BIOMASS FOR POWER GENERATION AND COMBINED HEAT & POWER (CHP) The State of the Art

BIOMASS FOR POWER GENERATION AND COMBINED HEAT & POWER (CHP)The State of the Art

01-10-2010 Wilson Jordão Filho

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Biomass for Power Generat ion and Combined Heat & Power (CHP) The S tate of the Ar t ( tu tor ia l paper) W I L S O N J O R D Ã O F I L H O

Abstract

This paper is focused the use of biomass for producing heat and electricity in industrial and power plants solely or in combined heat & power schemes.

Biomass is becoming a typical agribusiness in the last decades stimulated by the search for new sources of energy (heat and electricity) and fuels. A wide an complex framework for biomass can be found today involving different raw materials, processes and byproducts delivered to market. The figure which follows illustrates that. However evidence is clear for the concentration of investments in energy and biofuels.

Special concerns are made to woody and bagasse biomass which can be converted into useful forms of energy (solid, liquid, or gaseous fuels) as well as useful products (polymers, bio-plastics, char, pellets, and acids) at a biorefinery. A biorefinery is a facility that uses biomass conversion technologies to convert biomass into fuels, power, and more recently to value-added chemicals. Each biorefinery process yields different amounts and types of co-products and by-products. Co-products describe the useful and marketable by-products, other than energy, that are produced simultaneously during biomass conversion. Many of today’s co-products may have traditionally been defined as waste or by-products. Biorefinery process technologies include thermochemical (gasification, pyrolysis), biochemical (fermentation), or chemical (chemical synthesis) pathways.

Sugar cane bagasse which goes straight to combustion to produce bioelectricity and bioheating, is recently investigated for further processing to obtain cellulosic ethanol. That may change trends of steam power generation with bagasse as it may come raw material for additional biofuel as ethanol.

An overview of the state of art in biomass for heat and power is presented with the main attitudes in EU and US. It also includes a summary for biomass applications as well as a description of the main woody and bagasse biomasses defining peculiar schemes for plants.

Also the emerging concept of biorefineries is presented consisting on the co-production of a spectrum of bio-based products (food, feed, materials, chemicals) and energy (fuels, power, heat) from biomass.

A biorefinery is a facility that integrates biomass conversion processes and equipment to produce fuels, power, and value-added chemicals from biomass. The biorefinery concept is analogous to today’s petroleum refinery, which produces multiple fuels and products from petroleum.

B i o m a s s f o r P o w e r G e n e r a t i o n a n d C H P - T h e S t a t e o f t h e A r t

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1 - Introduction

Biomass for heat and power holds a large potential as a source of renewable energy and greenhouse gas emission reductions, but this potential is only being realized at a slow pace today. A global effort to remove barriers is needed to accelerate this way of low carbon development. To ensure such a development does not come at the expense of a sustainable use of natural resources, reinforced environmental frameworks and legislative processes are needed.

Combined Heat and Power (CHP) is the simultaneous generation of two or more forms of energy from a single fuel source. By recycling valuable heat from the combustion process, CHP results in far greater efficiencies than centralized power generation. The recovered thermal energy may be used for industrial processes, space heating, and refrigeration or space cooling through an absorption chiller. CHP is considered the most viable and economical use of distributed generation (DG) when implemented at or near the point of use. CHP offers many benefits to energy consumers choosing to adopt this technology including:

Modern equipment is environmentally friendly

Uses available heat (thermal energy) to improve fuel-use efficiency

Diversifies electric supplies to the end-user and enhances energy security

On-site generation alleviates geographical transmission and distribution loadconstraints

In US the US Environmental Protection Agency CHP Partnership Program was established as a voluntary program seeking to reduce the environmental impact of power generation by promoting the use of CHP. The partnership works closely with energy users, the CHP industry, state and local governments, and other clean energy stakeholders to facilitate the development of new projects and to promote their environmental and economic benefits. (www.epa.gov/chp/index.html)

Also, the US Department of Energy Distributed Energy Program supports cost-effective research and development aimed at lowering costs, reducing emissions, and improving reliability and performance to expand opportunities for the installation of distributed energy equipment today and in the future. (www.eere.energy.gov/de)

In Europe, according to the report "Biomass for heat and power - Opportunity and Economics" from European Climate Foundation and others, most biomass energy applications reduce carbon dioxide emissions between 55% to 98% compared to fossil fuels, even when transported long distances, as long as the biomass production does not cause any land-use change. Also it emphasized that contrary to common belief, there is a large inherent cost improvement potential in biomass-generated power and heat as volumes and experience grow – 15% to 40% compared to today. Capturing these cost improvements will be challenging but would make biomass cost competitive with coal and gas in a broad range of applications at a carbon dioxide price of 30 to 50 EUR per ton in 2020.

