Bioenergy systems for the future 16 biomass gasification producer gas cleanup

Bioenergy systems for the future 16 biomass gasification producer gas cleanup Bioenergy systems for the future 16 biomass gasification producer gas cleanup Bioenergy systems for the future 16 biomass gasification producer gas cleanup Bioenergy systems for the future 16 biomass gasification producer gas cleanup Bioenergy systems for the future 16 biomass gasification producer gas cleanup Bioenergy systems for the future 16 biomass gasification producer gas cleanup

Biomass gasification producer

gas cleanup

S Adhikari*, N Abdoulmoumine†, H Nam*, O Oyedeji†

*Auburn University, Auburn, AL, United States,†University of Tennessee,

Knoxville, TN, United States

16.1 Introduction

Biomass producer gas contains nonnegligible amount of impurities such as fine partic-ulates, condensable organic compounds known as tar, sulfur-containing compounds, nitrogen-based compounds, hydrogen halides, and trace metals The presence of these impurities in biomass producer gas hinders its utilization and must therefore be removed

to meet stringent environmental emission regulations and minimize deleterious effects

on equipment and catalysts in downstream processes Producer gas cleaning is a vital step for large-scale commercial deployment of biomass gasification as it has the most impact

on the cost of clean syngas and its potential use for several downstream processes

16.2 Producer gas impurities

Particulate matter (PM) impurities emanates from biochar, soot, and elutriated bed materials, if fluidized-bed systems are used, with particle diameter up to 100μm The quantity of particulates in the producer gas prior to cleaning depends on the particle size of the starting feed, type of gasifiers (i.e., moving bed vs fluidized bed) and the process conditions (residence time and temperature) Particulate impu-rities are primarily composed of residual solid carbon and inorganic elements emanat-ing from biomass ash Particulate impurities are classified accordemanat-ing to aerodynamic diameter with PM10, for example, representing “particulate matter” with diameter smaller than 10μm This classification is used to indicate particulate cleanliness requirements for specific applications For example, gas turbine applications require particulate concentrations less than 30 mg/m3for PM5and above Particulates in pro-ducer gas lead to air pollution, fouling, corrosion, and erosion, which adversely affect human health, efficiency, and safety in gasification plants

Tars are remnants of volatile compounds of biomass devolatilization and are a com-plex group of organic compounds that condenses in transfer lines, conduits, and other equipment downstream of the gasifier The definition and classification of tar is not

Bioenergy Systems for the Future http://dx.doi.org/10.1016/B978-0-08-101031-0.00016-8

© 2017 Elsevier Ltd All rights reserved.

uniform in literature because of the complex chemical nature of tar However, tars have been defined as “all gasification organic compounds with molecular weight greater than benzene” and classified into five classes of tars in several works:

l Class I: This class represents tar compounds with seven or more rings Tar compounds belonging to this class are considered the heaviest and tend to condense even at high tem-perature and moderately low gas-phase concentrations These tar compounds exhibit low volatility and are seldom detected during gas chromatographic analysis

l Class II: Class II tar compounds encompass heterocyclic hydrocarbons with heteroatoms Examples of these compounds are phenol, cresol, and pyridine These compounds are highly water-soluble

l Class III: This class is composed of light aromatic compounds that do not readily condense

on surfaces and have poor solubility in water Examples of such compounds are toluene, styrene, and xylene

l Class IV: Light polyaromatic compounds with two or three rings belong to this class Unlike Class III compounds, these compounds readily condense at intermediate temperatures Common tar compounds in this class are naphthalene, phenanthrene, and anthracene

l Class V: This class covers heavy polyaromatic hydrocarbons with four to six ring compounds that condense even at high temperatures and low concentrations Examples of these compounds are fluoranthene and pyrene

Tar is the most notorious impurity existing in producer gas from biomass gasification because of its high abundance and potential to polymerize As a result, tar has been the focus of most producer gas cleanup researches (Abdoulmoumine et al., 2015)

