Category Archives: Technologies for Converting Biomass to Useful Energy

COFIRING FB WITH COAL

Additional literature concerned with co-firing FB with coal may be found in papers by Anna — malai et al. (2007a, 2007b), Sweeten et al. (2003), Frazzitta et al. (1999), Arumugam et al. (2005), Annamalai et al. (2003) and Annamalai et al. (2003b). However, most co-firing experi­mental results with FB seem to indicate that NOX emissions did not increase and sometimes even decreased. Therefore, it is hypothesized that FB may also be used in reburning facilities to both partially supplement coal and to reduce NOX emissions.

3.11.1 NO emissions with longer reactor

The comparison of the NO measurements normalized to 3% O2 in the exhaust with that of Frazzitta et al. (1999) is given in Figure 3.36. The measurements are depicted for the co-firing of 80:20 coal:FB blend with a 97% burnt fraction. Under both study conditions, a similar fuel N conversion efficiency of ~ 16% was observed. However, a lower NO value was measured as compared to the previous study, which might be attributed to the longer residence time available in the modified reactor. The residence time in the prior study was ~0.5 s at hot conditions as compared to ~1sin the modified longer reactor. The higher duration available in the longer reactor aids the reduction of NO to further lower values. The increased residence times with modified reactor attributes 17% reduction in the normalized NO emission.

image146

Figure 3.37. Variation of normalized exit NO emissions with fuel nitrogen content (adopted from Arumugam et al., 2005).

Policy-oriented biomass gasification in China

In order to ensure the steady development of Biomass gasification industry, the Chinese govern­ment issued a series of laws, regulations and policy measures to actively promote the development of biomass gasification technology.

4.3.9.1 Guide public awareness

China promulgated the “New Energy Law”, “renewable energy industry catalogue” in 2005, issued the “Renewable Energy Law”, “renewable energy for the generation of regulations”, “renewable energy prices and cost-sharing management pilot scheme” and “Renewable energy development special fund Interim Measures” in 2006. A sound legal framework has been estab­lished through a series of laws and regulations. Then the management of various aspects, such as energy development, investment, production and consumption can be conducted under the protection of the law. The “long-term renewable energy development plan” released in November 2007 developed a specific strategy and goals for sustainable energy development, and clearly proposed taking advantage of biogas and waste gasification technology to improve the proportion of gas used in rural areas. What’s more, it also identified the biomass gasification technology as an important measure to solve the problem of rural waste and industrial organic waste. Since then, the biomass gasification technology has developed into a new stage.

GASIFICATION SYSTEMS

Flexible utilization of biomass or black liquor to useful products by means of gasification is carried out in more or less complex systems, including beside the gasifier a variety of upstream and downstream process components selected depending on the requirement of the application and also use of fuel. Examples of components in gasification systems are illustrated in Figure 6.5.

In the subsequent sections below different gasification as well as downstream process technologies are briefly described.

image302

•Chipping

• Fixed bed

•Tar reforming

• Gas turbine

• Sizing

• Fluidised bed

• Filtration

• Combined heat &

• Torrefaction

• Entrained flow

•Scrubbing

power

• Pyrolysis

• Gas cooling

• Fuel cells

•etc.

1 1

•WGS upgrading •etc.

1___

• Synfuel applications

1

!

I

Upstream processing

Downstream processing

Figure 6.5. Different steps included in a gasification system.

Table 6.2. Comparison of the three fixed bed gasifier technologies

(adapted from Knoef, 2005).

Fuel (wood)

Updraft

Downdraft

Crossdraft

Moisture wet basis [%]

<60

<25

10-20

Dry-ash basis [%]

<25

<6

0.5-1.0

Ash melting temperature [°C]

>1000

>1250

Fuel size [mm]

5-100

20-100

5-20

Application range [MW]

2-30

1-2

Gas exit temperature [°C]

200-400

700

1250

Tar [g/Nm3]

30-150

4.5-5.0

4.0-4.5

Gas LHV [MJ/m3N]

5-6

85-90

75-90

Hot-gas efficiency [%]

90-95

3-4

2-3

Fermentation

The biomass can also be fermented to yield liquid alcohol fuels. If one assumes that 50% biomass is cellulose and all cellulose is converted into fermentable glucose, then with reac­tion C6H12O6 + 3H2O [L] = 3C2H5OH [L] + 3O2 or 1 kg cellulose + 0.3 kg of H2O = 0.77kg

Table 3.1. Biogas yields from different biomass.