The expected growth is likely to require successfully mobilizing biomass demand in both the energy industry and in residential and commercial heating applications. European biomass supply for heat and power could be doubled through 2020 in an aggressive mobilization

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scenario, releasing approximately 1000 TWh of domestically produced primary renewable energy – but this mobilization is not happening today.

A much faster global biomass supply mobilization than today is needed to avoid supply shortage in the transition phase – and the resulting negative consequences on food, feed and the forestry industry – even though there is technically enough unused land and forest/agriculture residues available globally to meet demand without compromising other stakeholder needs.Policy makers must take action to unlock biomass value chains and ensure some level of profitability, companies will need to make investments for the long-term and assume some risks, and sustainability aspects will need to be carefully managed.

However many energy experts say that Combined Heat and Power (CHP) have being mostly applicable to cogeneration systems for producing both heat and electric power. They are generally mature and really can reduce emissions of CO2 compared to other fossil-fuel technologies. But they say there are two problems with large adoption of CHP:

1) Fossil-fuel-based CHP cannot be a long-term solution on climate or energy because they still burn fossil fuels, and therefore still emit a lot of CO2. Reducing that by 20% or even 50% is not enough; there is a need to take steps that over the next 30-40 years will bring fossil CO2 emissions close to 0.

2) Efficiency claims for CHP systems are frequently greatly overstated. Heat is lower-quality energy than electricity, and only at high temperatures does it become close to comparable. Efficiency claims for CHP systems that use high-temperature heat are not so far off, but CHP systems that make use of low-temperature waste heat have much lower thermodynamic efficiencies than usually claimed.

The inflated efficiency claims often lead to assertions that CHP is the "largest" or one of the largest potential solutions. But the number of applications that require high-temperature heat where CHP efficiency really is quite high are limited. And the modest efficiency gains with low-temperature waste heat use, which could be much more widely applied, don't lead to very much improvement in overall energy use. The combining of heat and power production in CHP systems can reduce the fossil CO2 emissions in some percent, but much more than that is needed in coming decades.

Emphasis on CHP technology for applications in Europe is increasing and in particular for biomass fueling. Another growing movement in the EU and elsewhere is one towards decentralized production of electricity. Various arguments underlie this move such as, higher CHP potential and lower grid system power losses. In any event decentralization is better suited to biomass fuel resources since by its very nature biomass is often created over disperse areas and transport costs are relatively high due to the low energy density compared to fossil fuels. The vision for the EU is to increase decentralized electricity production from the current level of 9% to 20% by 2020.


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Following is a typical biomass CHT plant for 5 MW installed capacity which illustrates the energy & power & heat schemes from Erneuerbare Energien in Internet.

Typical biomass CHT plant

2 - Biomass classification and bioproducts market

Following the US report "Biomass feedstock for bioenergy and bioproducts industry ... etc.", an overall classification made for biomass resource is presented on the figure below (with adaptations made by the author for the sugarcane bagasse):

Types of biomass for heat & power production

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From the above figure one can see that it is largely adopted the woody products from forestry as well as sugar cane bagasse as the current collected and used biomass products. With the world engagement on the low carbon economics all forms of biomass can be contemplated for generating heat and power, solely or combined.

Biomass, processing and bioproducts may comprise a large variety of alternatives. These alternatives can be increased by new advances in biotechnology which may provide access to other wastes and residues as raw materials.

Regarding bioproducts market it is presented a simplified diagram which follows presents a summary of possible applications of biomass inserted on the larger scope of biofuels as a raw material. The graph does not include yet the sugar cane bagasse that goes straight to combustion to produce bioelectricity and bioheating, and more recently through new processing the cellulosic ethanol.

Biomass processing and bioproducts

Source: http://www.extension.org/pages/Woody_Biomass_Properties

Special concerns is made to woody biomass which can be converted into useful forms of energy (solid, liquid, or gaseous fuels) as well as useful products (polymers, bio-plastics, char, pellets, and acids) at a biorefinery. A biorefinery is a facility that uses biomass conversion technologies to convert biomass into fuels, power, and value-added chemicals. Each biorefinery process yields different amounts and types of co-products and by-products. Co-products


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describe the useful and marketable by-products, other than energy, that are produced simultaneously during biomass conversion. Many of today’s co-products may have traditionally been defined as waste or by-products. Biorefinery process technologies include thermochemical (gasification, pyrolysis), biochemical (fermentation), or chemical (chemical synthesis) pathways.

