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Converting CFB boilers from coal to biomass combustion VGB PowerTech 4 l 2017

Authors

Kurzfassung

Umbau von Wirbelschichtkesseln von Kohlefeuerung auf 100 % Biomassefeuerung

Mit der Forderung, den Anteil erneuerbarer Energien in der Tschechischen Republik zu er-höhen, rückt die Nutzung von Biomasse in der Stromerzeugung verstärkt in den Fokus. Ein Schema für die nationale finanzielle Förderung in der Tschechischen Republik wurde entwi-ckelt. ČEZ, a.s. hat die Umstellung von Wirbel-schichtkesseln auf Biomassefeuerung geprüft, wobei zwischen 25 und 100 % Biomasseeinsatz möglich sein sollen. Die technischen Aspekte der Umstellung betreffen neben der Feuerung auch Fragen der Logistik in den Bereichen Brenn-stoffantransport, Lagerung sowie Aschehand-habung und -abtransport. Die schon umgesetz-ten Umstellungen haben einen positiven finan-ziellen Beitrag für die betreffenden Anlagen in der derzeitigen Marktsituation bewirkt. l

Converting CFB boilers from coal to 100 % biomass combustionJan Žižka, Ludvík Pavlík and Hynek Lang

Ing. Jan Žižka Coal and Hydro Plants Engineering DirectorIng. Ludvík PavlíkHead of Boilers and FGD Engineering Department ČEZ Inženýring, s.r.o. (ČEZ Group)Ing. Hynek LangHead of Mechanical Engineering Department ČEZ, a.s. (ČEZ Group), Czech Republic

With the requirement to increase the share of renewables in the Czech Republic there is an aim as much as possible to uti-lise biomass for electricity generation. Scheme of subsidy was created with the different levels according to type of bio-mass and type of combustion. This differ-entiation is reflected in a level of subsidy, when the biomass with other utilisation with co-combustion was subsidised in level of 1,000 CZK per MWh (about 37 EUR per MWh) as minimum subsidy and biomass planted for combustion with 100 % biomass combustion in level of 2,800 CZK per MWh (about 103 EUR per MWh) as maximum subsidy. ČEZ, a.s. evaluated this opportunity and decided to refurbish circulated fluidised bed boilers at Power Plant Poříčí and Power Plant Hodonín. Refurbishments enable to increase the amount of biomass to be burnt in the combustion chamber from 25 % to 100 % of heat value in fuel. These projects were focused to increase capacity to unload biomass from trucks, delivering biomass from storage yards to storage bunkers close to the boiler and delivering biomass from bunkers to the combustion chamber. Addi-tionally, in the Poříčí power plant there was a change in the technology of ash cooling and ash transportation. In Power Plant Poříčí this project enabled to increase the amount of biomass burned from 100,000 t/year to 170,000 t/year (2015), alternatively in Power Plant Ho-donín from 160,000 t/year to 270,000 t/year (2015). This reconstruction means for both sites receiving of additional revenues which support positive financial results of power plants in market situation with low electricity price. Burning biomass planted for combustion had impact to availability of these boilers. Cleaning of superheater of the boiler had to be done after several months of operation and hence was decided not to use only this biomass in Power Plant Hodonín. Forest residues burning in Power Plant Poříčí led to intensive abrasion in the area of the bot-tom belt of the membrane wall, and hence it was decided to cover it by metal coating material – Castolin 595. Each site has an opportunity to decide to operate boilers with fuel (biomass or lignite) which gener-ates the highest gross margin from electric-ity and heat supply.

The state regulatory authority changed the subsidy scheme last year with the aim to support parallel combustion and co-com-bustion of biomass only with the precondi-tion of electricity generation in high effi-ciency cogeneration.