Feedstock (manure)

CH4 [Yield, liters/ liter of VS added]

Dairy

0.034-0.118

Beef

0.055-0.082

Swine

0.058-0.111

Wheat stems

0.071

Wheat roots

0.047

Tomato

0.060

Table 3.2. Estimation of growth of biomass with 6%

conversion efficiency.

Solar radiation and biomass growth

1 gallon in m3

0.003785

Feed density [kg/m3]

800

Ethanol density [kg/liter (lb/gallon)]

0.788 (6.6)

HHV ethanol [MJ/kg (BTU/lb)]

167 (76000)

Solar irradiance [W/m2]

1200

Conversion efficiency [%]

6

HHV biomass [kJ/kmole]

2801000

Molecular weight biomass

180

Cellulose% in biomass

50

Conversion to ethanol [kg/g glucose]

0.77

Cellulose to fermentable glucose

1

Biomass production [tonnes/hectare/year]

1459

Glucose [tonnes/hectare per year]

730

Ethanol production [tonnes/hectare]

562

Ethanol production [gallons/hectare]

185525

Ethanol [kg per tonne biomass]

385

Ethanol [gallon per tonne biomass]

127

ethanol + 0.53 kg O2, ethanol production per tonne biomass is 125 gallons per tonne of biomass which is close to the reported theoretical value of 124 gallons/dry tonne of corn grain (Ragauskas). With 1 hectare = 10,000 m2, and solar irradiation of 1200 W/m2 and photosynthesis efficiency of 6%, the biomass production is 1460 tonnes per year per hectare assuming a heat value of

2,800,0 kJ/kmole, and with a molecular weight M = 180 kg/kmole. Comparing it with switch grass whose production is 37 tonnes per hectare per year, the maximum ethanol yield is 48.1 m3 (183,000 gallons) per hectare per year with a heating value (HV) of 7.37 kJ/m3 (28,000 kJ/gallon). With a higher heating value (HHV) of gasoline about 32.9kJ/m3 (125,000kJ/gallon), 1 liter of ethanol is equivalent to 0.22 liter of gasoline. Estimation of biomass growth with 6% conversion efficiency and the yield of ethanol from biomass are tabulated in Table 3.2.

Livestock manure

Livestock manure refers to animal dung and waste which has been used for centuries as a fertilizer for farming. According to the Renewable Energy Development Project (REDP, 2005), nearly 80 billion cubic meters of biogas, which equals 57 million tonnes of standard coal equivalent are generated from farming and the agriculture industry in China (Li et al., 2001).

With the great change of food choice on Chinese people’s tables, livestock production has been expanded to meet increasing demand for meat, egg and dairy products. Due to N2O and CH4 emission from ammonia utilization and untreated manure, as well as CO2 emission from a large reliance on fossil fuels and traditional biomass, and anaerobic digestion as a biological waste treatment, technology to integrate the energy system and agricultural system into a manure management system has now attracted attention from the public. Of special concern in this task is the setting up of a manure-biogas-digestate model and evaluating its greenhouse gas (GHG) emission abatement compared to a reference system. Due to differences in livestock production, energy consumption pattern and agricultural land distribution, household biogas systems and livestock farm-based biogas systems are encouraged strongly in suburban and rural areas in China, respectively (Liu, 2010).

