COGENERATION POWER PLANT ON THE BASIS OF BIOFUEL FIRING BOILER Текст научной статьи по специальности «Электротехника, электронная техника, информационные технологии»



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COGENERATION POWER PLANT ON THE BASIS OF BIOFUEL

FIRING BOILER

A. V. Soudarev

Research-Engineering "Ceramic Heat Engines" Center named after A. M. Boyko, Ltd (NIZ KTD, Ltd») Polyustrovsky Av. 15, block 2, St.Petersburg, 195221, Russia Phone/fax: 7 (812) 2253453; e-mail: soudarev@boykocenter.spb.ru

Cogeneration is a process of fuel combustion that allows:

— simultaneous production and supply of heat and electric power;

— simultaneous solution of challenges associated with power saving and environment protection;

— commercialization on the basis of the existing boilers including a bio power fuel firing.

The paper presents a survey of the existing heat schemes for cogeneration plants with the fossil types of the hydrocarbon fuel fired therein. An impracticability of using the naturally sustained bio fuels as a heat power source for the similar power plants is illustrated. Thus, an expediency of update of the operated power-and-heat boilers with bio fuel fired as wood chips to ensure generation of both the heat and electric power is underlined. It is noted that for the sake of the target above outlined it would be necessary to have a ceramic air heater built into the bio boiler design, this component being at the same time a part of the electric generator gas turbine engine drive.

Introduction

Development and production of devices aimed to reduce the toxic gases (NO^, CO, SO^) exhaust into atmosphere have assumed a status of a main trend in activities of research-engineering bodies all over the world, this trend being focused on mitigation of affects of the global use of the hydrocarbon-fuel on the environment. This trend became especially explicit after adoption of the Pure Air Act in 1990 and the Kyoto Protocols signed on the eve of the 3rd millennium.

The objective of the present project is to decrease the gross output of the toxic gases that are contained in the boiler exhaust. This is achieved by an update of the operated bio power boiler rooms whereas their efficiency increases through generation of the electric power with no amount of the supplied heat power reduced.

Bio power is the most environmentally friendly power generation source as it is based on woods waste firing. This applies primarily to the bio power boilers that are already de4signed and manufactured to produce and supply only the heat power. It should be underlined that with such technology the fuel combustion products heat is far from being transferred to the water under heating completely as its notable portion is discharged into atmosphere.

At the same time, the current bio power technology allows realization of cogeneration given the bio fuel combustion products are, first, utilized for heating of the air that supplies the turboelectric generator turbine and only then for water heating. In this event, the heat efficiency of a plant increases substantially. A consumer gets both the electric and heat power while the boiler owner gets a big profit since the cost of the electric power is notably higher compared to that of the heat power.

The existing cogeneration plants

At present, the cogeneration is evolving and applied with two mostly typical heat schemes.

1. Cogeneration plants with gas turbine engines for electric generator driving, the exhaust from the electric generator utilized as a heat carrier for water heating that supplies into the pow-er-and-heat generation system (Fig. 1).

2. Cogeneration plants with power-and-heat water boilers where exhaust gases from electric generator driving gas turbine (Fig. 2) or diesel engines are used as oxidizer for the fuel fired.

Merits of the similar systems are their high efficiency and capability to control the ratio between the heat and electric power generation.

A principal flaw of the operated power cogen-eration plants is impracticability to fire a solid bio

Доклад на Первом Всемирном конгрессе «Альтернативная энергетика и экология» WCAEE-2006, 21—25 августа 2006 г., Волга, Россия.

Paper at the First International Congress "Alternative energy and ecology" WCAEE-2006, August 21—25, Volga, Russia.

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W "WV

He

W 20 °C

r(H2Ob - 90 °C

AH

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TT - 600 °C 1;

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Fig. 1. Cogeneration gas turbine plant with standardized heat exchanger ("exhaust gases — water"). Designations: C — compressor; T — turbine; EG — electric generator; St — stack; CC — combustion chamber; AH — air heater; HE — heat exchanger for power-and-heat water

TF - 20 °C

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Fig. 2. Cogeneration plants with fuel fired in the exhaust gas flow from the electric generator driving gas turbine engine. Designations: C — compressor; T — turbine; EG — electric generator; WB — water boiler; CC — combustion chamber

fuel since both the gas and diesel engines are operated, as a rule, only with gaseous and liquid fuels. Attempts to utilize a solid fuel as coal dust result in an excessive design complicating and a dramatic increase in the engine cost which is related with problems of gasification, filtration, cleaning, heating and other operations to treat fuel, erosion wear-out of the flow passages, etc. Also, the toxic components emissions from heat engines in such plants are rather high.

Cogeneration bio fuel firing plants

The objective of the project is to increase the efficiency of the existing bio boilers through use of the bio fuel combustion products firing to generate the electric power. Fig. 3 shows the heat scheme for conventional boilers that fire solid fuels and supply hot water to the power-and-heat systems of residential areas, industrial objects, hotels, trade centers, industrial enterprises, etc.

