1 CO-FIRING

1 CO-FIRING

In this introductive chapter, the technique of co-firing will be explained by framing the subject.

After a brief explanation about co-firing and its driving forces, the state of art will be analyzed and different co-combustion options will be presented.

The chapter concludes with an overview on the future perspectives for this technique.

1.1 Definitions and reasons

Like the name suggests, co-firing is the simultaneous combustion of two, or more, different fuels, with one of these usually considered as the primary fuel and the others consequently considered as secondary, or auxiliary, fuels, taking place in a plant originally designed for the combustion of the sole primary fuel.

Nowadays the term co-firing is more specifically used as a definition for co-combustion of a fossil fuel, especially coal, with renewable energy resources such as biomass, from different origins like dedicated energy crops, industrial or agricultural by-products or residues and from urban green and forest maintenance, and with fictitious non biological renewable energy sources like municipal solid waste (MSW) and other industrial or urban wastes (e.g.: black liquor and tires). A more accurate definition of the actual co- firing practice, according to the International Energy Agency [1], would therefore be:

"Partial substitution of coal as a main fuel in a utility boiler with biomass or waste"

The mayor driving forces that push towards an increasingly larger amount of co-firing percentage in the existing plant for electricity or heat production are summarized by the following:

• International protocols for greenhouse gases (GHG) emissions reduction [2];

• Renewable energy sources (RES) share targets achievement [3];

• Other pollutants emissions reduction (e.g. SO

).

Those objectives can be fulfilled through co-combustion technologies, mostly because

the combustion of biomass is widely accepted as CO

-neutral: the amount of carbon

dioxide released into the atmosphere by the biomass combustion, unlike that from fossil

fuel combustion, is part of a short-term biological cycle in which the CO

from the

combustion is lately stored in the new growing biomass within one or few year (see Figure 1.1).

Figure 1.1: The short-term CO

cycle of biomass.

Furthermore, co-firing has become an interesting solution to meet the aforementioned goals, thanks to other significant advantages that contribute to make this practice also technically and economically feasible:

• Many types of fuels were tested with essentially every type of boiler technology available (T-fired, Wall fired, Cyclone and Fluidized Bed);

• Installation costs are low, whether the overall plant capacity remains unaltered, ranging from 50-300 $/kW of installed capacity;

• Biomass-coal conversion efficiency ranges from 30-38 %, thus easily exceeding efficiencies in dedicated biomass systems;

• The subsequently emerging bio-fuel collecting, transporting and handling chain will benefit local, rural areas where the biomass is produced, turning it from a waste to a valuable by-product [4].

In synthesis, co-combustion of biomass and coal blends allows to benefit from biomass

advantages listed above, avoiding meanwhile some of the problem encountered in

dedicated biomass combustion systems. On the other hand, retrofitting coal-fired boiler

for co-firing with biomass arises some problems related to heat exchange and flue-gas treatment devices, that will be discussed later on.

1.2 State of the art

1.2.1 Biomass co-firing routes

A typical classification method that can be applied to subdivide the various co-firing plants is based on the way the biomass, or a product from biomass utilization such as syngas or steam, is blended, fired with coal or mixed prior expansion in the steam turbine. At the present state there are three basic co-firing options for biomass materials in coal-fired boilers, and all of them have been demonstrated on the industrial scale [5- 7]:

Direct co-firing: combustion of biomass and coal takes place in the very same boiler.

Indirect co-firing: biomass undergoes thermo-chemical conversion (typically gasification) before being fired in the boiler with the coal.

Parallel co-firing: biomass is fired in a dedicated boiler which usually share the steam system with a coal boiler.

1.2.1.1 Direct co-firing

Direct co-firing is the least expensive, most straightforward, and most commonly applied approach. The biomass and coal are burned in the coal boiler furnace and, depending principally on the biomass characteristics, three different ways of plant configuration are possible (see routes 1, 2 and 3 in Figure 1.2):

1. blending biomass and coal on the fuel pile, using the same mills and burners;

2. milling coal and biomass with different mills lines and mixing the fuels before the same shared burner;

3. milling and burning the biomass with different mills lines and burners of that of

the coal.

Figure 1.2: Direct co-firing configurations.

Co-firing less than 2-3 % (thermal basis) biomass or waste may be possible using existing pulverizers by previously blending the fuel (route 1). Co-firing greater than this amount of alternative fuel will require a separate feed system to act in parallel with the coal-feed system (route 2 or 3). The blended-feed system requires lower capital costs but the separate-feed system has the advantage of the individual control of the biomass, and coal, feed rates.

