Technical large –scale test for biomass

Technical Large –Scale Test for Biomass Co-Combustion in a Lignite Power Plant 1

Goals and Basics

The Bavarian State government aims at increasing considerably the share of renewable raw materials for energyproduction. Biomass is the renewable resource to increase the most. One option to reach this aim is to test and later use on the biomass co-combustion in large power plants.

Supported by the Bavarian State government the biomass co-combustion was tested at the power plant Schwandorf of the Bayernwerk AG. The series of tests comprised four 24-hour tests (April and May 1996) and a long-term test (November/December 1996).

The side conditions of the series of tests.were for example

Transparency 1 -

use of brick-shaped biomass (briquettes)

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thermal value of biomass comparable to coal

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no change of the existing plant technique

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no financial investments

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handling of biomass causes additional expenditure

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minimisation of expenditure for large-scale tests

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no exceeding of licensed limits for flue gas and residuals (dumping in insitu-deposit)

Most important for the series of tests were the goals as follows (Transparency 2)

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clarification of biomass unloading, transport, mixing and grinding ability

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clarification of the combustion characteristics of biomass

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influence of various biomass fractions on above-mentioned criteria

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limits of biomass share due to plant or process technology

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influence of biomass co-combustion on the emissions

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clarification of the influence on power plant residuals

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clarification of the operational influences on the denitrification plant and FGDP

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hints to the corrision and sludging behaviour

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Unit B of Power Plant Schwandorf

The co-combustion of straw briquettes in existing plants of the Bayernwerk AG is technically sensible only for lignite power plants. Thus the lignite-fired power plant Schwandorf is suited for this task.

The power plant with a total electrical output of 532 MW (gross) is situated approximately 150 km north-east of Munich.

Since 1982 the power plant has been firing a Czech hard lignite from the Moster site instead of the originally used local raw lignite.

Adapting to this higher-caloric fuel, then the hot (approximately 950° C) flue gas withdrawal from the furnace was modified to a warm (approximately 380° C) flue gas withdrawal including flue gas extraction before the circulating air preheater.The unit B with a total electric output of 108 MW was chosen for the tests due to separate flue gas cleaning plants.

The power plant ash is mixed with the gypsum suspension (approximately 50% water) from wet desulphurization. The product is named „stable material“. It is deposited on the dumping site of the power plant and hardens to a concrete-like material.

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Basic Fuel and Biomass Fuel

The biomass for the tests was provided by C.A.R.M.E.N. e.V. Three fractions of biomass bricks were used . (Transparency 5) Sample tests of the lignite and the biomass used during the test periods were analysed extensively.

Apart from the values of heat capacity and components of combustion, heavy metals, metals, alkaline earth metals and further trace elements were recorded.

The following picture ( Transparency 6)compares the heating values and some fuel components.

All indicated values refer to the raw materials. The values for coal varied considerably. Therefore

average values were used for the test operations.

(Transparency 6) Comparison of Some Fuel Components of Coal and Biomass The water contents of the biomass fuels were a fourth to a third of those of coal. Originally, biomass with water contents of up to 15% produced by mobile presses (so-called biotrucks) had been envisaged the technical use of which has still to be awaited.

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Results

3.1

Supply, Handling and insitu Transport of Coal and Biomass

The biomass was delivered by trucks. Considerable dusting while unloading could not be prevented.

For the short-term tests the dumped biomass was separated from the coal and transported by train to an outside bunker. The mixture and transportation into the boiler bunkers was conducted by adjustable bunkers and conveyors. As more dust came up with an increasing share of biomass in the coal-feeding area, the internal logistics were changed for the long-term test. A premixing of lignite and biomass was performed in the coal store area. Dusting within the power plant could thus be minimised by a slightly increased handling. However, considerable dusting in the coal store area emerged.

The mechanically instable bricks from the wet plant drying station were used to safeguard the test dates. Regarding the internal handling within the power plant these bricks are not suitable for longterm operation. If a long-term biomass co-combustion is to be executed without investments in additional plant technique for handling, highly mechanically stable briquettes with low fines contents are necessary.

3.2

Influence of the Biomass on the Operational Behaviour of the Steam Generator

3.2.1

Influence on the Water Steam System and the Flue Gas System of the Boiler

During the tests no essential changes in the operational conditions as compared to the normal operational state occurred.

3.2.2

Grindings and Operational Behaviour of the Mills

The existing mills in unit B are rigid-hammer crushers with gravity separation. The fuel is dried with flue gas at 380° C. The mills were operated with biomass shares between 10 and approximately 40%. Shares of more than 30 up to 40% cannot be handled by the existing grinding system. For permanent co-combustion a share of up to 10 % biomass is technically feasible.

(Folie 10) Fractions of the Fuel after Mill In general, due to it´s structure biomass was grounded worse than coal. Fragments of up to 1-3 mm, were combined with a kind of straw flour and singular 5-10 mm long straws. The wetter coal was partially bound by the dry biomass. As expected, grinding gets coarser with increasing biomass share.

Furthermore, grinding is coarser for whole plants and hay as compared to straw.

It can be summarized that using the existing mills up to a biomass fuel content of up to 10% the operation is similar to that with mere coal. Exceeding this limit the grinding behaviour worsens even for coal and the more fibrous and voluminous properties of biomass dominate. The grinding gets coarser.

