the afterburner, which gives it a temperature at the inlet of as much as 900°C. There is some uncertainty as to whether this is possible, as such high temperatures place great demands on the properties of the material; however, as this is the "best case scenario", it is assumed that this problem can be solved.

The simulations showed that this direct combustion of hot LHN gas resulted in the temperature of the flue gas rising from 608°C to 651°C before the first heat exchanger in the steam generator. The mass flow rate of flue gas through the steam generator increased by 1.35%, from 665.6 kg/s to 674.6kg/s. This increase is the same as the amount of added LHN gas.

Trial parti: Production of electricity only. The important results from the first set of runs have been listed in the table below.

Table 6: Results of simulations

Efficiencies.

In order to evaluate the results of the simulation, a system efficiency is defined:

η = system _ efficiency = — -

P_eι - net elektrical power out of the generator Qi_n - total fired thermal effect (LHV) to the system

Table 7: Efficiencies

The right hand column of the table shows the efficiency for the bark alone, and this is calculated by finding the difference between the two alternatives. It states how much extra electricity is generated as a result of the gasification and the supplementary firing in a waste heat boiler. An electricity efficiency of more than 50% is very good for bark, and for comparative purposes, mention may be made of the fact that the efficiency in the case of combustion of bark in a steam boiler is around 20 %.

Trial part 2: CHP. Drawing of both electricity and heat.

The expression CHP is an abbreviation of Combined Heat and Power. This simulation investigated the effect of supplementary firing of gasified fuel in a CHP plant. In order to get as close to reality as possible with regard to a combination with a process plant, 25 kg/s of low pressure process steam was taken off at 3.5 bar and 155°C. These values are equivalent to drawing 69.1 MW of heat. Runs were first carried out without combustion of LHV gas, so as to enable the results to be evaluated against a reference.

The offtake of steam is constant and independent of the amount of fired fuel. Thus any supplementary firing to the system will result in the generation of additional electrical energy. This experiment aimed to measure the additional generation of electrical power relative to the amount of fired bark.

Furthermore, the effect of the temperature of the LHV gas was investigated. By running six simulations at different temperatures, the effect of reducing the temperature could be investigated.

Results

Table 8: Results of CHP simulation

Bark efficiency (el.) here refers to the additional electricity generated relative to the fired thermal effect of the biofuel. Comments:

Figure 2 shows that there is a linear relationship between the temperature of the fired LHV gas and the efficiency of the biofuel in the process. A lower temperature in the LHV gas gives a deteriorating efficiency. However, these simulations do not take into account the heat that results from the cooling, which can also be used in the boiler, and so in reality, the efficiencies will be higher than those found here.

What gives such a high efficiency? Intuitively, a 50% efficiency in a steam cycle may seem an unrealistic result. It is therefore very important to emphasize the fact that this is the marginal effect of the gasification process.

The supplementary firing causes the temperature of the flue gas to increase from 608°C to 651°C, and this will improve the performance of the steam cycle, cf. the Carnot cycle efficiency.

The results show the actual steam cycle without firing of hot LHN gas to have an efficiency of over 30%, and with this firing, this value increases to 33%. This is a result of the supplementary firing coming "on top", i.e. the conditions for high pressure steam are improved.

Although the present invention has been described in reference to a specific plant and specific choices, it will be obvious to a person skilled in the art that different technologies for gasification of biomass may be used. In addition, other internal combustion engines and other steam engines than the gas turbines and steam turbines described here, may be envisaged. The internal combustion and/or steam engines may alternatively be any kind of internal combustion or steam engine, such as piston engines and Wankel engines.

The present invention has also been described using bark as biomass. Bark is a waste product in the production of cellulose and paper, and is therefore the preferred biomass when the present combined heat and power plant is co-located with a cellulose and/or paper mill. Chips is yet another preferred biomass which may be combined with bark.

Claims

C l a i m s

1.

A method of using biomass as supplementary firing in a combined cycle combined heat and power plant comprising an internal combustion engine fired with fossil fuel and a steam engine, where steam for the steam engine is generated in a waste heat boiler for the flue gas from the internal combustion engine, and where the biomass is gasified in a gasification reactor to produce a biomass gas, wherein the biomass gas is carried hot from the gasification reactor and into an afterburner in the waste heat boiler, and combusted there.

2.

A method according to Claim 1, wherein the biomass gas is carried unconditioned from the gasification reactor and into the afterburner in the waste heat boiler.

3.

A method according to Claim 1 or 2, wherein the temperature of the gasification reactor is between 600 and 1000 °C, preferably between 700 and 900 °C.

4.

A method according to one or more of the preceding claims, wherein the energy of the biomass constitutes between 1 and 50%, preferably between 2 and 30%, of the total added energy.

5.

A method according to one or more of the preceding claims, wherein the biomass consists of bark or chips.

6. A combined heat and power plant of the combined cycle type, comprising an internal combustion engine fired with fossil fuel, a steam engine and a waste heat boiler for generating steam for the steam engine, wherein the power plant further comprises a gasification reactor for gasification of biomass in order to produce a biomass gas; an afterburner for combustion of the biomass gas in the waste heat boiler, as well as means of passing the gas uncooled and unconditioned from the gasification reactor to the burner.

7.

A combined heat and power plant according to Claim 6, wherein the internal combustion engine is a gas turbine.

8.

A combined heat and power plant according to Claim 6 or 7, wherein the steam engine is a steam turbine.

9. A combined heat and power plant according to one or more of Claims 6 to 8, wherein the gasification reactor is a fixed bed, fluidized bed, cyclonic or vortex reactor, preferably a fixed bed or fluidized bed reactor, most preferably a circulating fluidized bed reactor.

10.

A combined heat and power plant according to one of Claims 6 to 9, wherein the waste heat boiler is constructed as a suspended structure in order to make the distance between the gasification reactor and the afterburner in the waste heat boiler as short as possible.