3 - Conversion process for biofuel, bioheating and bioelectricity

Biomass resources include primary, secondary, and tertiary sources of biomass. Primary biomass resources are produced directly by photosynthesis and are taken directly from the land. They include perennial short-rotation woody crops and herbaceous crops, the seeds of oil crops, and residues resulting from the harvesting of agricultural crops and forest trees (e.g., wheat straw, corn stover, and the tops, limbs, and bark from trees).

Secondary biomass resources result from the processing of primary biomass resources either physically (e.g., the production of sawdust in mills), chemically (e.g., black liquor from pulping processes), or biologically (e.g., manure production by animals). Tertiary biomass resources are post-consumer residue streams including animal fats and greases, used vegetable oils, packaging wastes, and construction and demolition debris. There are various conversion technologies that can convert biomass resources into power, heat, and fuels for potential use in tropical countries.

Following it is presented a frame for the biomass power plants for heat and electricity. The simplified diagram which follows presents a summary of possible applications of biomass inserted on the larger scope of biofuels as a raw material. The graph does not include yet the sugar cane bagasse that goes straight to combustion to produce bioelectricity and bioheating, and more recently through new processing the cellulosic ethanol.

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Biomass energy systems to generate electricity are able through methods such as:

Gasification

Pyrolysis

Anaerobic digestion

Modular

According to EIA there are alternatives for direct and indirect combustion in biomass power plants. Direct combustion burn the biomass fuel directly in boilers that supply steam for turbines connected to the-electric generators. In the indirect combustion (biomass gasification), biomass is converted into a gas - methane - that can then fuel steam turbines steam generators, combustion turbines, combined cycle technologies or fuel cells. The primary benefit of biomass gasification, compared to direct combustion, is that extracted gasses can be used in a variety of power plant configurations. For converting biomass into energy there are direct and indirect combustion.

Direct Combustion methods: The most common way of converting biomass to heat energy is through straight combustion, and this accounts for around 90% of all energy attained from biomass. However there is a number of different technologies available that can be used for biomass combustion divided into two groups: fixed bed combustion systems and fluidized bed combustion systems.

Fixed Bed Combustion - There are two types of fixed bed combustion: underfeed stokers and grate firings. With these methods of combustion air is primarily supplied through the grate from below, and initial combustion of solid fuel takes place on the grate and some gasification occurs. This allows for secondary combustion in another chamber over the first one where secondary air is added.

Fluidized Bed Combustion Systems - Fluidized bed furnaces operate in quite a different manner from fixed bed furnaces and have a number of advantages associated with them. There are two main types of fluidized bed furnace, Bubbling Fluidized Bed (BFB) and Circulating Fluidized Bed (CFB).

Bubbling Fluidized Bed (BFB) Furnaces - The fundamental principle of a BFB furnace is that the fuel is dropped down a chute from above into the combustion chamber where a bed, usually of silica sand, sits on top of a nozzle distributor plate, through which air is fed into the chamber with a velocity of between 1 and 2.5m/s. The bed normally has a temperature of between 800 and 900°C and the sand accounts for about 98% of the mixture, with the fuel then making up a small fraction of the fuel and bed material. There are two main advantages in terms of fuel size and type over more traditional fixed bed systems. Firstly they can cope with fuel of varying particle size and moisture content with little problem, and secondly they can burn mixtures of different fuel types such as wood and straw. BFB’s are only a practical option with larger plants with a nominal boiler capacity greater than 10 MWth.

Circulating Fluidized Bed (CFB) Furnaces - If the air velocity is increased to 5-10m/s then a CFB system can be achieved, where the sand is carried upwards by the flue gases and a more thorough mixing of the bed material and fuel takes place. The sand is then separated from the gas in a hot cyclone or U beam separator at the top of the furnace and fed back into the


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combustion chamber where the whole process begins again. CFB’s deliver very stable combustion conditions but it comes at a cost. Due to their larger size compared to other combustion methods the cost is relatively high and there are problems involved with fuel size, which must be very small, and the difficulties involved in running them at partial load. All of this means that they are really only feasible for plants with a boiler capacity of over about 30MWth.

Indirect combustion methods: The indirect combustion methods have the capability to convert raw biomass into gases, liquid fuels, or solid fuels so that it can be used directly in the power plants for generating electricity. Biomass usually contains carbohydrates, which are composition of oxygen, carbon, and hydrogen and that can be broke down into different types of chemicals from which some are very useful fuels.