Introduction

The Czech Republic has a renewable ener-gy target with a share of 13 % in the year 2020. One of the supported area to achieve this goal is biomass utilisation in electricity generation. CEZ, a.s. has different kinds of boilers in its portfolio. Several tests and studies were carried out to understand limits and bottle-necks in the technology of pulverised coal and circulated fluidised bed boilers. There were found the limits of biomass co-com-bustion for both kinds of boilers with any technology modernisation. Limitation for PF was 5 to 10 % of the biomass in heating value and limitation for CFCB was 20 to 25 % of the biomass in heating value.On the other hand not only technological limitations was important to take into ac-count, but also subsidy scheme legislation to achieve highest gross margin from elec-tricity generation from biomass. The first steps to prepare biomass co-firing in the form of wood chips were done in 2003 when the government introduced an en-dowment system of so-called “green bo-nus” for a MWh of electricity generated from renewable sources, in this case from biomass. Now the subsidy scheme is quite complex. Subsidies are at different finan-cial levels, according to type of biomass and type of combustion. First type (1) of biomass is defined as biomass with other utilisation (e.g. saw dust), the second type (2) of biomass is defined as biomass with difficult other utilisation (e.g. wood chips and straw) and the third type (3) of bio-mass sis biomass planted for energy utilisa-tion , e.g. energy crops). Type of combus-tion is defined as 100 % biomass combus-tion (O), parallel combustion (P) and co-combustion (S).This differentiation is reflected in the level of the subsidy, when the biomass with oth-er utilisation with co-combustion was sub-sidised in level of 1,000 CZK per MWh (about 37 EUR per MWh) as minimum and biomass planted for combustion with

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VGB PowerTech 4 l 2017 Converting CFB boilers from coal to biomass combustion

100 % biomass combustion in level of 2,800 CZK per MWh (about 103 EUR per MWh) as maximum, see F i g u r e 1 . After the evaluation how to achieve the best utilisation of biomass in CEZ, a.s. port-folio fleet, it was defined , that reconstruc-tion of two CFCB from 25 % to 100 % bio-mass combustion is the most effective way to increase electricity generation from bio-mass.

Reconstruction of CFCB in the Poříčí power plant

The Poříčí (EPO) power plant was built in 1950s to utilise black coal from the nearby mines for electricity production. In 1970s, it was reconstructed so that it could supply local towns and villages with as much as 200 MWt of heat in a combined cycle for power generation and heating. At present its installed electric capacity is 165 MWe and it provides the district heating network with 1,6 TJ of heat a year on average.From 1996 to 1998 the original technology of the boilers was replaced by two new flu-idised bed boilers, with original design from Foster Wheeler with a steam output of 250 t/h, with the options for fuel: lignite, hard coal, wood chips and agro-pellets (original design of boiler FK 7 see F i g -u r e   2 ) . In spring 2004 both the fluidised bed boil-ers in the Porící power plant were tested for the first time with positive results, but the covering was limited to a mixture with coal containing 20 to 25 % of the heat content of biomass. It was caused by the capacity of fuel transport lines and boiler input pro-files, which were not able to transport enough fuel with lower heat content into the combustion chamber. The originally designed coal had a heat value between 16 and 18 MJ/kg and the real heat content of  wood chips is based between 8 and 11 MJ/kg.

In the following years new improvements of the fuel lines for safe but limited co-fir-ing were made. However, gradual legisla-tive changes degraded co-combustion to the lowest level of support and thus made it economically ineffective. Unfortunately, all discussions about the lack of economic and ecological arguments failed. In 2008 new studies focused on the conversion of one of the boilers into a boiler for burning 100 % of biomass.The project was prepared with following design principles and conditions:

– The real heat content of wood chips, which is the most suitable fuel for the current plant location, as far as its suffi-cient supply and its price is concerned, is between 8 and 11 MJ/kg. The typical characteristics of wood chips (mostly for-est residue) for conversion project were:

– particle size: 50 x 50 x 50 mm, excep-tional length 150 mm,

– moisture: 35 to 60 %, – ash/soil/sand: 1 to 15 %.

– There is no dumping ground for 2 to 4 week´s operations, including a neces-sary handling area near the boiler. The only place meeting these demands is lo-cated at the distance of approximately 600 metres, and therefore it was neces-sary to solve the problem of fuel trans-port from that location to the nearest boiler No. 7.

– The boiler was not constructed for the combustion of fuel with the highest con-tent of alkali salt in the ash. As the super-heaters in the second duct could be clogged with that kind of ashes, the pro-ject submission did not take into account using these sorts of biomass (especially straw and hay) in the mixture with wood chips in a bigger rate than 20 % (heat content).