Nitrogen in biomass is converted to ammonia (NH3), hydrogen cyanide (HCN), and oxides of nitrogen (NO, NO2, N2O, and other NOx) (Zhou et al., 2000) Nitrogenous impurities in biomass producer gas generally emanate from the decomposition of pro-tein and/or heterocyclic aromatic structures in the biomass feedstock (Hansson et al.,

2003, 2004) It has been hypothesized that biomass protein is first decomposed to form 2,5-diketopiperazines that are later decomposed to form HCN and HNCO, while NH3

is mainly form in the solid phase (Hansson et al., 2004) The nitrogen-containing compounds in producer gas may deactivate catalysts and cause air pollution Among all nitrogenous impurities, NH3is the most abundant with widely varying concentra-tions typically between 350 and 18,000 ppmv depending on the nitrogen content in the feedstock and process conditions The primary incentive for NH3removal is the reduc-tion of NOxemissions in downstream applications such as burners, gas engines, and turbines

Sulfur in the biomass is converted primarily to hydrogen sulfide (H2S), carbonyl sulfide (COS), carbon disulfide (CS2), and other minor sulfur-containing compounds with H2S being the most dominant in gasification The quantity of sulfur-based impu-rities in biomass producer gas is in lower quantity compared with coal producer gas Most downstream applications require the removal of sulfur-based impurities to avoid

equipment corrosion and catalyst deactivation and to comply with emission regula-tions Since concentrations as low as few parts per million of sulfur in gas stream can severely deactivate catalysts, producer gas requires extensive cleaning of H2S and other sulfur-based impurities if it is intended for use in catalytic chemical processes downstream

Halogens, like chlorine in biomass feedstocks, are released as hydrogen halides such

as hydrogen chloride (HCl) and their respective salts by binding to metals in biomass such as alkali metals Among all hydrogen halides, HCl is the most abundant (Ohtsuka

et al., 2009) Hydrogen halide removal is very important to minimize corrosion and fouling/slagging of equipment such as filters, turbine blades, and heat exchanger surfaces and to meet often stringent requirements for downstream application Fuel cell applications particularly have very stringent requirements for hydrogen halide concentrations due to the susceptibility of electrolytes and electrodes attack by halogen ions

Metals such as Ca, Mg, P, Si, Na, K, Fe, Al, Cu, Mn, Fe, Zn, Mo, As, Cd, Hg, and Pb are contained in biomass intrinsically and sometimes added through technogenic activities Intrinsic biomass metals are taken during the plant growth from soil, water, and air, and technogenic biomass metals are added to biomass during pregasification processes such

as harvesting and transportation During gasification, intrinsic biomass metal is par-titioned into char and gas product The gas-phase metals are the major sources of concern

as they must be captured downstream prior to exhausting the gas into the atmosphere Consequently, they must be removed as they are a source of concern for human health and environmental pollution In addition, some of these elements can contribute to cat-alyst deactivation and corrosion and fouling of equipment Technogenic biomass metal may be detached from biomass especially in fluidized-bed systems causing slagging and severe defluidization issues Trace metal impurities during gasification may also origi-nate from catalysts and bed material in the case of fluidized-bed systems

Several heavy metals such as Hg, As, Se, Cu, Pd, Cd, and Zn are found in producer gas

in trace quantities In the context of the environment and human health, these metals are highly toxic Biomass in its raw form contains less than 40 ppb of Hg (Thy and Jenkins, 2010) However, Hg receives special attention being a severe environmental pollutant that is capable of accumulating in the ecosystem and causing serious human health problems Vaporized Hg may remain in the atmosphere for months and move across intercontinental distances Hg exists in producer gas mainly as elemental Hg and rarely as HgCl2and HgS depending on gasification operating conditions In down-stream applications, Hg forms amalgams with metals, especially aluminum, leading to

the failure of metal components of equipment Dioxins and dioxin-like compounds (DCLs) are another toxic impurity in producer gas DCLs are formed at low gasification temperatures by hydrocarbons and chlorine, in the presence of oxygen and metals Sim-ilar to Hg, DCLs have the capacity to bioaccumulate in the tissue of animals and have been found to cause cancer and damage to the human hormonal and immune system