Biomass gasification means incomplete combustion of biomass resulting in production of combustible gases consisting of Carbon monoxide (CO), Hydrogen (H2) and traces of Methane (CH4). This gas. can be used to run internal combustion engines (both compression and spark ignition), can be used as substitute for furnace oil in direct heat applications and can be used to produce, in an economically viable way, methanol.

The advantages of the biomass gasification system are:

The power generation efficiency of this system is higher than that of the traditional systems even at a small capacity.

The product gas can be used as a substitute for a fossil fuel.

The product gas can be converted to a raw material with a high added value.

The figure illustrates a gasification system by Takuma - Japan.

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Schematic view for a biomass gasification system

4 - Focus on biomass for power generation and CHP (Combined Heat and Power)

Combined Heat and Power is a very efficient way of utilizing the energy released from steam. Via the turbine for power and by extracting heat from the uncondensed, condensed or indirectly heated water of the steam cycle.

Rather than producing as much power as possible, the approach is combined to ensure that power is produced at the correct voltage, and heat at the temperature required to be used by the developer at their discretion. This method can be adopted across the range of 1 MWe to 20 MWe modules. Electrical output will reduce in accordance with the heat required, the lower the grade of heat needed the more power you will produce. As situations change, society can progress with more effective and greener options being available as we all improve our ethics and green credentials

Following information released by OECD and IEA in 2007 one can say the following about CHP projects:

Processes – Biomass combustion is a carbon-free process because the resulting CO2 was previously captured by the plants being combusted. At present, biomass co-firing in modern coal power plants with efficiencies up to 45% is the most cost-effective biomass use for power generation. Due to feedstock availability issues, dedicated biomass plants for combined heat & power (CHP), are typically of smaller size and


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lower electrical efficiency compared to coal plants (30%-34% using dry biomass, and around 22% for municipal solid waste). In cogeneration mode the total efficiency may reach 85%-90%. Biomass integrated gasification in gas-turbine plants (BIG/GT) is not yet commercial, but integrated gasification combined cycles (IGCC) using black-liquor (a by-product from the pulp & paper industry) are already in use. Anaerobic digestion to produce biogas is expanding in small, off-grid applications.

Typical costs – Costs of biomass power vary widely. Co-firing in coal power plants requires limited incremental investment ($50-$250/kW) and the electricity cost may be competitive (US$ 20/MWh) if local feedstock is available at low cost (no transportation). For biomass typical cost of $3-$3.5/GJ, the electricity cost may exceed $30-$50/MWh. Due to their small size, dedicated biomass power plants are more expensive ($1500-$3000/kW) than coal plants. Electricity costs in cogeneration mode range from $40 to $90/MWh. Electricity cost from new gasification plants is around $100-$130/MWh, but with significant reduction potential in the future.

Status – Resources availability and favorable policies are enabling biomass power to expand in Northern Europe (mostly co-generation from wood residues) as well as in US and countries producing sugar cane bagasse like Brazil. Proliferation of small projects, including digesters for off-grid applications, is recorded in both OECD and emerging economies. Global biomass electricity capacity is in the range of 47 GW, with 2–3 GW added in 2005. Associated investment accounted for 7% of total investment in renewable energy capacity in 2005 ($38 billion excluding large hydro).

Potential & barriers – In the short term, co-firing remains the most cost-effective use of biomass for power generation. In the mid-long term, BIG/GT (which includes gasification) plants and biorefineries could expand significantly. IEA projections suggest that the biomass share in electricity production may increase from the current 1.3% to some 3%-5% by 2050. This is a small contribution compared to the estimated total biomass potential (10%-20% of primary energy supply by 2050), but biomass are also used for heat generation and to produce fuels for transport. Main barriers remain in production and transportation costs; conversion efficiency; feedstock availability (competition with industry and biofuels for feedstock, and with food and fiber production for arable land); lack of supply logistics; risks associated with intensive farming (fertilizers, chemicals, biodiversity).

Feedstock & processes – Biomass resources include agricultural residues; animal manure; wood wastes from forestry and industry; residues from food and paper industries; municipal green wastes; sewage sludge; dedicated energy crops such as short-rotation (3-15 years) coppice (eucalyptus, poplar, willow), grasses (Miscanthus), sugar crops (sugar cane, beet, sorghum), starch crops (corn, wheat) and oil crops (soy, sunflower, oilseed rape, iatropha, palm oil). Organic wastes and residues have been the major biomass sources so far, but energy crops are gaining importance and market share. With re-planting, biomass combustion is a carbon-neutral process as the CO2 emitted has previously been absorbed by the plants from the atmosphere. Residues, wastes, bagasse are primarily used for heat & power generation.