The aims of this chapter are to assess the environmental benefits from a manure treatment per­spective, energy perspective and agricultural perspective ofthe entire biogas system and to analyze whether biogas system implementation is a good choice to achieve sustainability. Three steps are in focus to achieve the research aim: (1) Calculating GHG emission abatement from household biogas systems in rural areas and assessing which contributes to environmental impacts; (2) Assessment of environmental impact made through comparison between energy-environmental biogas systems and energy-ecological biogas systems; (3) Comparisons of these two types of manure-biogas-digestate systems with changes of energy consumption pattern and agricultural land area are then made. Through investigation of a household biogas project in western China and a livestock farm-based biogas project in east, the basic data used for assessing environmental benefits ofthe two systems were collected. In the household biogas system, CO2 emission abate­ment is the largest in biogas substitution but CH4 is produced in large amount from an uncovered anaerobic lagoon after anaerobic digestion (AD). As for livestock farm-based biogas systems, AD selection and manure treatment process design play an important role in the GHG emission mitigation potential, which are based on the main purpose of project implementation. Both energy substitution and agricultural land acceptable capacity are considered as constraint conditions of large-scale biogas system development (Liu, 2010).

COMBUSTION BASICS

5.2.1 Introduction

Combustion can be defined as “an exothermic oxidation process occurring at a relatively high temperature” (Basu, 2001). A simplified stoichiometry of the reaction for a biomass of generic composition is the following (Tillman, 1991):

CpHqOr + (p + q/4 — r/2)O2 ^ p CO2 + 1 /2q H2O + heat (5.11)

O2 used as an oxidant is usually provided through combustion air therefore assuming a standard volume composition of air (79% N2 and 21% O2) so also (3.76 x n) N2 has to be considered, n being the number of moles of oxygen required to complete the combustion of the fuel.

Stoichiometric or ideal combustion for a biomass (with the following composition: p carbon mass fraction; q hydrogen mass fraction, r oxygen mass fraction) can therefore be simplified as:

1 kgCpH? Or + 1/0.233(8q — r + 8/3p)kgairst

^ 11/3pkgCO2 + 9 x qkgH2O + 0.767/0.233(8q — r + 8/3p)kgN2st (5.12)

The combustion of biomass can be described as the steps followed by the biofuel to undergo a complete oxidation. Four steps can be identified (Browne, 1958): drying and heating, solid particle pyrolysis, char oxidation and volatile oxidation (Fig. 5.3) (van Loo and Kopperjan, 2002; He et al., 2006).

A biomass particle that enters a hot combustion chamber is rapidly heated from the outside to its internal core. Heat is transferred from the furnace to the particle outer layer through radiation- convection from flame and flue gases and conduction from the hot biomass bed while conductive heat transfer brings heat inside the particle. The temperature increases abruptly in the outer layer but slowly towards the core of the particle therefore humidity evaporation begins in the external layer and proceeds towards the inside with an evaporation front which is considered to happen conventionally when the layer reaches 105°C. Water evaporates and its expansion cracks the particles producing micro and meso-pores through which steam is ejected. The dried layers increase further their temperature, but cannot burn because oxygen does not reach the inner layers, eventually hemicellulose first and cellulose-lignin after start to decompose thermally. Long polymeric chains are cracked into smaller ones which vaporize or become permanent gases leaving the particles through the same paths followed by steam. This mixture of permanent gases

image203

Figure 5.4. Schematization of the combustion of a solid biomass particle: (a) heating and drying; (b) devolatization; (c) combustion.

(syngas) and vapors (tars) constitute the volatile content of the biomass and when ejected into the combustion chamber reacts with oxygen producing a flaming combustion.

Volatile extraction and combustion continues while the pyrolysis front moves towards the inner core of the particles, the same as the evaporation front did previously, leaving a charred layer on the outside which burns when in contact with oxygen. Char combustion does not produce a flame (glowing combustion) and it is particularly slow since oxidation happens only in the solid-gas boundary layer leaving a layer of insulating ashes which will eventually be removed by mechanical actions of the flue gases combined with gravity to allow oxygen to attack a new fresh layer of char.

This series of events which happen continuously during the combustion of a solid fuel, such as biomass, depends on the temperature reached by a certain area of the particle and on the exposure to oxygen, and are illustrated in Figure 5.4.