Merits of such boiler room:

— Operation with bio power fuel firing (organic origin vegetable mass as a resultant of the photosynthesis), e.g. wood waste,

its flaws are:

— a high amount of heat are lost with high temperature exhaust gases;

— relatively high emissions of the toxic components (NO , ^ ^H^ ete);

— considerable expenses to buy the electric power required to drive the bio boiler auxiliaries (fans to supply primary and secondary air, augers to supply solid fuel to furnace, fuel tank vibrators, smoke exhausters, signal and control hardware, control, governing and monitoring system instruments, lighting, etc); the total consumed power amounting to 5-10 % of the total heat power of a bio boiler.

With firing of a gaseous or liquid fuel in the water heating boilers, switch over to the cogen-eration technology is a comparatively simple procedure. In this case, no fuel is supplied to the boiler* and there is no combus-

* If required, an independent boiler operation could be envisaged over the periods when a Customer needs only the heat supply

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T - 200 °C

H2O

T - 500°C

T - 1300°C

T - 20 °C

T; - 500 °C

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WB

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Fig. 3. Heat scheme for solid fuel firing bio boiler. Designation: WB — water boiler

tion process in the furnace, only a hot water generation occurs through utilization of heat power of the high temperature exhaust gases from the turbo- or diesel electric generator operated using only a liquid or gaseous fuel.

With the bio boiler available, when wood processing, forest cleaning, etc waste is fired, it would be the most useful to have the bio boiler and the electric generator driving gas turbine engine (GTE) integrated. In GTE, an air heater placed downstream of the bio boiler combustion zone, i. e. before combustion products entry into the heat exchange water heating surface functions as a conventional combustor (Fig. 4).

Fig. 4. Heat scheme for cogeneration plant water boiler + GTE driven electric generator. Designation: WB — water boiler; T — turbine; C — compressor; EG — electric generator; AH — air heater; B — bypass

In updated bio boiler, an air heat exchanger is mounted between the furnace and the water heat exchanger inside tubes thereof the compressed air supplies.

The air is heated in the heat exchanger by heat bleed-off from the combustion products at the boiler outlet and enters the air turbine that drives the compressor and AC electric generator. After expansion in the turbine, the air has temperature over 750 °C with pressure close to the atmospheric one; with such parameters, it supplies inside the boiler space. The total amount of air in an updated design is, certainly, 2.5-3.0 times higher that in an existing design since air is required not only to ensure the fuel combustion but, also, to operate the turbine that drives the electric generator.

Prior to entering inside of the boiler, all the air is separated into two parts:

— the 1st part supplies the boiler furnace via burner; the air at the same time is supplied along the same channels and holes and in the same amount as in an existing design, i. e. with the excess air coefficient being 1.3-1.5, therefore, all the operation burn-out process is not frustrated, it can be but better since a higher temperature of the air supplied (not 20 °C but higher 750 °C) will result in a more complete burning-out of the woods waste;

— the 2nd part enters, in the amount necessary for the turbine, the space at the air heat exchanger inlet, i. e. actually behind the woods fuel burn-out zone. In an existing design ( with the excess air coefficient being 1.3-1.5), the temperature within this zone behind the boiler furnace will be around 900 °C, i. e. the temperature of the combustion products will be roughly 900 °C higher that of the air that enters the combustion zone. Since the excess air coefficient in an updated design is the same as in an existing design, then the working media heating in the combustion zone will be maintained and will be the same 900 °C. This means the temperature behind the combustion zone will be over 1600 °C. To ensure the same conditions for the water heat exchanger operation as in the existing design, it would be necessary to decrease this temperature. This is achieved by solution of the combustion products in the air that supplies from the turbine exhaust (with the pressure being roughly 0.11 MPa, the temperature being over 750 °C). In this case, the temperature at the air heater exit, i. e. before the water heat exchanger is the determining parameter.

As follows from the above comparison, the updated design maintains all the conditions required for:

• a normal proceeding of the operation processes in the boiler furnace;

• target water temperatures at the water heat exchanger exit;

• a normal operation of the strength elements of the boiler design.

It should be emphasized that for putting the cogeneration with solid fuel firing within the boiler furnace into effect, the air to rotate the turbocom-pressor turbine and to ensure its acceptability must have temperature not lower 800-850 °C. Given the temperature of the combustion products that heat air is around 1100-1300 °C, the average temperature of the material of the heat transfer tubes in the air heater is on the level of 950-1050 °C.