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5 - Survey on biomass applications for heat and power

Most biomass-fired steam turbine plants are located at industrial sites that have a steady supply of biomass available. These include factories that make sugar and/or ethanol from sugarcane at pulp and paper mills. At these sites, waste heat from the steam turbine can be recovered and used for meeting industrial heat needs—further enhancing the economic attractiveness of such plants.

Referred to as combined heat and power (CHP) facilities (also called cogeneration facilities), these facilities are highly resource efficient and they provide increased levels of energy services per unit of biomass consumed compared to facilities that generate power only. Following a summary of the paper for biomass applications in EU is presented. The paper is referred to an approach for updating information based on a complete survey in the subject focusing the power applications of biomass.

(http://www.nri.org/projects/biomass/conference_papers/recent_advances_in_biomass_energy_in_europe.pdf)

Typical capacity/efficiency/resource data for biomass power systems

System Power KW Energy efficiency

%

Biomassdemand tons/yr

Comments

Small down draft gasifier/IC engine

10 15 74 High operation & maintenance,and/or low availability, low cost

Large down draft gasifier/IC engine

100 25 442 High operation & maintenance,and/or low availability, low cost

Stirling Engine 35 20 177 Potential good availability, under development, high cost

Steam Engine 100 6 1840

Good reliability, high cost

Indirect-fired gas turbine 200 20 1104 Not available commercially

Pyrolysis/IC engine 300 28 1183 Under development

Rankine Organic Cycle 1000 18 6133 Commercial

Updraft gasifier/IC engine

2000 28 7886 Commercial

Fixed grate or fluid bed boiler/steam turbine

2000 18 12270 Commercial

Fluidized bed (BIG/CC) – 8,000 + 28 29710 Demonstrated

Biomass for heat: The market uptake and use of biomass as heating fuel in Europe varies greatly between different countries, due to various, climate, cultural, historical and infrastructure


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issues. Heat-only application of biomass is more suited to rural areas without gas grid supplies. Certain counties, notably Austria and the Scandinavia states, have far above the average use of biomass and it is considered that there is an undeveloped potential for biomass fuelled heating systems in other countries. In UK there is a growing lobby for effective governmental support of biomass derived or ‘green’ heat. In Europe commercial activities in this technology are largely for domestic or institutional heating purposes that do not apply for Sri Lanka and neighboring countries, so it is not easy to match types of equipment. Traditional European equipment suppliers to the plantation industries have now largely vanished and equipment needs are increasingly met from within the region. Any equipment sourced from the EU-15 is likely to be of higher cost, although this may not be the case for suppliers in the new accession countries some of which have extensive forestry related industries with related equipment suppliers.

Biomass for power: There is a large number of different routes for power generation from biomass at various scales of power output. These scales of output can be classified as: around 100 kW, around 2 MW, and above 2MW.

5.1 - Power Systems 100 kW Range

Small steam engine - This technology is technically robust and reliable but more suited to about the 100 kW scale. However an efficiency of only 6% would be typical. Cost was high but a complete steam engine system manufactured in a developing country could be very much cheaper to one manufactured in Europe.

Small-scale gasification with an IC engine - Downdraft gasifiers are available for use at the 100 kW scale or smaller and this is a technology that has essentially been available commercially since the 19th century and pre-dates the widespread use of liquid fossil fuels. There has been resurgent interest in times of fuel shortage and more recently as aviable alternative form of renewable energy. For improved maintenance and operation, a dual-fuel compression ignition engine using 10% diesel may be preferred but spark-ignition engines will operate without auxiliary fuel. Diesel engines are more tolerant of the tars and alkali metals deposition in the combustion chamber that can markedly reduce engine reliability. With either engine type, the gas must be cleaned to attain reliable operation and long engine life. Using such gasifier systems at this scale offers an energy conversion efficiency of around 20%. Maintenance requirements are more demanding than for steam systems.

Essentially the choice for smaller scale systems at around 100kW until recently has been either:

i. low efficiency steam systems coupled to either steam turbine or for smaller scale to steam engines, with high reliability and low maintenance or

ii. for higher efficiency, gasifier based systems using producer gas for fuelling of internal combustion engines but with lower availability and higher operational and maintenance requirements.

