Ene CGT / Fall 2015 / Model Solution Homework 2 V2.0. Introduction M.M.

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1 M.M. Ene CGT / Fall 2015 / Model Solution Homework 2 V2.0 Kindly note that the model solutions are intended to help and complement the learning process. The prepared solutions are not to be used as detailed descriptions of grading principles. Therefore, most answers are provided more comprehensively, additional examples are illustrated and far more details are explained than what is required and expected from a graded good homework solution. Introduction The Winkler system was the first developed commercial scale direct-gasifier back in the early 1920s. The developed patent by Fritz Winkler of Germany was first for an atmospheric bubbling fluidized bed where a portion of the fuel is combusted for gasification demand within the singular unit. For historical context, the earlier applications were for powering gas engines, (Basu, 2013) especially during World War II, where a motivation for self-sufficiency to counter depleted resources because of warfare led to the deployment of many coal and biomass fuel based gasifiers. (Meyers, 1984) Later on more than 70 reactors went into commercialization for synthetic fuels production under the traditional Winkler gasifier configuration which remained dominant till the early 1960s, with a total capacity of over 20 million Nm 3 /d. (Higman and Burgt, 2003) The wide deployment of the technology was attributed to its fuel flexibility, as it was first developed for the gasification of coal and then later on, with the introduction of design improvements, was adapted for biomass. (Leckner, 2015) The bubbling bed configuration utilizes bed material to assist in maintaining rapid and uniform heat transfer. The selection criteria for appropriate material is based on being an abundant non-combustible one, with some applications looking into even the catalytic effect (Basu, 2013) The fluidization phenomena is induced through the grate at the lower end of the reactor by the introduction of air, oxygen, or steam. As the name suggests, a BFB configuration operates with a fluidizing velocity high enough to allow the bed to boil, but not so high that the solids are swept out of bed and entrained in the product syngas. Typical fluidization velocities range between 1 to 5 m/s. (Higman and Burgt, 2003) The utility of the bubbling bed design lies in its ability to gasify a wide range of fuels, with only moderate tar production and the potential for reliable operation. However, reactor size requirements can be an issue with this technology, as reactor diameters must be sufficiently large enough to keep gas velocities below the point of entraining solids in the product gas. (Meyers, 1984) Also it was mentioned in (Higman and Burgt, 2003) that when operating

2 with the more uniform coal fuels (in compared to biomass), milling pretreatment was required to drop the particulate size to below 10 mm. Furthermore, as some solids carryover is unavoidable, the carbon losses detected in the ash could reach values as high as 20% on a feed basis depending on the type of fuel used. In order to mitigate carbon loss in more recently developed configurations, the char-bearing ash can be circulated either into a recovery boiler or back to reactor as fuel for internal combustion. (Basu, 2013) Where the traditional Winkler gasifier operates at atmospheric pressure, high-pressure configurations up to 30 bar were introduced as additional adjustments to realize carbon conversion improvements. (Higman and Burgt, 2003) The high-temperature Winkler (HTW) process operates at both elevated temperatures (>800 C) and elevated pressures (>10 bar). (Leckner, 2015) High temperature Winkler reactors require lockhoppers on the fuel feed and on the ash/char discharge to maintain elevated pressures. Further improvements in the HTW process were brought about by injecting the fluidizing medium both below the reactor grate and into the freeboard over the bed. (Meyers, 1984) The additional fluidizing gas added over-bed raises the temperature in that reactor zone to more selectively produce the desirable CO and H 2 products, rather than methane and heavier hydrocarbons. Finally, high-pressure operation results in a pressurized syngas that should require less downstream compression energy for subsequent unit operations. (Basu, 2013) Figure 1. Simplified gasifier schematics. Right: Traditional Winkler Gasifier, Left: High temperature Winkler-HTW gasifier. (both are bubbling beds despite the cyclone) (Leckner, 2015)

3 For the sake of comparison, Figure 1 adapted from (Leckner, 2015) shows the schematics of both Winkler gasifier configurations. With the spurred energy crisis because of the war in the Middle East at that time, the HTW gasifier was introduced commercially in 1974 for power applications and still is considered a success, however synthesis plants, for example one for Methanol in Germany was reportedly shut down later in 1997 due to economic reasons. As a result, more recent developments have been directed toward power production in higher efficiency Rankine cycles. One example is the 2*80 MW circulating fluidized bed gasifier in Lahti that employs sorted solid waste. (Power Plant / Home / Lahti Energia, URL) Problem 1 The developed mass and energy balance is based on the data provided in table 5.6 in reference (Higman and Burgt, 2003) for the Winkler gasifier. General assumptions are based on information collected in literature about the process and references are provided accordingly along the line of the solution. Part A: Mass balance formulation Step 1: Define the mass inlet into the gasifier The information provided in table 5.6 doesn t specify the type of biomass so two assumptions: ash content is neglected as it is assumed the ash that will enter the mass balance will exit in its existing nature (the only effect will be the accompanying char, which will be discussed later) and the water content is assumed to be 10 wt.% of the feed, as this a very conservative estimate given that it is the operating limit for Winkler gasifiers. (Check introduction part)