16.3 Operating conditions and their implications

on producer gas impurities

Particulate matter is primarily affected by the temperature that causes morphological, physical, and chemical changes As temperature is increased, particulate diameter decreases following a shrinking core phenomena However, particle-size reduction

is more strongly impacted by elutriation, especially when pellets and chips in fluidized-bed and entrained-flow gasifiers by fragmentation Furthermore, tempera-ture significantly alters the chemical properties of particulates and increases residual solid carbon and inorganic content ER and steam do not influence the morphology or physical properties and mildly affect the chemical properties

The quantity and nature of tar in producer gas can be affected by temperature, resi-dence time, equivalence ratio (ER), and steam-to-biomass ratio by promoting thermal cracking, oxidation, and steam-reforming reactions Temperature plays a vital role on tar content and on the nature of tar compounds in producer gas by promoting thermal cracking and favoring the formation of multiring aromatic tar compounds In general, tar concentration usually decreases as temperature increases (Narvaez et al., 1996), but tar refractoriness increases with temperature The role of residence time is similar

to that of temperature because the severity of reaction is directly proportional to both temperature and residence time ER has a significant effect on tar reduction by pro-moting oxidation of volatiles formed during devolatilization In general, an increase

ER decreases tar concentration Tar concentration was decreased from 10 to near 2.5 g/Nm3by increasing the equivalence ratio from 0.26 to 0.45, everything else being equal (Narvaez et al., 1996) Catalyst has been found to affect tar yield and compo-sition with tar yield following Co/Al2O3>Fe/Al2O3>Ni/Al2O3when corn stover was gasified in a microwave-assisted system at 900°C (Xie et al., 2014)

Since nitrogen impurities evolve due to the presence of nitrogen in biomass, it is expected that biomass feedstock type will play a major role in their concentrations

In addition, biomass gasification operating parameters impact the yield of individual nitrogenous impurities When gasification temperature is increased, the decomposition

of NH3 to N2 is increased For example, using sawdust, the concentration of NH3 decreased by half over temperatures ranging from 700°C to 900°C at ER¼0.25 (Zhou et al., 2000) Similar trends are observed for HCN and NO (Zhou et al., 2000)

in sawdust at ER¼0.25 Besides temperature, ER can also affect the proportion of

NH3, HCN, NO, and other nitrogen-containing species in producer gas It was observed that NH3, HCN, and NO decrease as ER is increased slightly from 0.18 to 0.37 at 800°C (Zhou et al., 2000) However, it is evident that temperature, rather than ER, has the most impact in reducing nitrogen impurities as illustrated inFig 16.1

Besides temperature and ER, the type of gasifying medium also plays a role in the concentration of NH3, HCN, and other nitrogenous compounds The presence

of steam in gasification enhances the formation of NH3 A comparison of two fuels with similar nitrogen contents gasified in a circulated fluidized-bed reactor with air and steam at similar temperature showed that NH3concentration is doubled when steam is used (Wilk and Hofbauer, 2013; Van der Drift et al., 2001) Fuel nitrogen conversion to HCN is also affected by gasifying agents As the use of steam increases the concentration of H2in the syngas, it is likely that the increasing reducing environ-ment favors the formation of NH3

There are diverging reports of the effect of temperature on H2S content in syngas In a study of a mixture of 70% refuse-derived fuel and coal (0.62 wt% dry ash-free basis sulfur), it was reported that H2S concentration in syngas increased from 808 to

1081 ppmv from 720°C to 850°C but subsequently decreased to 823 ppmv as temper-ature was further increased to 900°C (Dias and Gulyurtlu, 2008) However,Carpenter

et al (2010)reported an increase in H2S for switchgrass, Vermont wood, and wheatgrass

as temperature was increased from 650°C to 875°C Gasifying agent (air, steam, air/ steam, or O2/steam) significantly affects the composition of primary gases as discussed earlier These primary gases, in turn, can be involved in various reactions with H2S and

0

20

40

60

80

100

Gasification temperature (°C)