Table 1

Technical-economical calculations for comparison of water bio-boiler (WBB) in the existing design

and cogeneration plant (WBB+ TEG)

Versions Notes

1 — Water bio-

Name Symbols Dimension boiler (WBB)

1 2 3 2, 3 — WBB + turboelectro-generator

Initial data

1.1 Fuel - density - humidity - cost Qi Wf cf t/m3 % Euro/t 0.65 8-10 53.3 —

1.2 Design heat power Nd KW 120 —

1.3 Total consumed power by electric equipment Na KW 1.95 — —

1.4 Annual operation hours T 1y h/year 5040 —

1.5 Supplied heat power Ety MW/year 317 —

1.6 Annual fuel consumption for heat power production Bfy t/year 67.50

1.7 Cost of heat power Ct Euro/MW 30 —

1.8 Cost of consumed electric power Се Euro/MW 70 —

1.9 Air flow across WBB Gc kg/s 140 196 —

1.10 Air pressure ratio Pc — 1.1 3.0 5.0

1.11 Temperature downstream of flame tube Tto2 K/h 1373/1100 1573/1300 —

1.12 Van (compressor) efficiency EfJC % 65 (65) —

1.13 Electric engine efficiency Effd % 65 — —

1.14 Turbine efficiency Eft % — 70 —

1.15 Regeneration ratio for recuperative airheater (AH) Er % 0 80 —

1.16 Total pressure losses AE % 12.68 16.99 —

1.17 Bypass ratio for AH ¥ % — 20 —

1.18 Heat efficiency for WBB Efe % 65

1.19 Electric turbogenerator efficiency Eetg % — 30 —

Economic values

2.1 Average annual heat power N KW 62.90 62.84 61.67 N=Ey / Ty

2.2 Supplied electric power Ne KW — 8.15 13.19 —

2.3 Fuel specific consumption at heat power generation bfi kg/KWh 0.213 bft = Bft / Ety

2.4 TEG efficiency Пе % — 30 —

2.5 Fuel specific consumption at electric power generation bfe kg/KWh — 0.462 bfe = bfi nt / Пе

2.6 Fuel annual consumption for production of: - heat power - electric power - total Bfi Bfe Bfy t/year t/year t/year 6.75 6.75 67.46 18.98 86.44 66.20 30.71 96.41 Bfi = f Ty N Bfe = bfe Ty Ne Bfy = Bft + Bfe

2.7 Expenses on: - fuel - electric power purchase - total f Cfe СУ Euro/year Euro/year Euro/year 3598 688 4286 4607 4607 5166 5166 Cy = cf By Cfe = Се Na Cy = Cfy + Cfe

2.8 Gains from selling of: - heat power - electric power - total С Ce CZy Euro/year Euro/year Euro/year 9510 9510 9501 2875 12376 9325 4653 13978 С = C, Ty N, Ce = ce Ty Ne

2.9 Profits: - absolute - relative AC Ac Euro/year 5224 1 7769 1.487 8812 1.687 AC = Cy — Cy AC = AC / AC

Hence, the cogeneration plant operation reliability can be ensured when it is applied to heat the air of the ceramic heat exchanger.

The heat surfaces were developed using the ceramic tubes with the outer finning of various

geometry (Fig. 5) which technology was elaborated for the gas industry, automotive, etc GTEs [1—3].

To update the bio-power boilers and to convert them into cogeneration low-toxic and highly effective plants for combined thermal and electric

powers, it is necessary also to have turboelectro-generator plants of power not less 5-10 % of the boiler's heat power at the production of electric power to meet only its own standby systems and equipment needs. At the electric power supply to the outside electric power consumers, the cogen-eration plant's electric power can amount to 3050 % of the boiler's heat power.

Technical-economical comparison

As an illustration, the technical-economical calculation results are summarized for the boiler bio-plant prior to and after updating (Table 1).

Summary

ft As follows from the results of the economic

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t effectiveness with use of WBB to produce com-s bined heat and electric power, profitableness of

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^ the plant is 1.5-1.7 time that of the base-line

| version. This is naturally referred to the current

| price ratio for these kinds of power.

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References

1. Soudarev A. V., Souryaninov A. A., Souda-rev B. V. Compact tubular ceramic heat exchangers for micro gas turbine engines // Advanced Computational Methods in Heat Transfer VIII, 2004 WIT Press. P. 253-261.

2. Soudarev A. V., Tikhoplav V. Yu. Research-Engineering Center "Ceramic Heat Engines (NIZ KTD) at the Research-Technological Institute for Power Engineering (NITI EM)". / Edited by Mark van Roode, Mattison K. Ferber, David W. Richer-son, Gas Turbine Design and Test Experience, Progress in Ceramic Gas Turbine Development, ASME PRESS, Vol. 1, ch. 32, PP. 683-707, 2002.

3. Soudarev A. V., Grishaev V. V. Structural Alumino-Boron-Nitride Ceramic Material for Gas Turbine Engine Application. / Edited by Mark van Roode, Mattison K. Ferber, David W. Richerson, Gas Turbine Design and Test Experience, Progress in Ceramic Gas Turbine Development, ASME PRESS. Vol. 2, ch. 13, PP. 245-257, 2003.