Other type of boilers, such as Cyclone boilers and Fluidized Bed Combustion (FBC) boilers, can assure higher biomass share (up to 15 % on a thermal basis for Cyclone and over 50 % for FBC) thanks to their higher flexibility and lower biomass pre-treatment requirements in terms of humidity and particle distribution [8].

An European example of direct co-firing plant running at commercial operation is the 124

MW

power plant at St. Andrä in Austria (see Figure 1.3), where bark and wood chips

are burned on two grates, reaching a biomass share of 3 wt% [9].

1.2.1.2 Indirect co-firing

It is possible to install a biomass gasifier to convert the solid bio-fuel into a fuel gas, which can be burned in the coal boiler furnace (Figure 1.4). This approach can offer a high degree of fuel flexibility and the fuel gas can be cleaned prior to combustion to minimize the impact of the syngas combustion product on the performance and integrity of the boiler.

Nevertheless, injection of typical LHV gas obtained from gasification of biomass or waste (2.5-5 MJ/Nm

) will also have some negative effects although less pronounced than in the case of direct co-firing. These negative effect will, however, occur in the gasifier and can be better managed there than in the boiler itself.

Figure 1.4: Indirect co-firing.

A proven working plant that uses this kind of technology, reaching up to 15 %, on an energy basis, of biomass/waste co-firing share, is the THERMIE demonstration project in Lathi, Finland (Figure 1.5), a CHP plant that provides 167 MW

[10].

Another way of co-firing a bio-waste originated gas in a coal-fired boiler is to inject the

gas from the gasifier, without applying any kind of cleaning nor cooling operations, as a

reburning fuel. An industrial scale example of this technical solution is the BioCoComb

demonstration project in Zeltweg, Austria (Figure 1.6). The plant generates 137 MW

by

burning hard coal in a T-fired PF furnace, and gasified biomass fuel gas (3 % on an

energy basis) for reburning purposes [11].

Figure 1.5: Lathi, THERMIE demonstration project.

Figure 1.6: Zeltweg, BioCoComb demonstration project.

1.2.1.3 Parallel co-firing

It is also possible to install a completely separate biomass boiler and integrate its steam system with a coal-fired boiler one. This solution is, unsurprisingly, the less adopted mainly because it requires high investments to install the boiler and all the auxiliary systems, which makes it a more attractive solution for new plants rather than for retrofitting existing ones. The advantages in this technical solution is the possibility to design the biomass boiler specifically for the biomass itself, and to avoid fly ash

contamination in the coal-fired boiler from the biomass fly ash, thus utilizing both types, the first for the concrete industry and the second as a fertilizer. In addition, in the biomass boiler the steam can be produced using the biomass at lower temperature and then can be sent back to the coal boiler for temperature enhancement or mixed directly with the steam flow from the coal boiler prior expanding in the steam turbine. The latter solution, especially, allows the steam from the bio-boiler to be expanded in a more efficient turbine without affecting the overall steam quality considerably. The only two known power plants operating with this technological configuration are the Avedøre unit

#2 CHP (16 % heat on total steam production) firing straw and wood chips pellets [12], and the Ensted #3 (see Figure 1.7) CHP unit (6 % heat on total steam production) firing straw and wood chips as well; both plants are located in Denmark [13].

Figure 1.7: Ensted, parallel co-firing CHP plant.

Figure 1.8 shows an overview about the main co-firing routes adopted nowadays.

Routes 4 and 5 represent respectively indirect and parallel co-firing, while route 6 represent torrefaction, which is to be explained later on in this section.

Figure 1.8: The main co-firing routes.

1.2.1.4 Retrofitting boilers for 100% biomass combustion

In addition to the explained co-firing routes, recently some projects for retrofitting coal- fired boilers to rely solely on biomass fuel have been proposed, studied and applied.

Two examples are the Drax power plant in England [14] and the Rodenhuize "Max

Green" demonstration project in Belgium [15]. Both power plant are currently co-firing biomass with pulverized coal but some modifications to the plant layout are to be made in order to successfully run the power plant with biomass only.

1.2.2 Worldwide distribution

At the present, there are 236 known plants that are practicing, or have practiced for experimental or demonstration purposes, co-combustion of coal and biomass or waste.

As Figure 1.9 shows, Finland is the leading country, with around 33 % of the total co- firing plants, followed by United States, Germany, United Kingdom and Sweden [1].