According to the temperature gradient the combustion of a solid fuel may then be divided into four steps that occur at different temperatures (Williams etal., 2012):

STEP 1: Below 200°C (heating and drying) biomass absorbs heat in the heating and drying process. The sample loses weight steadily, but it does not ignite.

STEP 2: From 200°C to 280°C (torrefaction) the sample continues to increase its tempera­ture while releasing preliminary volatiles deriving from low temperature decomposition, mainly hemicellulose; gases evolved are still not fully ignitable, however some exothermic reactions happen. The temperature at which the reaction of pyrolysis and oxidation become exothermic can be considered as the definition of the ignition point of wood. There are several studies examining the ignition point (Janssens, 1991; Li and Drysdale, 1992; Masank, 1993; Fangrat et al., 1997; Babrauskas, 2001) that could be considered to happen at a temperature of around 250°C.

STEP 3: From 280°C to 500°C (pyrolysis and volatile combustion): pyrolysis is the ther­mal degradation of a solid in the absence of oxygen; the global pyrolysis combustion model is represented in Figure 5.5. Pyrolysis is a process that is mainly endothermic and happens in two phases:

• the primary reactions are endothermic reactions that transform biomass in GAS (syngas),

CHAR (fixed carbon + ashes) and TAR (condensable gases);

• the secondary reactions are exothermic reactions (cracking) that break tar in syngas and char.

During this phase pyrolysis gases copiously evolve from the particle and when they meet oxygen they burn with a flaming combustion in the gas phase, provided that the mixing with air happens within the lower and upper limits of flammability (LaGrega et al., 1994). Self-sustaining diffusion flames from biomass can burn at 1100°C and more; one-half to two thirds of the heat of combustion is due to flaming combustion, the rest to glowing combustion of char. If pyrolysis

image204I

GAS ——— 1—— ►—- FLAMING COMBUSTION

♦ I

I

Подпись: BIOMASSTAR

CHAR ——— і—— >— GLOWING COMBUSTION

I

Figure 5.5. Simplified pyrolysis and combustion process (Tillman, 1991).

gases are liberated rapidly they consume oxygen around the particle surface therefore there is no oxygen left for char combustion which then accumulates.

Since char has only one-third to one-half the conductivity of wood (Browne, 1958) the layer of char decreases the progress of the pyrolysis front towards the inside of the particle (Fig. 5.5) and a temperature decreasing trend is observed passing from the surface of the particle to the center. This turns into a diversified timing of combustion within the particle which may be still expelling water from the inside core while the mid core is pyrolyzing and the outer layer is already charred. For this reason usually a strong initial flaming is followed by a decrease until sufficient heat has reached a deeper portion of wood to activate pyrolysis reaction.

STEP 4: above 500°C: Glowing combustion begins and it occurs with and without flame. When the surface temperature has reached 1000° C the char at the surface reacts as fast as the pyrolysis layer moves to the center of the particle (Martin, 1956). The luminous diffusion flames due to primary pyrolysis gases and tars are substituted by non-luminous diffusion flames due to the combustion of carbon and hydrogen. When even the production of those gases is ended the remaining char glows almost without flame.

The four steps of biomass combustion will be described with more detail in the following sections.

Dolomite catalysts

The use of dolomite as a catalyst in biomass gasification has attracted much attention because it is a low cost material that significantly can reduce the tar content of the product gas from a gasifier. With suitable ratios of biomass to oxidant agent, almost 100% elimination of tars can be achieved. Dolomite is a magnesium-calcium ore with the general formula MgCO3 • CaCO3 that contain additional minerals at trace levels, such as SiO2, Fe2O3 and Al2O3. The chemical composition of dolomite, as well as many of its characteristics, such as surface area and pore diameter, varies from source to source. Dolomites are the most active with the calcination reaction (MgCO3 • CaCO3 = MgO • CaO + 2CO2). The activity can be directly related to the surface area, pore size and pore distribution. A higher activity has also been observed when iron oxide is present.