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In order to expand and develop the options for power from biomass at this scale there has been a significant range of continuing development work underway. Other options that may now be considered include:

Pyrolysis with an IC engine - Various processes exist for pyrolysis of wood to produce bio-oil and, after suitable treatment, this can be burnt in a compression- ignition engine. In the flash pyrolysis process, yields of bio-oil as high as 80% can be achieved. For a 300 kW system, the capital cost is very high. However it has a good energy conversion efficiency at about 28%, and there may be substantial economies of scale. This technology is still under development.

Indirectly-fired gas turbine under development – Use of indirectly-fired gas turbines eliminates problems of corrosion and erosion of the turbine blades encountered with gasifier systems. It offers a potentially high availability and reliability. In the basic system wood-fuel is burnt in a furnace and the hot combustion gases pass through a heat exchanger to provide clean hot air. This clean hot air from the heat exchanger takes the place of the combustion chamber in a directly-fired gas turbine. Benefits are expected through use of automotive technology for the turbine unit.

Stirling Engine- Stirling engine systems have been the subject of some impressive development work over the past 5-10 years by the Danish Technical University in association with Bios Energiesysteme GMBH, Austria on biomass fuelled stirling engines. This has resulted in trial operation of a 35kW power output system for over 7,000 hours. Also a pilot plant with a 75 kWel stirling engine has been put into operation in autumn 2003 and has been operated for more than 2,000 hours (until March 2004)6. These Stirling engines are modern technology versions of an old engine design that involves externally fired reciprocating engines. They and have the potential for high availability with good conversion efficiency and can be used in small-scale CHP applications. Small series manufacture of such engines is now underway and is expected to considerable reduce engine cost.

The 25-50 KW unit of biomass plant as an residential boiler by Biomatic


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5.2 - Power Systems from 100 KW to up 2 MW

2 MW Steam Turbine - These systems represent the most readily available technology at this scale and this technology is well established with proven reliability. The boiler systems use various types of horizontal grate or fluidized beds in more advanced designs for larger scale. Fuel to power energy conversion efficiencies are at this scale are maximum 18%.

2 MW Updraft gasifier with an IC engine - This technology has scope for gains in efficiency and lower capital costs compared to steam technology at this scale. Complex gas clean-up trains are required and are subject to further development, but essentially this is old and proven technology. It could use large marine type diesel engines operating in dual fuel mode or large modified spark ignition natural gas engines. Manufacturers have quoted power energy conversion efficiencies of around 28%. Maintenance and operational labor requirements will probably be more demanding than for steam systems. At 2 MW, a technical choice of the gasification process system could be made between either updraft fixed bed gasification or fluid bed gasifiers. However, at this output, cost considerations favor the choice of updraft fixed bed gasification.

The 100-750 KW unit of biomass plant as an industrial boiler by Swebo

An economic analysis of these systems at 2MW concluded that the price of electricity generated by the steam boiler/steam turbine and updraft gasifier/ IC engine systems is similar. The steam technology is well established with little scope for cost reduction and process improvement. Gasifier power systems using fixed bed updraft gasifier technology were selected as they have more potential for improvement, particularly in terms of capital cost and efficiency.

Organic Rankine Cycle (ORC)- It is relevant to mention here recent commercial developments of Austrian technology with the Organic Rankine Cycle (ORC) system6. This is the same engine cycle as used by conventional steam turbines but with a closed system organic fluid instead of water as the working fluid. It is presently being run for a CHP application with a net electric conversion efficiency of 18% on a 1 MW electricity power

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output. This available commercially in ranges of 0.4 to 1.5 MW electricity output and has good power-turndown characteristics that are important for flexible operation in CHP applications.

5.3 - Power Systems over 2MW

Above 2 MW the choice of systems is essentially between steam boilers coupled to steam turbines, or gasifiers coupled to gas turbines. As the scale increases, more advanced combustion and gasifer technology is appropriate and fluid bed systems are utilized. This would apply for around 5MW and above.

Fluidized Bed Gasifiers - Fluidized bed systems offer larger unit capacity and economies of scale compared to conventional grate boilers and fixed bed updraft gasifiers. Fluid bed combustion has become widely used over the past 20-30 years and is now standard technology from such companies as Foster Wheeler, Lurgi and others. For power applications these are normally linked to steam turbines. Potentially more efficient systems result from coupling gasifiers to direct fired gas turbines as will be described below.