0.18 0.25 0.32

ER

Fig 16.1 Effect of temperature and ER on fuel-bound nitrogen conversion to NH3

Reproduced from tabulated data Zhou, J., Masutani, S.M., Ishimura, D.M., Turn, S.Q., Kinoshita, C.M., 2000 Release

of fuel-bound nitrogen during biomass gasification Ind Eng Chem Res 39(3), 626–634

other sulfur impurities, thus affecting their final concentrations Several reactions are involved in H2S and COS formation and conversion during gasification:

H2S + CO2! COS + H2O

H2S + CO! COS + H2

H2S + 3=2O2$ SO2+ H2O

COS + H2S$ CS2+ H2O

In a study of corn straw gasification in a downdraft gasifier, it was observed that as ER increased from 0.20 to 0.40, H2S first increased from 473 to 512 ppmv from 0.20 to 0.30 ER and subsequently decreased to 459 ppmv from 0.30 to 0.40 ER (Gai et al.,

2014).Dias and Gulyurtlu (2008)also observed that as ER is increased from 0 to 0.40 in a mixture of 70% refuse-derived fuel and coal (0.62 wt% dry-ash-free basis) and H2S concentration increased from 672 to 1204 ppmv However, as S/B ratio increased, H2S concentration decreased according to various studies (Gai et al., 2014; Meng et al., 2010)

Temperature impacts hydrogen halides concentration in syngas by enhancing the for-mation of alkali halides using fuel-bound metals (Kuramochi et al., 2005) At ER of 0.20, it was noticed that HCl concentration decreased from 95 to 65 ppmv as temper-ature was increased from 720°C to 900°C for a mixture of 70% refuse-derived fuel and coal with a weighted average of 0.07 wt% dry-ash-free basis chloride (Dias and Gulyurtlu, 2008) The impact of gasifying agent on hydrogen halides is not very clear due to the lack of information particularly in air/steam and steam gasification As ER

is increased from 0 to 0.40, HCl concentration increased from 78 to 85 ppmv at 850°C (Dias and Gulyurtlu, 2008)

Temperature plays a crucial role in the level of trace metals detected during gasifica-tion Alkali and other trace metals are commonly bound to halogen and other inorganic elements in biomass with the formation of alkali halides commonly increasing as tem-perature is increased due to favorable thermodynamics (Porbatzki et al., 2011; Dolan

et al., 2012).Porbatzki et al (2011)investigated the release of metals in wood and miscanthus during gasification at 800°C, 900°C, and 1000°C in a fluidized bed and observed that the release of potassium decreased as temperature increased for wood but increased as temperature increased for miscanthus In a thermodynamic modeling

of fluidized-bed gasifier, it was reported that Na, K, Fe, and Mn are not appreciably release into the gas phase even as gasification high temperature is increased to 1000°C (Konttinen et al., 2013) Similar conclusions were drawn by Froment et al (2013) where Ba, Mg, K, P, and Mn are not released at temperatures lower than 1000°C

On the other hand, Pb, As, Zn, Hg, Sd, Sn, and Cd are completely released in the gas phase at 750°C (Konttinen et al., 2013)

16.3.7 Mercury and other toxic impurities

The form in which Hg exists in producer gas is important and determined by the gas-ification operating conditions It is more difficult to remove elemental Hg than it is to remove oxidized Hg in the form of HgS or HgCl2 When reducing conditions is prev-alent during gasification, the oxidation of Hg to form Hg2+compounds is inhibited This is typically the case for gasification process Gasification conditions do not favor formation of DCLs; therefore, the amount of DCLs in producer gas may be in few parts by quadrillion The low oxygen levels in gasification condition inhibit the for-mation of DCLs Hence, the forfor-mation of DCLs is more prevalent in combustion con-ditions (Cheng and Hu, 2010) In addition, DCLs are decomposed at gasification temperatures higher than 850°C (Kalisz et al., 2008) There is however the potential for the formation of DCLs during cold gas cleanup process where biomass producer gas is cooled down The presence of particulate matter at the producer gas cool phase may facilitate the formation of DLCs (Lemmens et al., 2007)