Dolomites are generally used in a secondary bed downstream from the gasifier, but they can be used as a primary catalyst, dry-mixed with the biomass. Claimed dolomites can be placed in the gasifier with good result, but they tend to be brittle and thus erode easily.

Dolomite decomposes in principle all tar compounds but naphthalene, thus naphthalene has been identified as the most abundant condensable compound after reforming of tars over dolomite at 800-900°C. This point to a limitation of the dolomites as catalysts if the overall aim is the total elimination of all tars from the product gas. The naphthalene conversion varies with the steam partial pressure, reaching a maximum for a steam concentration of 5-15% and overall naphthalene conversions have been reported to be 80-95% with dolomite (Sutton, 2001).

An additional drawback with dolomite is its eventual use at pressurized conditions. This because dolomite must be calcined for performing acceptably and calcined dolomites can be re-carbonated depending on the temperature and partial pressure of the carbon dioxide. For example, at 900°C, CaO will be carbonated to CaCO3 if the partial pressure of carbon dioxide exceeds 100 kPa. Thus, the use of dolomites as catalysts are, in principle, restricted to biomass gasification applications at atmospheric pressure.

Deactivation due to carbon deposition has been also reported. Nevertheless, the relatively high amounts of steam used in gasification can be effective in maintaining the activity of the dolomite catalysts. The catalyst is also sensitive towards chlorine in the gas phase since it easily forms CaCl2 at the actual temperature present in the gasifier or the tar cracking reactor (Nemanova,

2011) . CaCl2 has a relatively low melting point at 782°C, which is lower than the temperature normally used in the gasifier or a cracking reactor, > 800°C. The CaCl2 will form a soft outer layer at the bed particles. This layer will be more easily abraded and will also cause a blocking of the pore structure, i. e. the material will be completely non-active in terms of catalytic activity.

CHEMICAL KINETICS

3.8.1 Activation energy from single reaction model

The activation energies of five different biomasses, Low ash raw manure (LARM), low ash partially composted manure (LAPC), high ash raw manure (HARM), high ash partially composted manure (HAPC), and Texas Lignite coal (TXL), were determined by using the single reaction model. Tests were performed to see the variation of activation energy with changes in volatile matter (VM) of the fuel and mean particle diameter. Five fuel types were tested in a thermo gravimetric analyzer. Tests were conducted on three particle sizes: as received, 75 ^m, and 45 ^m. The activation energy for each of the fuels from the single reaction model is shown in Figure 3.17. The results indicate that the activation energy for low ash biomass is higher than that of high ash biomass for both raw and partially composted samples. In addition, the raw manure samples have slightly higher activation energies than the partially composted samples. It is noted that a uniform

90

□ AR

image111

LAPC HAPC LARM HARM TXL

Type of Biomass

Figure 3.17. Activation energy results obtained using the single reaction model (adopted from Martin, 2006): AR: as received, 75 microns and 45 microns, B = 1.67 x 1013 1/s. LARM: Low ash raw manure, HAPC: High ash partially composted manure, LARM: low ash raw manner, HARM: high ash raw manure, TXL: Texas Lignite coal.

Table 3.5. Activation energies from parallel reaction energy model, adopted from Martin (2006); B = 1.67 x 1013 1/s.

Fuel

Parallel reaction model (KJ/kmol)

Dairy biomass

61316

Sorghum

129548

LARM

169000

LAPC

175000

HARM

172000

HAPC

173000

TXL

22500

particle temperature assumption has been used. The size effect on pyrolysis values comes through the temperature gradient within the particle; however, the particle sizes here are extremely small. In addition, the heating rates are low; thus, the size effect may not be responsible for different activation energies.

Heilongjiang Jiansanjiang Heating and Power Plant

In Bureau of Heilongjiang Agricultural Reclamation Department Sanjiang Branch, a 35 t/h cir­culating fluidized bed boiler of a thermal power plant burns the mixture of coal and rice husk instead of burning the original bituminous coal. It was found that the boiler saves about 20-40% coal. In the boiler, nearly 200,000 m3 of straw were burned every year and saved the cost of coal about 2.1681 million Yuan (Wang, 2004). The branch has a rice planting area of about 1.6665 billion m2, with a rice output of 1.25 million tonnes per year. The thermal power plant had done a lot of research work in how to utilize rice husk to combust mixed with raw coal. As a result, a new way was found that a small and medium-sized circulating fluidized bed boiler is used for producing heat and generating power with co-combustion of coal and rice husk. The information on the boiler is shown at Table 4.7.