Figure 1.9: Worldwide co-firing plants.

What emerges by deepening into the available information about co-firing worldwide is, as shown in Figure 1.10, PF is the most used combustion technology (51 %).

Explanations to this preponderance of the PF, on the other combustion technologies, are

to find in simple techno-economic considerations: retrofitting existent PF coal-fired boiler

by adapting the burners or the mills to co-fire with bio-fuel or RDF is by far the most cost

effective solution.

Figure 1.10: Boilers used and type of co-firing.

Fluidized bed combustion, both Bubbling bed (BFB) and Circulating bed (CFB), is the second more common plant configuration and represent mostly newer plants built specifically for co-firing purposes (e.g. the Finnish experience with multy-fuel FBC for large scale paper mills and CHP plants) [16].

Finally the least used technology, even though proven to present good fuel flexibility and low pre-treatment efforts requirements, is the grate (or stoker) combustion, mostly used in the United States together with cyclone boilers [8].

Besides, it can also be noted from Figure 1.10 that the most spread co-firing route nowadays, covering alone 96 % of the total plants, is the direct one. This is due to the fact that the direct co-combustion route is the more proven and, together with the parallel route, the only one commercially available for industrial scale. Despite its reliability, parallel co-firing in not as widely used as the direct route (only 2 power plant running at present state) because it needs to be accurately planned and presents the highest investment costs, thus involving political and economical large scale planning (see the Danish experience).

As for the indirect co-firing routes, this solution is still immature and exists in the form of

few internationally financed demonstration project.

Table 1.1: Summary of worldwide co-firing plants.

Continent Country Total plants PF BFB CFB BFB,CFB GRATE Direct Indirect Parallel

Asia Indonesia 2 2 2

Taiwan 1 1 1

Thailand 1 1 1

Oceania Australia 8 8 8

Europe Finland 78 11 44 13 6 4 77 1

Germany 26 18 1 4 3 26

U.K. 18 16 2 16 2

Sweden 15 3 3 7 2 15

Denmark 9 4 1 4 7 2

Italy 7 6 1 7

Netherlands 6 6 5 1

Austria 5 1 3 1 4 1

Hungary 5 4 1 5

Spain 2 2 2

Belgium 5 5 5

Norway 1 1 1

North America Canada 7 7 7

U.S.A. 40 29 1 5 5 39 1

Total 236 120 48 37 11 20 228 6 2

1.2.3 Logistic chains

As well as for fossil fuel supplies, also bio-fuels present several logistic options that must been taken into account, as they can affect severely the final energy production costs, both under economic and energetic point of view, especially in a CO

-saving driven scenario.

Exploiting biomass resources available in the surroundings of a thermal power plant, or any other combustion based utility, is logistically very different than importing a

considerable amount of bio-fuel via the international trade channels.

The process of producing energy from biomass is a chain of interlinking stages, which

are mutually dependent. Actually there are six main points that need to be carefully

predicted and arranged in order to achieve a successful, economic and CO

-reductive

bio-energy plant (the last two are discussed respectively in section 1.3.2 and 1.2.1):

Resource and harvest: what type of bio-fuel is available (energy crops, forestry residues, agricultural residues, etcetera), how much is available and how the biomass is collected, with which frequency and at which rate.

Storage: how the storage is realized (piles, silos, open air, indoor, and so on).

Transport: how the fuel is moved from the production point to its final destination.

Pre-treatment: whether the biomass needs sizing, drying or other

mechanical/thermo-chemical treatments that aim to improve its quality as a fuel.

Final conversion: which technology is demanded to convert the fuel energy in a more convenient energy carrier.

It is also to be noted that these steps are not always all necessary, and neither is the order in which they were presented. To recall the comparison made before, the plant with available biomass in the surroundings would virtually have the chain shown in Figure 1.11, A., while a plant that needs to import its fuel from oversea would be represented by Figure 1.11, B..

Figure 1.11: Examples of biomass chains.

1.2.3.1 Resource and harvest

When talking about bio-fuels, even some type of urban waste, or other type of human activities non biological residues, can be classified as "biomass" or, more properly, as a waste material convertible to fuel supply (e.g. tires), exploitable for energy purposes.