4 1- Biomass Table 1. Break down of Biomass input to gasifier. (Ultimate analysis and total biomass input =893 kg/1000nm3 of H2+CO are reported on dry ash free basis) Ultimate Analysis as reported at inlet Mass input - kg per 1000 Nm 3 of H 2 + CO C H S N O ash neglected moist total Air Notes: Moles input - kmol per 1000 Nm 3 of H 2 + CO - The chemical formula (based on reported Ultimate Analysis) is C H S 0.03 N 0.06 O Per 1000 Nm3 of H 2 + CO, and at normal conditions of Temperature = 20 ⁰C & Atmospheric pressure Mass air = Volume * Density = 1358 [Nm 3 ] * [kg/m 3 ] = kg, Mole Air = moles Mass N 2 = [kg] * [wt.%] = kg, Mass O 2 = [kg] * [wt.%] = kg Mole N 2 = moles Mole O 2 = moles Another perspective to evaluate the air flow rate is to look at its contribution in the gasifier operation, The stoichiometric oxygen needed = n C + (1/4 n H) (1/2 n O) = moles And thus the equivalence ratio of air supplied = /37.89 = 0.31 which falls within accepted supply ranges for the gasification process to take place. Step 2: Perform a stoichiometric balance for the gasifier Please refer to appendix pages 1-4 for the detailed formulation of the stoichiometric balance. The final reaction is

5 C H S 0.03 N 0.06 O H 2O [O N 2] 7.51 C CO CO CH H H 2O N H 2S The final mol.% of dry gas is compared to the data given for the Winkler gasifier in Table 5.6 in (Higman and Burgt, 2003). The discrepancies between both data set fall within an acceptable range and are attributed to either calculated estimation errors or different feed configurations for the actual gasifier. The calculation errors might have been a result of using the molar yield for CH 4 on dry basis for calculation on a wet basis, but as mentioned in the appendix; given that CH4 has the smallest mol.% to the other carbon balance compounds, setting CH 4 as a starting point for estimation minimizes the influence of the error on the other compounds. The different feed configuration errors stem from either the over estimation of Char (assumed to be solid carbon) losses with ash (20 wt.%) or the higher level moisture content used for feed (10 wt.%). It is clear from the data in the reference that the Winkler gasifier is from the HTW generation operating at 30 bar and higher temperatures while those assumptions would be more common for the traditional Winkler gasifier. Table 2. Comparison between reactor dry gas yield in reference and the estimated yield from the stoichiometric balance. mol.% on dry ash free basis Reference Calculated CO % 7.1 % CO % % H % % CH % 2.56 % Ar 0.50 % Neglected N % % H 2S 0.30 % 0.04 % Total % %

6 Step 3: Define the mass outlet from the gasifier Table 3. Mass and molar balance for the gasifier outlet Product gas Reported Norm. Volume at1000 Mass Out Mass MW Moles Out mol%-dry mol%-dry Nm 3 CO+H 2 kg wt.% kg/kmol kmol CO % CO 31.0 % H % CH % Ar 0.5 % N % H 2S 0.3% Total 100.3% In order to perform the balance for the gasifier outlet, a back end calculation approach needs to be performed. The molar weight of the dry product gas is shown in Table 3 (obtained from table 5.6 in (Higman and Burgt, 2003)). The sum of H 2 + CO was set to 1000 Nm 3, and based on the molar weights the volume for the balance of the gas products were calculated. Based on density values obtained from (NIST Data Gateway), the mass flow of each component was calculated and converted using the molecular weight to molar flow in the last column of Table 3. Mass and molar balance for the gasifier outlet Step 4: Define the mass balance of the gasifier When comparing the calculated values for inlet (Table 1) and outlet (Table 3) values for the gasifier, on dry basis, some discrepancies are found, For Carbon Balance: Inlet = kmol, while Outlet = n CO + n CO2 + n CH4 = kmol Carbon losses = 1 (33.26/37.54) = 11.4 %, and are expected to be carbon loss with ash or carbon compounds soluble in the removed condense water. It is worth to note that it is significantly lower than the 20 % reported in literature for traditional gasifiers. (Basu, 2013)