16.4 Producer gas cleanup

Prior to its utilization, biomass-derived syngas must be purified to adhere to the spe-cific downstream applications (Table 16.1)

Consequently, depending on the type of feedstock and gasification process, syngas purification and conditioning must be adapted to target desired impurities In addition

to aforementioned impurities, carbon dioxide and other light hydrocarbons might require removal for optimal operations in catalytic reactors

Particulate removal is essentially achieved by producer gas filtration since the mass of the particulate matter is larger than that of the producer gas Particulate cleanup technol-ogies are more mature relative to other cleanup technoltechnol-ogies and operate by inertial sep-aration, barrier filtration, and electrostatic interaction Many cleanup technology options

Table 16.1 Upper limits of impurities in gasification syngas

for selected applications ( Woolcock and Brown, 2013 )

Applications

Tars (mg/Nm3)

Sulfur impurities (ppmv)

Nitrogen impurities (ppmv)

Alkali (ppmv)

Halides (ppmv)

FT synthesis <0.1–1a

Methanol

synthesis

<0.1 <1‡

n/a means “not available”.

are available with selection of the appropriate technology dictated by the desired partic-ulate removal efficiency and the operating conditions of the filtration device The most common particulate filtration technologies are wet electrostatic precipitator (WESP), scrubbers, bag and candle filters, cyclones, and sand bed illustrated inFig 16.2

Tar impurities are eliminated from producer gas using thermal or catalytic cracking and physical separation with the latter performed at low temperature Tar cleanup can be carried out in situ and post gasification as primary and secondary measures, respectively Primary cleanup measures encompass thermal cracking at high gasifica-tion temperatures, tar oxidagasifica-tion by limited oxygen injecgasifica-tion, and catalytic cracking using catalytically active bed materials Producer gas tar content can be decreased

to concentration tolerable by direct combustion application (50 mg/m3) by apply one or several of these measures However, chemical synthesis applications, with more stringent tar content requirements, require secondary tar cleanup measures Some secondary tar removal measures utilize thermal or catalytic cracking of tar to form simple gases at temperatures within gasification conditions Steam reforming of tar compounds is also a valuable strategy for removing the tar content of producer gas Steam-reforming reactions are highly endothermic and are typically catalyzed with nickel-based and noble-metal-supported catalysts (Liu et al., 2010) Postgasification

900

750

200

WESP

Scrubber

PRE = 40−65

PRE = 95−99

PRE = 90−99

PRE = 45−70

PRE = 80−95

Bag/candle

50

Fig 16.2 Comparison of several particulate filtrations system at varying operating

temperatures and particulate removal efficiency (PRE) expressed as on a percentage basis

tar removal can also be conducted at temperatures lower than gasification conditions

by filtering condensed aerosols with cleanup technologies used for particulate matter removal Wet scrubbing of producer gas with liquid solvent is a widely used secondary measure for removing tar in producer gas Many polar and nonpolar solvents have been applied for wet scrubbing of producer gas, with each having varying cost and degree of effectiveness Nonpolar solvents such as diesel fuel, engine oil, and vege-table oil are more expensive but more effective for tar removal when compared with polar solvents such as water The evaluation of waste cooking oil as a cost-effective source for nonpolar solvent found that its tar removal efficiency drastically reduced from 88% in the initial 20 min to 25.3% after 10 h (Tarnpradab et al., 2016) In the last two decades, nonthermal plasma and plasma-assisted systems have been utilized to remove tar in producer gas through reforming or decomposition of tar (Tao et al., 2013; Torres et al., 2007) Plasmas consist of highly reactive atoms, ions, electrons, and radicals Cost and durability of devices are the major drawbacks to the technology

The removal of nitrogen-containing compounds from biomass producer gas has been mainly focused on NH3, being the most dominant nitrogen-containing compound in biomass producer NH3 removal strategies involve web scrubbing and catalytic decomposition Wet scrubbing with water is effective for NH3because of its high sol-ubility in water However, gas cooling prior to or during wet scrubbing leads to ther-mal inefficiencies and the disposal of wastewater may constitute environmental challenges Catalytic decomposition of NH3at temperatures ranging from 450°C to