– The fuel inputs connected to ash chutes from cyclones in the rear wall of the com-bustion chamber had to be retained. The only part for adding new chutes for bio-mass with sufficient output was the front wall of the chamber. However, it meant

Year

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Wh

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3,500

3,000

2,500

2,000

1,500

1,000

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Note: P 1 – 3, S 1 – 3 from 2016 only in high efficiency cogeneration

010203P1P2P3S1S2S3

2012 2013 2014 2015 2016

27 CZK = 1 EUR

Fig. 1. Subsidies for biomass combustion in the Czech Republic; 100 % biomass combustion (O), parallel combustion (P) and co-combustion (S).

Fig. 2. Original design of the Poříčí (EPO) power plant CFB boilers.

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Converting CFB boilers from coal to biomass combustion VGB PowerTech 4 l 2017

to remove the system of bottom ash out-lets and coolers, which were originally installed there.

– The target project output of the boiler fir-ing 100 % of wood was set to 70 to 80 % of the original rated output (for coal). The main reason for that, considering the increase of the specific volume of flue gases, was to avoid the capacity prob-lems or even necessary reconstruction of the second duct and the following com-ponents (ESP, ID fan etc.).

From the technological point of view the whole project comprises the following parts:

– A new assembly of the conveyor line from the biomass dump, which included 2 push floor receiving bunkers for tracks, star screener, magnetic separator, weigh-ing machine (belt type) and 550 m long covered belt conveyor up to the boiler biomass bunker with transport capacity 300 m3/hour and line transport time 8 minutes (see F i g u r e 3 ).

– New daily biomass bunker (about 600 m3) next to the boiler with 4 extract-ing screw conveyors and following chutes to the combustion chamber.

– Reconstruction of the bottom ash extrac-tion and cooling system by removing of original cooling system (fluidised bed coolers and screw coolers) and installa-tion of new bottom ash outlets and vi-brating coolers (see F i g u r e 4 ).

Project time schedule was executed with the aim to reconstruct the boiler as fast possible:

– 2009 to 2011Approving the project and a detailed preparation of contractual documents with the main supplier

– 27.9.2011Signing the contract

– 1.9. to 30.11. 2012Trial operation with final guarantee test, including at least one week´s operation using only coal and then a week´s opera-tion using only wood chips together with the overall load rate as well as dynamic change tests,

During guarantee and the trial operation period, no serious problems occurred and everything was according to the expecta-tions. The tests verified the proper func-tions of the boiler in the load range from 30 to 100 % for coal and from 30 to 75 % of biomass. No troubles with “switching” be-tween both fuels appeared. The diagram of fuel consumption, steam load and calorific value is in F i g u r e 5 .Regarding to emissions, there were achieved expected results during biomass combustion: SO2 emissions almost zero and NOx/dust emissions mostly below fu-ture limits, see F i g u r e 6 .After several years of operation there is no evidence of any troubles with salty ash de-

Fig. 3. New conveyor line of the Poříčí (EPO) power plant for 100 % biomass combustion.

Biofuel tankHydraulic push floorScrew conveyorCombustion chamber

Pneumatic plate gate

Dosing double plate gate

Vibration sieve

Expansion jointRotary feeder

Hand plate gatePneumatic plate gate

Biofuel feeding chutesSpiral conveyor

Vibration cooling conveyor

Fig. 4. New bottom ash extraction and cooling system of the Poříčí (EPO) power plant for 100 % biomass combustion.

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Coalconsumption(tons/day)

Biomassconsumption(tons/day)

Coal calorificvalue (Qir)(MJ/kg)

Biomasscalorific value(Qir)(MJ/kg)

Fig. 5. Fuel consumption, steam load and calorific value of the Poříčí (EPO) power plant during commissioning after reconstruction.

