Biochar Adsorber Design (graphical method)

 

 

 

Here we present a graphical method for estimating the design and operational parameters of biochar adsorbers for use in water treatment.

 

We have solved the design equations for a range of typical throughputs, char filter sizes, contact times and replacement frequencies, and presented the results in graphical format on logarithmic plots. The calculation methodology and assumptions are explained in detail below.

 

The user can locate their design parameters on the appropriate plot (color-coded by char type) and then determine char filter bed size or char replacement frequency based on their treated water requirement.

 

First, choose the option that best describes your char and download the corresponding plot:

high temperature (≥850 °C) gasifier char

intermediate temperature (550-850 °C) gasifier or retort char

low temperature (350-550 °C) retort char: not recommended

 

To find the minimum char replacement frequency for your desired char contactor volume: Locate your treated water requirement in units of liters per day on the y-axis. Follow this (colored) line horizontally until it intersects the (colored) vertical line corresponding to the volume of your char filter bed. The nearest black line above and to the left of this intersection corresponds to the minimum char replacement frequency.

 

To find the minimum volume of char needed to meet your treated water requirements at a given char replacement frequency: Locate your treated water requirement in units of liters per day on the y-axis. Follow this (colored) line horizontally until it intersects the black line corresponding to the desired char replacement frequency. (Some interpolation may be necessary if no black line exactly corresponds to your desired char replacement frequency.) Trace a vertical line down from this intersection to the x-axis. The nearest vertical line to the right of this intersection corresponds to the minimum volume of char needed (in units of liters).

 

Important Considerations

 

Low temperature chars     We do not recommend using low temperature (350-500 °C) retort char for water treatment purposes. Our research indicates that it has low adsorption capacity for trace organic chemical contaminants. For this reason, char contactor design and operation can be impractical and/or uneconomic. Also, low temperature char contains non-fully-carbonized compounds that can leach into water, causing undesirable taste, odor, and appearance.

 

Intermediate and high temperature chars     According to our research, high temperature (≥ 850 °C) chars, in particular those produced from high-draft top-lit gasifiers, have approximately ten times the dissolved organic contaminant adsorption capacity of intermediate temperature (500-850 °C) chars. This can make a great difference in the practicality and economics of building and operating biochar adsorbers. Our field experience suggests that high temperatures can be reliably achieved using small forced draft gasifier cookstoves with pelletized feedstocks, and using 55-gal drum pyrolyzers (such as the basic drum oven and “JRO” modified models described in our handbooks) with woody feedstocks chopped into approximately 1-2″ diameter by 4-6″ long segments, or 2-3″ blocks. If the capacity exists to measure temperature within the char bed (preferably at multiple locations) throughout the burn, this is the best method for ascertaining char quality in the field. In lieu of direct temperature measurement, we have also observed mass loss during pyrolysis (mass of char product / mass of raw feedstock) to be a reliable proxy for pyrolysis temperature. Optimal high temperatures correspond to 85-90% mass loss, while intermediate temperatures correspond to 75-80% mass loss. (Some adjustments may be made in consideration of the initial moisture content of the feedstock. However, this may be difficult to know with precision in the field. The general rule is that drier feedstocks are better.)

 

Contact time between char and water     In addition to char adsorption capacity, the amount of contact time between water and char is also an important factor in contaminant removal. Our ongoing research is establishing the optimal range of contact time for different adsorber system parameters. For now, we have taken a conservative approach using contact times that are much longer than those typically used for analogous systems employing granular activated carbon (GAC). We used empty bed contact times (EBCTs) of 2.5-12.5 hours to generate the plots provided above. Contact times shorter than this might be insufficient for achieving a high level of treatment. (Consider, for example, that passing twice as much water through the same volume of char while doubling the replacement frequency results in half the contact time.) Contact times longer than ~12 hours are likely to be uneconomic given the size of the char adsorber required for a given treated water requirement. Very long contact times (one to several days) may also promote “stagnant” (anaerobic) conditions within the char, leading to undesirable taste and odor in the water and potential pathogen re-growth. Replacement frequencies longer than a few years may present management challenges, especially in unstable communities or when community water managers frequently emigrate. For these reasons, replacement frequencies less than 1 year and greater than 5 years do not appear on the design plot for high temperature char, and frequencies less than 1.5 months and greater than 6 months do not appear on the plot for intermediate temperature char.

 

Char particle size     An important factor influencing the uptake of trace organic contaminants by char is particle size. Small char particles exhibit more efficacious uptake (in scientific terms, improved adsorption kinetics) of dissolved contaminants than large particles, owing to their greater exposed surface area and shorter distance of travel for contaminants migrating into pores. Thus char for use in water treatment should be crushed and sieved, ideally retaining the approximately 1-5 mm size fraction. This particle size range is larger than typical for GAC systems (0.5-2 mm) in consideration of practical challenges often encountered in developing community and disaster relief contexts. Efforts should be made to reduce char particles at least below 10 mm, if at all possible. Very fine char particles (powder) may lead to clogging, and so ideally should be removed by sieving and put to other uses (e.g. soil amendment, eco-sanitation cover material, etc.).