7 For Nitrogen balance: Inlet = nitrogen in air (calculated in step1) + nitrogen in biomass (Table 1) = kmol, While outlet = 34 kmol. This accounts for 25 % loss. For Oxygen balance: Inlet = oxygen in air (calculated in step1) + oxygen in biomass (Table 1) = kmol While Outlet = n CO + 2* n CO2 = kmol, which accounts for losses of 23% or kmol. For hydrogen balance: Inlet = kmol, while Outlet = 2* n H2 + 4* n CH4 + 2* n H2S = kmol, which accounts for losses of 18% or 9.4 kmol. A summary of elemental balance is shown in Table 4. Table 4. Elemental molar balance for Winkler gasifier input and output from reference (Higman and Burgt, 2003). Element In [kmol] Out [kmol] Balance C % H % O % N % Figure 1 below shows the mass balance on dry basis, (values are divided by 893 to give specific kg). For Product gas, m = (Table 3)/893 = kg per kg of dry feed. For carbon, m = (C in-c out)* MW C /893 = kg per kg of dry feed. For nitrogen balanced, 23.5% was losses in table 4. m = (N in-n out)* MW N2/893 = kg per kg of dry feed. For condensed water, assumed as missing balance from hydrogen found in dry biomass, m = ((H in-h out)/2)* MW H2O /893 = kg per kg of dry feed For oxygen, assumed as the remaining oxygen balance (after accounting for the needed moles to oxidize the hydrogen into condensed water), m = ((O in-o out)- ((H in-h out)/2))* MW O2 /893 = kg per kg of dry feed

8 Sum of outlets = kg/kg of dry feed, which has an error of 1.04% from the amount of feed on dry basis. This finding as well as the unaccounted balance for N 2 & O 2 could be mainly attributed to approximation errors during calculation or unaccounted for solubility of a portion of syngases with the removed condense water content. Another reason that should be considered is reporting errors by (Higman and Burgt, 2003) during their attempt to simplify the presentation of the gasifier. Biomass input 1 kg on dry basis Dry Air input is kg per kg of dry biomass Gasifier balance kg of product gas per kg of dry kg of carbon loss with ash presumably kg of condensed water based on hydrogen balance kg N 2 & kg O 2 not accounted for Figure 1. Schematic for the mass balance per dry basis of biomass feed for the gasifier system based on information provided in table 5.6 in reference (Higman and Burgt, 2003) Part B: Energy balance formulation The general balance equation for the gasifier based on the formulated mass balance is Q_input = Q_output Gasifier Q_biomass + Q_air = Q_productgas + Q_boundryloss Gasifier Where Q represents the heating values for each inlet and outlet and calculated as following. Note: all Q values are calculated at NTP conditions of 20 ⁰C and atmospheric pressure, all reactive heat loads are assumed to take place within the system boundaries of the gasifier. Step 1: Define the energy input into the gasifiers For the energy input on the right hand side of the equation, Q_air is assumed to equal Zero at NTP conditions. For the biomass, the higher heating value is estimated with the model developed by Gaur and Reed in 1995, (Boundy, 2011) HHV dry basis = (0.35*X C *X H + 0.1*X S 0.02*X N 0.1*X O 0.02*X ash) = MJ/kg

9 Where X represents the weight percentage of each elemental constituent of biomass from Table 1, and the HHV value is in MJ/kg. The lower heating value of feedstock, LHV could then be calculated from the formula; LHV dry basis = HHV dry basis - (8.93*m H2/m feed)h fg = MJ/kg Where (m H2O/m feed) is the moisture content of the feedstock from the proximate analysis and h fg is the standard latent heat of vaporization of water at NTP, which is MJ/kg. (Basu, 2013) So the amount of heat input on dry ash free basis, Q_biomass = 893 * = MJ Step 2: Define the energy outputs from the gasifiers For the energy output on the left hand side of the equation, Q_productgas is calculated based on the calculated outlet moles for each component provided in Table 3 and the molar heating values are obtained from the National Institute for Science & Technology online database (NIST Data Gateway, URL). Q_productgas = (n CO * Q CO) + (n H2 * Q H2) + (n CH4 * Q CH4) + (n H2S * Q H2S) = MJ Step 3: Calculate the cold gas efficiency, CGE of the gasifier on dry ash free basis CGE = Q_productgas / Q_biomass = % Problem 2 The students are expected to discuss and cover the historical background as well as the more recent technical development for the Winkler gasifier system in specific and that of fluidized bed technology in general. Students should be able from their review to present conclusions for the techno-economic drivers that led to the development of the HTW technology as well as offer a prediction for future applications.

10 Referenes: Basu, P., Biomass Gasification, Pyrolysis and Torrefaction: Practical Design and Theory. Academic Press. Boundy, B., Biomass Energy Data Book, 4th ed. Higman, C., Burgt, M. van der, Gasification. Gulf Professional Publishing. Leckner, B., Development of Fluidized Bed Conversion of Solid Fuels History and Future, in: 22nd Fluidized Bed Conversion Proceedings. Turku, Finland. Meyers, R.A., Handbook of synfuels technology. McGraw-Hill. NIST Data Gateway [WWW Document], URL (accessed ). Power Plant / Home / Lahti Energia [WWW Document], URL (accessed ).