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VGB PowerTech 4 l 2017 Converting CFB boilers from coal to biomass combustion

posits on superheaters or any other parts of the chamber or ducts. As a result of slight abrasion, which appeared in January 2013 (due to a high content of sand in the bot-tom ash), the bottom belt part of the mem-brane wall around the combustion cham-ber was covered in June 2013 by arc-spray-ing material Castolin 595, according to EN  14616 (the metal coating, high-alloy iron by chromium and boron, with an

amorphous structure). Thickness of coat-ing was min. 0.7 mm. (see F i g u r e 7 ).The bottom ash during biomass-only firing (forest residue) contains 70 to 80 % of sand and gravel with surplus balances of bottom material between 20 and 50 tonnes per day).In September 2012 the “cold” end of the air preheater (in the length up to 1,000 mm from the tube plate) was covered by spray-

ing ceramic material BG HitCoat 200 as a protection against low temperature corro-sion. Thickness of coating: 0.2 to 0.25 mm and protected area : 700 m2.

Reconstruction of CFCB in Hodonín power plant

The Hodonín (EHO) power plant was con-structed in 1951 to 1957 to utilise lignite from nearby mines for electricity genera-tion. In 1963 it was reconstructed so that it could supply local towns and villages with as much as 250 MWt of heat in a combined cycle for power and heat. At present its in-stalled electric capacity is 105 MWe and the plant provides the district heating network with 1,1 TJ of heat a year on average.From 1994 to 1998 the original technology of the boilers was replaced by two new flu-idised bed boilers, the original design from Austrian Energy and Environment with steam output 170 t/h, with possible fuel – lignite (original design of boiler FK 7 see F i g u r e 8 ) . After several trials of biomass co-combus-tion from 2000 it was decided to prepare a new project in year 2008 to achieve 100 % biomass combustion in boiler FK7. The scope of the project was defined for several technological areas:

– Modernisation of biomass unloading place and screw conveyor, see F i g u r e 9 .

– New hopper with rotary valves, screw conveyors and bunkers for biomass.

– Biomass classifier, see F i g u r e 10 H

600

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Guarantee tests – Emission

Avg. steam output [t/h]SO2 [mg/Nm3]NOx [mg/Nm3]CO [mg/Nm3]Dust [mg/Nm3 – not recalc.]

Fig. 6. SO2, NOx, CO and dust emissions.

Fig. 7. Bottom belt part of the membrane wall after covering by arc-spraying.

Fig. 8. Hodonín (EHO) power plant boiler design.

Fig. 9. Modernised biomass unloading place and screw conveyor at Hodonín (EHO) power plant.

Fig. 10. Biomass classifier at Hodonín (EHO) power plant.

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Converting CFB boilers from coal to biomass combustion VGB PowerTech 4 l 2017

The project time schedule was prepared ac-cording to internal rules to focus to achieve a CAPEX as low as possible:

– 2006 to 2007Approving the project and a detailed preparation of contractual documents with the main supplier

– 5/2008Signing the contract

Fig. 11. Fouling on the superheaters of Hodonín (EHO) power plant.

– 12/2008 – 3/2013Trial operation with final guarantee test, including at least one week´s operation using only coal and then a week´s opera-tion using only wood chips together with the overall load rate as well as dynamic change tests,

– 10/2013Installation of biomass classifier

After several months of using energy crops the availability of boilers decreased due to fouling on superheaters, see F i g u r e 11 . It was decided to reduce the amount of this type of biomass and increase amount of wood chips to eliminate the severity of fouling on superheaters.

Summary

The CFCB technology was evaluated as most suitable for 100 % biomass combus-tion in the power plant portfolio of CEZ, a.s. The CFCB in Power Plant Poříčí is the most powerful CFCB in the Czech Republic with the ability to burn 100 % of biomass. Technically, it is possible to burn 300,000 t/year. So far this limit is not utilised, be-cause the amount of biomass combustion is dependent to subsidy scheme and market conditions. Nevertheless, for both sites 100 % biomass combustion means to “keep sites on money” with low electricity market price. These two reconstruction projects enable CEZ to generate electricity from biomass in the range of 325 MWh in 2015 (117 MWh in Power plant Poříčí and 208 MWh in Pow-er plant Hodonín). This is about 98 % of electricity from biomass of the CEZ, a.s. fleet. About 400,000 t of biomass are com-busted, which is delivered mainly from suppliers from the Czech Republic. l

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