In the plant, coal is conveyed piecewise with a belt. Rice husk is transported by air conveying, which is monitored automatically by limit switches.

Under a condition of the same coal and the same boiler output, comparing with pure coal, the boiler using co-combustion of coal and rice husk could save coal amount between 20-40% (average heat of coal Q„et. v.ar = 18,924,135 kJ/kg). According to coal consumption of 6078t/h with a boiler rated evaporation capacity of 35 t/h, it can be calculated and obtained that the average coal-saving amount is 1823kg/h and the coal-saving amount is about 43.8t/d (lower calorific value of rice husk, Q„et. v.ar = 14,785 kJ/kg; density is 8-9. 26 m3/t, the moisture content of rice husk is 12-18%). If the plant works 11 months per year, the coal-saving amount is 14,4541. Annually it could save the cost of raw coal is 2.1681 million yuan with the average price of coal is 150 yuan/t. In environmental protection, the plant has burned nearly 200,000 m3 rice husk per year, which realizing the transformation of waste to treasure with significant social and environmental benefits.

Design and operation issues

The following section provides a brief discussion of the main issues regarding the different combustion technologies that have been described previously and an insight into main design procedures for combustion chambers and some operational criticalities in terms of corrosion, slagging and fouling.

5.3.4.1 Design principles

In grate furnaces primary air passes through a biomass bed in which drying, devolatilization and char combustion take place, while secondary air to oxidize volatiles is added in a combustion zone over the bed. In the fluidized bed biomass is burned in a suspension of gas and solid bed material in which combustion air enters from below (Basu, 2006; Van Loo and Kopperjan, 2002). A comparison between grate furnaces and fluidized beds is seen in Figure 5.22.

Mass flows of air and flue gases are determined by combustion calculations. Boiler efficiency (which includes combustion efficiency) and excess air are the parameters chosen by the designer. For a biomass pile burner an efficiency ranging from 65 to 75% may be expected, while a stoker

image261

Figure 5.21. Schematic of a Circulating Fluidized Bed (CFB).

• fuel

• bed material

image262

fixed bed combustion bubbling fluidised circulating fluidised

(grate furnace) bed combustion bed combustion

Fixed bed Bubbling bed Circulating bed

Figure 5.22. Comparison between grate furnaces and fluidized beds.

boiler an efficiency of 70 to 80% can be considered while for a biomass Fluidized Bed based boiler an efficiency ranging from 75 to 85% can be considered (US EPA, 2007). Dealing with excess air, an excess air coefficient of 40% can be considered for a bubbling fluidized bed (Basu, 2006).

Once the required power of the boiler is fixed, the amount of fuel can be determined knowing its Lower Heating Value LHV, hence assuming that the water vapors produced exit the furnace above their condensation temperature (Oka and Anthony, 2004).

Подпись:Q0

n0LHV

where:

m f

— fuel mass flow [kg/s]

Qo

— heat power output [kW]

По

— boiler efficiency [-]

LHV

— lower heating value [kJ/kg]

The furnace and combustion system have to be designed to optimize the following aspects: stable ignition, complete burnout; prevention of slagging and corrosion inside the furnace; prevention of fouling and corrosion on the convective heating surfaces.