3.2.3

Combustion Behaviour

As criteria for the combustion behaviour mainly the forming of Carbonmonoxid (CO), the unburnt materials of the grate and fly ashes, the fractions of the fly ashes, examinations of the furnace and the burn-out grate were considered. The following transparency compares the respective values.

(Transparency 11): Comparison of Unburnt Materials in the Fly Ashes, Fractions in the Fly Ashes and CO in the Flue Gas after FGDP The share of CO does not change significantly. The grading in the crude gas dust hardly change.

Bigger unburnt particles were hardly to be found in the crude gas flow. The share of combustibles in the crude gas dust was higher in case of biomass co-combustions than with mere lignite, which can be

explained by the coarser grinding and thus the lower density of the biomass particles. However, the considerably larger load of the burn-out grate proved that the very coarse biomass components as well as the coarser coal components are not transported through the furnace but are burnt on the grate.

3.2.4

Sludging and Corrosion

While additional sludging during the short-term tests was hardly observed, the long-term test showed an increased sludging tendency especially in the area of the burn-out grate and the evaporator.

Due to the predominant operation (closed at night and at weekends) the boiler cleaned itself.

3.3

Influence of the Biomass on the Concentrations of Noxious Matters in the Flue Gas and on the Operation of Components of the Flue Gas Treatment

During the tests the licensed emission limiting values according to the 13 th National Act on the Prevention of Immissions (BImSchV) were never reached .

3.3.1

NOx-Contents of the Flue Gas and the Influence on the Operation of the Denitrification Plant (DeNOx)

The next transparency (Transparency 12)shows the taken average values of the NOx-emission together with the average nitrogen contents of the mixed fuels for all tests.

(Transparency 12) : Comparison of the NOx-Contents in the Flue Gas There is a tendency of theoretically lower NOx-formation due to somewhat lower N-contents of the mixed fuel. However, it cannot be proved definitely. The tendency is confirmed by a lower consumption of ammonia (NH3.)

3.3.2

Dust Contents of the Flue Gases and the Influence on the Operation of the Electrostatic Filter

The clean gas dust contents after the electrostatic filter increased during the tests independently of the biomass fraction and the biomass share. During the long-term test this tendency increased as can be seen in the next transparency. (Transparency 13)

(Transparency 13):

Comparison of the Dust Contents in the Flue Gas

The dust separation of the desulphurization plant guaranteed in any operation meeting the admissible dust contents at the measuring point for the emissions.

As neither the crude gas dust contents nor the crude gas dust fraction changed significantly, the cause may either be the specifically ligther superfine dust contents or the electrostatic filter ash formed by biomass. On the other hand a further cause was the decrease of the ionization of the flue gas in the electrostatic filter (lower water contents and lower sulphur dioxid (SO 2 ) contents of the flue gas).

3.3.3

SO2 and Halogens Contents of the Flue Gas as well as Operation of the Desulphurization Plant (FGDP)

The sulphur contents of the biomass fuel is but a tenth of that of the contents of the used coal . The values shown on the next transparency (Transparency 14) confirm the lower sulphur dioxid values in the flue gas to be expected .

The biomass co-combustion did not influence the operation of the desulphurization plant and the desulphurization rate.

The higher contents of chlorides in the biomass fuel (especially straw) leads to a higher fraction of hydrogen chloride (HCl) in the flue gas after the steam generator. This increased fraction is however of a low value, if seen absolutely, that can be compared with the values of some hard coal-fired power plants.

Halogens in the flue gas were measured before entering the stack. Whereas the values for HC1 tripled to approximately 1% of the limiting value of the 13th Federal Rule on the Prevention of Immissions (13. BimSchV), the values for HF (hydrogen fluoride) were beneath detection. The very modest increases in halogen contents are due to the high setting degree of halogen compounds in the desulphurization plant. The chloride load of the desulphurization plant suspension increased especially during the long-term test to values that need further observation as far as the involved special steels are concerned.

3.3.4

Dioxins and Furans

In the blank experiment the emission of dioxins and furans could not be detected. During the longterm test the measurements stated values hardly over the detection limit.

3.4

Influence of the Biomass on the Residuals

The power plant residuals such as fly ash, furnace ash and gypsum suspension of the desulphurization plant are deposited. Before dumping, a stable material is produced in the ash mixing plant which hardens on that insitu-deposit to a concrete-like material. Apart from that the influence of the biomass co-combustion on the residuals had to be examined. Points of examination were ashes, the produced stable material and the eluate from stable material made of Proctor cylinders.

Cations and anions, the polycyclic aromatic hydrocarbons (PAH) as well as dioxins and furans were measured.

The analyses of the test samples did not show significant differences as compared to mere lignite firing.

In view of all results the performance of the co-combustion tests was evaluated to be quite successful. The permanent co-combustion of up to 10 % biomass is technically feasible regarding various side-conditions in the process.

At last some information about the situation in the power plant Schwandorf.

In 1997 and 1998 the co-firing of waste wood was tested. The results showed that there were hardly any significant changes for example in emissions or dumping abilities of the residuals as compared to the mere lignite combustion. According to the good results the BAYERNWERK tried to obtain a permission for co-firing waste wood in unit D .

In 1999 the units B and C (both with a total electric output of 108 MW) were shut down for economic reasons.

On the 8th June 1999 the BAYERNWERK got the license to co-fire up to 100.000 tons per year of waste wood of the categories B1 and B2 in unit D.