Steam Boiler and Steam Turbine Technology - Steam boiler and steam turbine technology has always been recognized as offering low maintenance and high availability. A number of systems have been implemented in Europe for combustion of biomass feedstocks. In UK there are now five such plants operating on agricultural wastes such as chicken litter and straw and ranging in capacity from 10 MW to 38.5 MW and with total capacity of 110 MW8. Recently plans were announced by SemCorp Utilities for a wood-burning 30MW capacity plant costing ₤60 million to be installed as part of a energy supply complex providing CHP for industrial activities at Wilton, Teeside9.

Fluidized Bed Gasification and Gas Turbines - Fluid bed gasification and gas turbine technology has the potential for higher conversion efficiencies and better economics especially for the relatively low capacity power systems that are envisaged for biomass at below around 50 MW. This gasifier/gas turbine technology has been proposed with variations including combinations with waste heat boilers and integrated steam turbines –known as biomass integrated gasification combined cycle (BIG/CC).

As described, there has been a particular focus in Europe on advanced and capital intensive technology for biomass-derived energy. The EC and various national renewable energy promotion schemes have supported this. These policies have also placed a much greater emphasis on power generation from biomass, rather than thermal energy or CHP systems.


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The 2000 Kw unit of biomass plant as an by GE

6 - Co-Firing biomass on Existing Coal thermal power plants

Co-firing or co-combustion of biomass in existing coal-fired thermal power plants is now a trend in market to comply with the sustainable development through a low carbon economy.

Current biomass methods for Co-Combustion on these plants are:

Pre-mixing - When the proportion of biofuel is rather low, it can be fed together with coal to the coal mills and then be burned together in the burners. In principle, this is the simplest option and involves the smallest investments. On the other hand, this technology also carries the highest risk of malfunction of fuel feeding systems. Premixing is associated with direct co-firing.

Direct injection together - The second option involves separate handling, metering and comminuting the biofuel and injection into the pulverized fuel upstream of the burners or directly at the burner. This option requires the installation of a number of biofuel transport pipes across the boiler front, which may already be congested. It may also prove to be more difficult to control and to maintain the burner operating characteristics over the normal boiler load curve. Direct injection is associated with direct co-firing.

Separate Burning of biomass and coal - The third option involves the separate handling and comminuting the biofuel with combustion through a number of dedicated burners. This approach represents the highest capital cost option, but involves the least risk to normal boiler operation. This method is associated with in-direct co-firing or parallel co-firing.

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Re-burn of biomass in upper furnace - The final option involves the use of biofuel as a re-burn fuel for NOx emissions control, i.e. the combustion of biofuel is a specially-designed re-burn system located in the upper furnace. This system is still in the development stage, although some small-scale tests have been carried out. This method is associated with direct or indirect co-firing.

Over the last years there has been a major shift with energy policy to promote use of biomass as fuel with the realization that co-firing of biomass on existing coal-fired power stations is a technically viable option. Earlier trials had given inconclusive evidence about the impact on ash formation, in-plant depositions, etc and also there was a general reluctance from in the power generators to go in this direction since it created additional operational problems.

The level of technology being applied varies. Some simply make addition of the biomass feedstock to the pre-milled coal for subsequent injection to the boiler in the pulverized fuel burners. Other systems use separate biomass fuel preparation and additional burners, and in other applications a biomass gasification system is installed and the resulting producer gas fed into the co-fired boiler.

One of the longest running schemes for biomass co-firing is the Lahti, Finland CHP plant11 which is rated for outputs of 216 MW electricity and 240 MW heat. Standard fuelling is a mixture of gas and coal-fired , but locally available biofuels and refuse fuels substitute about 15% of the fossil fuels burned in the main boiler, This process using Foster Wheeler biomass gasification demonstrates on a commercial scale the direct gasification of wet biofuel and the use of hot, raw and very low calorific gas directly in the existing design coal-fired boiler.

7 - Highlights for the sugar cane bagasse

In tropical regions sugar cane represents a major crop. Because of the increasing demand for sugar in the last century, large areas in the tropical and subtropical countries all around the world were allocated to sugar cane crops. Low level of maintenance and good productivity made sugar cane an attractive crop for farmers in these regions. Most of the high sucrose varieties are fully ripened and ready for harvest when they are 10 to 15 months old. Accordingly, sugar cane bagasse is very much an annually renewable resource. Sugar cane is a major commercially grown agricultural crop in Brazil, India and many countries in Africa. It is one of the plants having the highest bioconversion efficiency of capture of sunlight through photosynthesis. Crops gives 60 to 80 ton per hectare of crop on annually renewable basis.