The depth and breadth of the furnace must adapt to the flame form in a way that it can expand as freely as possible, also ensuring that the walls will not be touched. Beside this the design ofthe furnace must take into account the generation of heat (heat release) and the absorption of heat by the walls ensuring that the designed amount of fuel can be burnt in the given furnace volume, that is the fuel burnout must be completed (Baher, 1985). Besides the cross-section and height of the furnace the performance will depend on the fuel type and on the geometry and location of the heating surfaces. This means that the ash melting temperature of the fuels defines the necessary furnace outlet temperature at the furnace end before the convective heating surfaces in order to avoid sticky deposits on them (Khan et al., 2009).

image264 Подпись: (5.30)

Dealing with heat release, each type of fuel has its specific heat release rate (Prabir et al., 2000), which can be expressed in three different bases: furnace volume, furnace cross section area and water wall area in the burner region. The volumetric heat release rate is the amount of heat generated by the combustion of fuel in a unit effective volume of the furnace. It is given by:

where:

qv — volumetric heat release rate [kW/m3]

B — designed fuel consumption rate [kg/s]

V — furnace volume [m3]

LHV — lower heating value [kJ/kg].

A proper choice of volumetric heat release rate will ensure that fuel particles are burnt substan­tially and the flue gas is cooled to the required safe temperature before leaving the furnace. This temperature is known as furnace exit gas temperature (FEGT) and it is critical for safe operation of downstream heat exchanger surfaces.

Подпись: qF Подпись: B ■ LHV Fgrate Подпись: (5.31)

The heat release rate per unit cross-sectional area is also indicated as grate heat release rate or grate thermal load and is given by:

where:

qF — thermal load [kW/m3]

F — cross sectional area of the furnace grate [m2].

Typical thermal loads for coal fired furnaces are given in Table 5.8.

Подпись: Table 5.8. Typical thermal loads of fixed beds combustors in coal firing (Miller, 2011). Heat release [kW/m2] Underfeed stokers 800-1500 Spreader stokers Stationary grates 1200-1400 Reciprocating and Vibtrating grates 1800-2000 Traveling grate 2000-2200 Overfeed stokers 1200-1300

The burner zone heat release is based on water wall area in the burner region and it is defined as:

Подпись: (5.32)B ■ LHV

2(a + b)Hb

where:

qb = borner zone heat release [kW/m3]

a and b = width and depth of the furnace [m]

Hb = distance between top edge of the uppermost burner and lower edge of the lowest

burner [m].

The heat absorbed by the surrounding walls can be defined by the corrected Stefan-Boltzmann equation and thus the following equation can be written:

20.53 * Fr * BBSA

image271 image272 Подпись: (5.33)

Hout = Heat absorbed by furnace wall + Heat carried by the gas leaving the furnace

5.3.4.2 Deposit and slagging problems

Deposition (i. e. slagging and fouling) and corrosion problems are major issues in the design and operation of a combustion system. In solid fuels combustion the particulate and vapors formed during combustion can deposit on heat exchanger tubes or furnaces walls exposed to radiant heat. If the deposit is formed by molten ashes in the boiler section that is directly exposed to the flame (fire-side) the phenomenon is referred as slagging; if the deposit is formed by vapors that form a sticky layer in the convective part of the boiler the phenomenon is named fouling (Tortosa-Masia et al., 2005).

Table 5.9. Ash chemistry deposition indexes (Yin et al., 2008; Juniper, 1996).

Index Formula

Подпись:ase/acid ratio

Iron index Slagging factor Silica ratio

Critical viscosity at 1426°C Estimated ash temperature corresponding to and ash viscosity of 250 poise

Alkali index (the ratio of the total amount of sodium and potassium expressed as their corresponding oxides to the heating value of the fuel in the unit of kg/GJ Chloride-sulfate molar ratio

Подпись: KCl(cr) Подпись: O Подпись: 2 3

Multi-fuel fouling index Ash melting points

Also vapors and fine particles can cause deterioration of intrinsic properties of the materials in the combustion chamber (corrosion) (Khan et al., 2009) as it happens for example in straw — fired boilers. Based on fuel properties and in particular ash chemistry.

Several experimental tests have been performed to analyze the mechanism of fly ash and deposit formation and corrosion in grate-fired boilers (Zhou et al., 2007; Zbogar et al., 2006; Jensen et al., 1997; 2004; Michelsen et al., 1998; Montgomery and Karlsson, 1999; Nielsen et al., 1999; Hansen etal., 2000; Montgomery etal., 2002).