900°C can be used in place of the direct removal of NH3 This strategy increases the H2content of producer gas by disintegrating NH3into H2and N2 It has been suggested that NH3decomposition occurs through a reversed NH3formation reaction (Torres et al., 2007) NHxmolecules on catalyst active sites are dehydrogenated to form N and H atoms, which recombine to produce H2and N2before leaving the cat-alyst active sites Some of the catcat-alysts that have been studied for the catalytic decom-position of NH3are nickel, iron, ruthenium, and cobalt based with alumina, zirconia,

or activated carbon support The selective oxidation of ammonia in biomass producer gas has been proposed This will require high reaction precision by introducing con-trolled quantity of oxidizer to specifically react with NH3to form H2O and N2because oxidation of producer gas CH4, CO, and H2is disadvantageous

The state of the art for removing sulfur-containing species in biomass-derived producer falls under wet and dry scrubbing processes at low and high temperatures, respectively Wet scrubbing is a mature technology and has been used in large-scale applications, primarily in coal producer gas cleanup Effective sorbents for high-temperature desul-furization are still under development and have not yet been proved in large-scale applications Some laboratory-scale and pilot-scale testing has however shown signif-icant success using monolithic Zn-Co-Ti-O and zinc-oxide-based sorbents with

95%–99% sulfur removal efficiency (Broer et al., 2015; Chomiak et al., 2016) High-temperature desulfurization technologies operate by physical or chemical adsorption to sorbent materials On these sorbents, the physical adsorption (i.e., physisorption) phe-nomena involve weak van der Waal’s intermolecular interactions between sulfur spe-cies and sorbents, whereas chemical adsorption (i.e., chemisorption) involves covalent bonding of sulfur-containing species onto the surface of adsorbents Sorbents that oper-ate by physisorption might be more regenerable due to potential multilayer adsorption resulting in weaker bonding In contrast, chemisorption might lead to lower sorption capacity due to monolayer coverage and poorer regenerability as a result of stronger bonding

Halide cleanup technologies have not been as exhaustive investigated as other biomass producer gas impurities (Dolan et al., 2012) However, halide removal is very important

to minimize degradation on equipment such as filters, turbine blades (Ohtsuka et al.,

2009), and heat exchanger surfaces (Ohtsuka et al., 2009) and to meet often stringent downstream application requirements In particular, fuel cell applications have very stringent limits due to the susceptibility of electrolytes and electrodes attack by halide ions Especially, hydrogen halide (HCl) is associated with rapid catalyst deactivation and, given its abundance relative to other halides, is the primary target of halide cleanup Commercial large-scale halide cleanup operations employ cold gas cleaning to remove halides by wet scrubbing usually through a basic solution or, alternatively, simply water Gas cooling induces the condensation of most metals, particularly alkali metals that can be reduced by mild gas cooling below 600°C and subsequent adequate gas fil-tration However, this approach only serves as a primary measure as some metals will remain in gas phase and can be removed by wet scrubbing with water

High-temperature cleaning of halides has thus far been carried out mostly with activated carbon; alumina and oxides of calcium (Ca) and sodium (Na) in fixed beds Other sorbents of multiple oxides can be more efficient, but these sorbents are cost pro-hibitive (Sasaoka et al., 1993) Among thermodynamically suitable sorbent for halide removal, hydroxides of calcium (Ca) and sodium (Na) sorbents have shown great poten-tial and typically induce removal by dehydrohalogenation as shown in the following reactions for HCl using calcium-based sorbents:

Ca OHð Þ2+ 2HCl! CaCl2+ 2H2O

CaCO3+ 2HCl! CaCl2+ H2O + CO2

CaO + 2HCl! CaCl2+ H2O

The removal of alkali-metal vapors often occurs in conjunction with halides as alkali metals tend to combine to halides to form salts (NaCl and KCl) in syngas In a study focusing on alkali metals removal, fly ash, bentonite, kaolin, and bauxite were inves-tigated as potential sorbents The authors reported that all sorbents reduced the release

of alkali metals although fly ash and bentonite recorded the best performance (Bl€asing