The type of biomass, as first step, is crucial as it practically influences the whole logistic chain. In order to choose the proper harvesting technique, it is important to be aware of (to name a few) biomass output temporal windows, costs and characteristics but also the characteristics of the land where the harvest is supposed to take place. For instance, producing biomass from forest residues needs to account for type and size of tree, type of terrain (flat or sleep), rate of working (production capacity) of the individual items of equipment and of the overall system.

1.2.3.2 Storage

Storing bio-fuels is always a necessary step to be considered. Long-term storage is necessary if there is a time gap between production and utilization, which is typical, for example, for seasonal feedstock's like energy crops or agricultural residues.

Furthermore, fuel needs to be stored at the combustion plant in ordered to assure fail- safe operation. On the other hand, short-term storage with an automatic system is needed for feeding fuel to the combustion plant. The fuel handling between long-term and daily storage facilities is usually demanded to cranes or wheel loaders.

The simplest way of storing biomass is to pile it. When applying this method several aspects have to be taken into account. First of all it is crucial to prevent or reduce heat development that can cause self-ignition in certain cases. Secondly, dry-matter losses, changes in moisture content and health risks (fungi and bacteria) should be taken into consideration.

Other storage practices adopted at the present state are outdoor covered/roofed storage, bunkers and silos. It is clear that the choice of storage layout is strongly

influenced both by space and resource availability and in turn influences the adoption of

some early fuel pre-treatment, such as chipping or drying [17].

1.2.3.3 Transport

Biomass fuels have a relatively low energy density compared to fossil fuel. This means that bio-fuels face higher transportation costs. Hence, transport distances must be kept as short as possible to minimize total transport costs.

Different means of transport can be used, depending on distances and type of biomass fuel:

tractors with trailers;

trucks;

trains; and ships.

Tractors with trailers are normally used for short-distance transport of unchipped thinning residues, forest woodchips and different kind of herbaceous biomass fuels. Average distances are around 10 km.

Trucks are adopted for medium-/long-distance transport of all kind of woody bio-fuels (e.g. logs, thinning residues, woodchips, sawdust and bark) and herbaceous bio-fuels (bulk or baled). Trucks are frequently used for distance ranges between 20 and 150 km.

Rail transport is used for logs, bundles and industrial by-products in bulk form. Different wagons are available depending on the fuel to be transported. Herbaceous biomass fuels train transportation is of minor relevance.

Transportation of biomass by ship could be reasonable in case of long-distance, large- scale biomass trade. It is especially relevant for pellets, as pellets have now became an internationally traded product. However, also woodchips and bales or bundles could be transported by this means [17].

Economic calculations about long-distance, large-scale international bio-energy transport have been made [18]. According to this study (see Figure 1.12) the

international water transport of woody biomass fuels accounts for only a small part of the

total fuel costs, whereas international rail transportation is considerably more expensive.

Figure 1.12: Comparison between ship and train transportation cost for woody biomass.

1.3 Future perspectives 1.3.1 Potential overview

According to IEA, the total renewable and waste primary energy supply consumption accounted for 10 % of the total energy resources utilization in 2008 (see Figure 1.13) [19].

Figure 1.13: Evolution from 1971 to 2008 of the world primary energy supply.

What emerges from further analysis is that, of this 10 %, biomass is mainly used as a

traditional fuel (e.g. fuelwood, dong) accounting for 68,1 %, while the rest 31.9 % is used

for advanced energy carriers production (fuels, electricity and high grade heat) [20].

Table 1.2: Biomass potential and use distribution between regions, EJ/year.

BM potential

North America

Latin

America Asia Africa Europe

Middle East

Former

USSR World

Woody BM 12,8 5,9 7,7 5,4 4,0 0,4 5,4 41,6

Energy crops 4,1 12,1 1,1 13,9 2,6 0,0 3,6 37,4

Straw 2,2 1,7 9,9 0,9 1,6 0,2 0,7 17,2

Other 0,8 1,8 2,9 1,2 0,7 0,1 0,3 7,8

Total Potential 19,9 21,5 21,6 21,4 8,9 0,7 10,0 104,0

Use 3,1 2,6 23,2 8,3 2,0 0,0 0,5 39,7

Use/Potential [%] 15,6 12,1 107,4 38,8 22,5 0,0 5,0 38,2

Besides, as Table 1.2 shows, the actual ratio between biomass potential and use, at present state is 38.2 %, with Asia (107.4 %) and Africa (38.8 %) being the leading zones, mostly due to traditional bio-fuel utilization. In the other world zones this ratio is lower than the average, indicating a consistent development margin still available [21].