It has been observed that chlorine increases the volatility/mobility of alkali metals facilitating the formation of alkali silicates and alkali chlorides or hydroxides in the gas phase. When the flue gases are cooled passing the super-heater the alkali compounds condense on the heat exchanger tubes (Fig. 5.23). The initial deposit is formed by vapors and fine particles that arrive at the tube surface where alkali silicates melt at 700°C and thus provide a sticky condition on the entire surface of the tube. This sticky layer enhances further deposition of big fly ashes; this phenomenon affects only the upstream side of the tube.

The deposits consist mainly of fly ash particles and large amounts of condensable salts, forming a matrix that glues the fly ash particles together. In the deposit the presence of KCl and K2SO4 with a structure of iron oxide (FexOy) has been found. A suggested corrosion mechanism for chlorine corrosion is based on gaseous Cl attack, where Fe and Cl in the metal react with gaseous Cl, forming volatile metal chlorides. This mechanism can explain the shift in corrosion behavior observed experimentally (i. e. selective corrosion is negligible at 450°C while is significant if temperature is higher than 520°C) (Yin etal., 2008).

Corrosion mechanisms can be classified into three classes: corrosion associated with gas species; solid-phase corrosion and molten phase corrosion.

The same problems encountered in grate combustion can be found in fluidized bed boilers. Besides in fluidized bed reactors problems of bed agglomeration can be encountered. Agglomer­ation happens when a small part of the ashes remains in the bed, then ash sintering can happen through three main mechanisms: the presence of a liquid phase; solid-state sintering; and chemical reactions.

In biomass fired FBC partial melting is considered the main mechanism leading to bed agglom­eration. The first mechanism is also identified as partial melting and can be further classified as highly viscous melt and low viscous melt. Highly viscous melt happens in the presence of silica this kind of melt is the most problematic because a glassy phase is produced which does not crystallize upon cooling (Llorente etal., 2003; Skrifvars etal., 1998). In the case of low viscous melts alkaline compounds can melt and act like a bonding agent.

Possible remedies to fouling, slagging and corrosion problems are the use of additives or co-firing with coal, peat and sludge. High temperature corrosion can be mitigated also reducing the surface temperature of superheaters. The use of additives aims to raise the melting temperatures of ashes, prevent the release of gaseous KCl and/or react with KCl to form less corrosive compounds. The materials that have been found to raise the melting temperatures of ashes include Al2O3, CaO, MgO, CaCO3, MgCO3, and kaolin (Llorente etal., 2006; Van denBroek, 2000). For example the addition of 3% in weight of kaolin to chopped oats straw can raise the deformation of ashes from 700 to 1200-1280°C (Jenkins, 1994). ChlorOut (ammonium sulfate) a concept developed and patented by Vattenfall AB (Olanders and Steenari, 1995) is a very promising method. An aqueous solution of ammonium sulfate (NH4)2SO4 is sprayed in the combustion zone at a temperature of 800-900°C upstream of the super heaters. It effective converts alkali chlorides into alkali sulfates, that are much less corrosive. The spraying of ammonium sulfate can also reduce NOX formation. The main reactions involved are:

(NH4)2SO4 (aq) ^ 2NH3 (g) + SO3 (g) + H2O (g)

2KCl (g) + SO3 (g) + H2O (g) ^ K2SO4 (s) + 2HCl (g)

4NH3 (g) + 4NO (g) + O2 (g) ^ 4N2 (g) + 6H2O (g)

Possible remedies against bed agglomeration for FBC systems can be: co-combustion, pre­processing of the risky fuel, use of additives and use of alternative bed materials to increase the melting point of sintering compounds. As far as additives are concerned, kaolin, dolomite, lime­stone, lime, alumina have been tested even if they have not been so effective. The use of alternative bed material is the most promising solution; dolomite, magnesite, ferric oxide, alumina, feldspar and aluminum rich materials have been tested (Laitinen et al., 2000; Daavitsainen et al., 2001; Silvennoinen, 2003).