As a further illustration of the role of char particle size in adsorber design, consider the following simplified approximation: Adsorption kinetics typically scale to the inverse of the particle diameter squared. This means that for every doubling of particle size, adsorption processes are slowed by a factor of four. This necessitates proportionally longer contact times (EBCTs) to achieve similar levels of contaminant uptake. As a simplified approach, we can apply a scaling factor comparing a range of char particle sizes to standard GAC adsorbers with average particle size approximately 1.15 mm and operated at an EBCT of 10 minutes. Considering the recommended char particle size range of 1-5 mm and making the conservative assumption that the average particle size is skewed well towards the larger end of the range (4.5 mm), we calculate a scaled EBCT of 2.55 hours. Considering the larger char particle size range of 1-10 mm and making a similar conservative assumption that the average particle size is skewed well towards the larger end of the range (9.5 mm), we calculate a scaled EBCT of 11.4 hours. These modeling results are in agreement with the recommended EBCTs for char adorbers in the field (2.5-12.5 hours), and stress the importance of crushing and sieving char to a relatively small size range whenever practicable. The plot below depicts scaled EBCTs as a function of average char particle size using this scaling method.

 

 

The use of engineering “safety factors”     Calculations presented here do not include proportional “safety factors.” A conservative approach to filter design and operation is always advised. Our recommendation is to increase char volume for a given throughput and replacement frequency, or increase replacement frequency for a given throughput and char volume, by at least 25%.

 

General disclaimer     The effective lifetime of the char filter media depends upon the quality of the char (a function of the pyrolysis temperature and feedstock), as well as the characteristics of the source water (e.g., dissolved background organic matter), and the efficacy of upstream treatment steps (such as gravel and bio-sand filters). In the rural developing community or disaster relief contexts, these factors are typified by high degrees of variability and uncertainty. Since char can be generated locally and inexpensively, a conservative approach is always recommended, designing for a large filter size and frequent char replacement.

The biochar adsorber design estimates derived using methods presented here should be taken as rough guidelines. Our ongoing research is refining filter system design specifications and recommended operation protocols. However, it is ultimately up to the discretion of the community water system operator(s) to consider factors such as variability in community water demands and seasonal source water quality concerns (e.g. turbidity and dissolved organic matter increase during the rainy season, local agricultural cycles and pesticide application periods, nearby industrial development that may impact sourcewater, etc.) in determining an appropriate char filter size and replacement frequency for each installation.

 

Calculation Methodology

 

Based on our laboratory research and fieldwork, we make the following conservative assumptions:

high temperature (≥850 oC) gasifier char: use rate 50 mg/L

intermediate temperature (550-850 oC) gasifier or retort char: use rate 500 mg/L

low temperature (350-550 oC) retort char: use rate 5000 mg/L

char bed density ≈ 175 mg/L

optimal empty bed contact time (EBCT) range 2.5-12.5 hours

 

Equation 1

Char lifecycle (in units of bed volumes, BV) = bed density (mass per volume) / char use rate (mass per volume)

 

Equation 2

Char lifecycle (in units of time) = char lifecycle (in BV) x bed volume / flow rate (volume per time)

 

Equation 3

Char bed size = char lifecycle (in units of time) x flow rate / char lifecycle (in units of BV)

 

Equation 4

Empty bed contact time (EBCT) = volume of char bed / flow rate (volume per time)

 

 

Sample Calculations

 

Consider high temperature gasifier char (use rate 50 mg/L), and 175 kg/m3 (g/L) bed density.

 

Equation 1:                     175 g/L / 0.050 g/L = 3500 bed volumes (BVs)

 

Scenario (a): Specify char filter size and calculate replacement frequency. Assume treated water requirement (flow rate) = 4,750 L/day, and char filter bed volume = 1 m3 (1,000 L).

 

Equation 2:                     Char lifecycle = (3,500 BV x 1,000 L) / 4,750 L/day = 24.2 months

Char replacement every 2 years.

 

Equation 4:                     EBCT = 1,000 L / 4,750 L/day = 5.1 hours

Check: 2.5 hours ≤ 5.1 hours ≤ 12.5 hours ? OK

 

Scenario (b): Specify replacement frequency and calculate char filter size. Assume flow rate = 2,000 L/day, and desired char replacement frequency = 1 year.

 

Equation 3:                     Char filter bed volume = (365 days x 2,000 L/day) / 3,500 BV = 209 L

Char filter bed size required = 209 L or about 37 kg.

 

Equation 4:                     EBCT = 209 L / 2,000 L/day = 2.5 hours

Check: 2.5 hours ≤ 2.5 hours ≤ 12.5 hours ? OK

 

Scenario (c): Specify replacement frequency and calculate char filter size. Assume flow rate = 2,000 L/day, and desired char replacement frequency = 3 months (91 days).

 

Equation 3:                     Char filter bed volume = (91 days x 2,000 L/day) / 3,500 BV = 52 L

Char filter bed size required = 52 L or about 9.1 kg.

 

Equation 4:                     EBCT = 52 L / 2,000 L/day = 0.62 hours = 37 minutes

Check: 2.5 hours ≤ 37 minutes ≤ 12.5 hours ? NO – START OVER