Furthermore, several studies on bio-energy [22-24] estimate, from now to 2050, a potential biomass use margin spanning from a few hundred to more than 1000 EJ/year.

All these data suggest biomass to play an important role in the future energy production scenario and co-firing technologies will have a crucial part in triggering the transition from fossil to renewable fuels in the next decades. Even though such a long-term predictions are not that reliable, due to multiple factors influence on the unfolding of events, it is possible to consider these predictions to be truthful if a proper infrastructure and market systems will see a development as substantial as the technologies are experiencing at the moment.

1.3.2 Novel pre-treatment technologies

In order to increase the biomass fraction for co-combustion application, there has been an increasing interest in the thermal pre-treatment of biomass materials in the recent years [16,25]. These pre-treatments include:

pelletisation;

pyrolysis;

torrefaction.

1.3.2.1 Pelletisation

Pelletisation can be defined as drying and pressing of biomass under high pressure to produce cylindrical pieces of compressed and extruded biomass. Pellets have a smaller volume and a higher volumetric density compared to wood chips and are hence more efficient to store, ship and convert to energy. Pelletisation non only produces a uniform and stable fuel, but also the amount of dust produced is minimized. Another advantage of pelletisation is that it enables free flowing which facilitates material handling and rate of flow control, important for loading and unloading operations.

Pelletising is a commercial practice since many years. Finland and Sweden are the two leading countries in pelletising technology in Europe and other countries are becoming pellets net exporters, helping this type of fuel to turn in to a commodity fuel, with a proper international market.

Figure 1.14: Typical pelletisation process.

1.3.2.2 Pyrolysis

Another technology that is already commercially used is pyrolysis. It consists in a thermal treatment of the biomass at modest temperatures, typically in the range from 400 to 1000°C, with oxygen partial pressures below those usually needed for proper gasification. The products of this process are gas, liquid and solid char, and their relative proportions depend on the pyrolysis method, the characteristic of the biomass and the reaction parameters.

Flash pyrolysis gives high specific bio-oil yields, but the technical efforts needed to

produce pyrolytic oil are still challenging at the present state of development. The liquid

fuel produced during the reaction is emulsified with water, is corrosive and contains

polycyclic aromatic compounds, which makes it unattractive for storage and handling.

Full scale commercialization has not yet been achieved, and therefore there is no practical experience of the long-term operation of industrial scale pyrolysis processes.

1.3.2.3 Torrefaction

Torrefaction is a thermal treatment technology performed at atmospheric pressure in the absence of oxygen. Temperatures between 200 and 300°C are used, which produces a solid uniform product with very low moisture content and high calorific value compared to fresh biomass.

Figure 1.15: ECN's torrefaction process [26].

Even thought torrefaction of biomass is in its infancy, several studies show [26] that it increases the energy density, hydrophobic nature and grindability properties of biomass.

Torrefied biomass typically contains 70 % of it initial weight and 90 % of the original energy content. The moisture uptake of these materials is very limited, varying from 1 % to 6 %.

After this treatment, biomass becomes more porous and fragile as it loses its mechanical

strength, making it easier to grind or pulverize. Thanks to increased hydrophobic nature,

the new material can be stored like coal in open space piles, without incurring in the risk

of high moisture content at the time of utilization. On the other hand this treatment

makes the biomass produce dust and reduces its volumetric density, which makes this

technology particularly attractive if combined with post process pelletisation, to make the

Torrefaction is a growing practice with several reactor technologies (see Table 1.3). One of the largest working plant, producing 36000 t/yr of torrefied biomass, is Torr-Coal production facility in the Netherlands [27]. In addiction, another plant, 60000 t/yr, is in construction by Topell [28].

Table 1.3: Emerging torrefaction technologies [29].

Reactor technologies Parties in advanced stage of Commercialization Rotary drum dryer Torr-Coal (NL)

CDS (UK) BIO3D (FR) Andritz/EBES (DK/AT) Vertical mechanical transport

through temperature zones

Wyssmont, Integro (USA) CMI-NESA (BE) Toroidal Fluidized Bed Topell (NL)

Screw Conveyor ETPC (S)

BTG, FoxCoal, Biolake (NL) Agri-Tech, RTF (USA) Belt Conveyor Stramproy (NL)

New Earth Eco Technology (USA) Vertical Compact Bed ECN (NL)

Thermya (FR)

Buhler (USA)

Fixed Bed Alterna (CA)

Microwave Rotawave (UK)