Brazil is increasingly turning to alternative biomass power generation in order to increase electricity supply and reduce its dependence on hydropower. According to Frost & Sullivan, biomass power represents approximately 4.1% of the total installed capacity in Brazil and most biomass cogeneration is based on sugarcane bagasse. In 2005 the Brazilian potential for sugar cane biomass used to obtain energy cogeneration was 385, 106 tons of sugar cane crushed, during 200 days/season. The Brazilian sugar and alcohol sector envisages to market electricity surplus to the national grid in order to produce marketable amounts of electricity. There is a clear trend toward the implementation of boilers with higher steam-production capacity. New boilers and steam turbines with higher capacity and efficiency would substantially increase the


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electricity surplus the plants could sell. By 2007, sugarcane bagasse cogeneration accounted for 3.03% of the total Brazilian energy matrix.

Statistics indicate that when crushing one ton (1,000 kg) of sugar cane, there is 250 - 300 kg of bagasse with heat value of 1850 kcal/kg. This bagasse is the best raw material for steam boiler to produce electricity and steam. According to records of Brazilian experts on magazine Biomassa e Energia (vol. 2; n.3, 2005).

Biomass resourcePotential power

(MW)

Energy generated (MWH X106)

Bagasse only (50%) 4,363 20.94Bagasse and crop residue minimum 5,871 28.18

Bagasse and crop residue maximum 8,638 41.46

The Bagasse is an agricultural waste from the sugar & ethanol industry. A typical chemical analysis of bagasse might be (on a washed and dried basis) reveals: cellulose 45–55%; hemicellulose 20–25%; lignin 18–24%; ash 1–4%; waxes <1%.

During the production of sugar cane ethanol, a large stream of bagasse is released. Bagasse is a fibrous, cellulose rich biomass material that is most often burned in co-generation electric power plants on site to run operations at the mill. Excess is sold as green and renewable electricity to nearby cities and industries. To cope with the bagasse residue, many of Brazil's sugarcane mills have even installed out-of-date, inefficient blast furnaces so they would not be left with excess biomass, for which they would otherwise have to pay for disposal. But today things are being changed.

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8 - Biorefineries

A biorefinery is a facility that integrates biomass conversion processes and equipment to produce fuels, power, heat, and value-added chemicals from biomass. The biorefinery concept is analogous to today's petroleum refinery.

The IEA Bioenergy Task on Biorefineries has defined biorefining as the sustainable processing of biomass into a spectrum of bio-based products (food, feed, chemicals, materials) and bioenergy (biofuels, power and/or heat) .

By producing multiple products, a biorefinery takes advantage of the various components in biomass and their intermediates therefore maximizing the value derived from the biomass feedstock. It can generate electricity and process heat, through combined heat and power (CHP) technology, for its own use and perhaps enough for sale of electricity to the local utility. The high-value products increase profitability, the high-volume fuel helps meet energy needs, and the power production helps to lower energy costs and reduce greenhouse gas emissions from traditional power plant facilities. Although some facilities exist that can be called bio-refineries, the bio-refinery has yet to be fully realized. Future biorefineries may play a major role in producing chemicals and materials that are traditionally produced from petroleum.

The figure below shows a schematic diagram of a biorefinery.

.Source: http://www.york.ac.uk/res/gcg/GCG/images/Biorefinery.JPG


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According to ICB, 2009, Europe seems to be losing global battle for biorefineries, as State funding for the development of biorefineries is tiny in this region, compared with the vast sums thrown at their development in the US and China. Europe has generated a lot of knowledge on technologies for biorefineries and their raw materials, but much of this knowledge is now being transferred abroad because of the lack of development projects in Europe. In the EU's latest round of R&D programs), the biggest program will be the development of technologies for the entire value chain. They will extend from biomass production, logistics and pretreatment, to biomass conversion into bio-based energy, fuels and chemicals.

The biorefinery program in the United States started from concerns about the rising cost of crude oil and a desire to reduce dependence on foreign oil for U.S. transportation fuels. The U.S. program focus is primarily on transportation fuels, with about equal efforts on gasification and Fischer–Tropsch reforming, and cellulose hydrolysis with fermentation to ethanol. This emphasis is partially dictated by availability of raw materials. About two-thirds of the biomass available in the United States is dedicated annual crops and unused stalks and leaves of cereal grain agriculture. The program emphasis could be also a response to challenges of climate change.

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Author:

Wilson Jordão Filho

Civil Engineer and Business Consultant

Rio de Janeiro, Brazil

The Sustainability Crusade Observatory

[email protected]

[email protected]

Documents

BIOMASS FOR POWER GENERATION AND COMBINED HEAT & POWER (CHP) The State of the Art