Mineral Nutrient Recovery from Pyrolysis Systems

Jatara Wise,

a

Donald Vietor,

a

Tony Provin,

a

Sergio Capareda,

b

Clyde Munster,

b

and Akwasi Boateng

c

a

Texas A&M University, Soil & Crop Sciences, College Station, TX; [e-mail] (for correspondence)

b

Texas A&M University, Biological And Agricultural Engineering, College Station, TX

c

United States Dept. of Agriculture-Agricultural Research Service (ARS), Eastern Regional Research Center, Wyndmoor, PA.

Published online 27 March 2012 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ep.11631

Bioenergy plants such as sorghum, bioenergy rice, corn

stover, and switchgrass can be thermochemically converted by
pyrolysis to produce bio-oil, synthesis gas from noncondensable
gases, and biochar. The biochar fraction can be recycled back
to the production field to improve soil physical qualities and
nutrient status. Although previous publications have described
the beneficial effects of pyrolysis biochar on soil physical
properties; relatively little has been published on the recovery
of mineral nutrients from pyrolysis co-products. This work
quantified the recovery of nutrients (P, K, Ca, and Mg) from
pyrolysis coproducts from various feedstocks using two distinct
reactors. Nutrient mass balances, on a feedstock basis, were
calculated for comparison of the two reactors’ efficiency in the
recovery of the nutrients. The results revealed the recovery
of nutrients varied by (1) species, (2) reactor design, and (3)
correlated highly with nutrient mass loss in biochar. Computa-
tions also revealed P recoveries of 93% (fixed-bed reactor) and
58% and 55% (fluidized-bed reactor) for pyrolyzed sorghum.
The recovery of key mineral nutrients in pyrolysis coproducts
(primarily biochar) is directly related to the feasibility of
nutrient recycling through biochar.

Ó 2012 American Institute of

Chemical Engineers Environ Prog, 31: 251–255, 2012*

INTRODUCTION

Pyrolysis is defined as the thermochemical decomposition

of organic matter in a near oxygen-free environment to pro-
duce liquid, gaseous, and solid coproducts. These respective
coproducts are termed bio-oil, synthesis gas (molecular
hydrogen and carbon monoxide fractions), and biochar. The
pyrolysis of agriculturally grown bioenergy crops is currently
being researched as one method for the generation of fluid
transportation fuels. The resulting solid biochar is being con-
sidered for the sequestration of carbon and as a soil amend-
ment. The bio-oil from biomass pyrolysis can be used
directly in home heating applications and large-scale power
plants or catalytically upgraded to yield specialized fuels and
chemicals. The noncondensable gaseous (NCG) coproduct of
biomass pyrolysis comprises mostly H

2

(molecular hydro-

gen), CO (carbon monoxide), CO

2

(carbon dioxide), CH

4

(methane), and C

2

H

6

(ethane). The combination of H

2

and

CO makes up what is commonly known as synthesis gas
(syngas) or producer gas.

A limited number of studies have focused on the recovery

of plant essential nutrients in the biochar. High recoveries of
P and K were reported for biochar derived from fluidized-
bed, fast pyrolysis of corn cob and stover feedstocks [1]. In
contrast, Schnell et al. [2] observed low recovery of K and

other nutrients when slow pyrolysis of sorghum biochar was
soil applied [2]. Recovery of

<10% of biomass K in biochar

derived from slow pyrolysis indicate K and other mineral
nutrients could be lost through condensed bio-oil and NCG’s
[2]. In contrast, studies of fast pyrolysis of stored hybrid
poplar, switchgrass, and corn stover feedstocks indicated
feedstock concentrations of P and K increased during feed-
stock aging [3].

Contrasting nutrient recoveries in biochar derived from py-

rolysis suggest definitive evaluations of mineral nutrient
recovery in biochar and other pyrolysis coproducts are needed
[1,2]. Studies of the fate of minerals during related combustion
and thermochemical conversion processes indicate losses
through airborne aerosols could occur. Buseck and Po

´sfai [4]

studied the effects of biomass burning on greenhouse gases
and climate change and concluded that between 20% and
50% of soot particles from both the Southern Ocean and Izana
regions contained significant K [4]. Li et al. [5] reported that
potassium salt particles were the most abundant inorganic aer-
osol constituent in the smoke from biomass burning [5].

While a number of investigators have found high recovery

of plant nutrients within the biochar, the existence of other
studies indicating significantly lower conservation of plant
nutrients within the biochar dictates further evaluation is
needed. This work aims to quantify the recovery of nutrients
(P, K, Ca, and Mg) from pyrolysis coproducts from various
feedstocks using two distinct reactors types.

MATERIALS AND METHODS

Preparation of Feedstock

High-energy

sorghum

(Sorghum bicolor),

switchgrass

(Panicum virgatum), corn stover (Zea mays), and high bio-
mass rice (Oryza sativa) were grown and harvested under
field conditions. Sorghum, switchgrass, and corn stover were
grown on the Texas AgriLife Research Farm near College
Station, TX, and rice was obtained from the Texas AgriLife
Research and Extension Center near Beaumont, TX. Harvesting
occurred in July 2009 for corn stover, September 2009 for sor-
ghum and rice, and January 2010 for switchgrass. Biomass was
rough-chopped to a length of 3 cm during harvest, air-dried,
then oven-dried (60

8C) before grinding to pass a 2-mm screen.

Before pyrolyzing, biomasses were redried at 65

8C.

Pyrolysis Using Fixed-Bed Pyrolyzer

A fixed-bed reactor system was used for slow pyrolysis of

feedstock samples under controlled conditions. The sealed
pyrolysis reactor comprised a 75-cm-long stainless steel pipe
(2.5 cm diameter) enclosed within a Thermolyne

Ò

tube

furnace. Compressed nitrogen (N

2

) entered the reactor pipe

Ó 2012 American Institute of Chemical Engineers *This article is a

US Government work and, as such, is in the public domain in the
United States of America.

Environmental Progress & Sustainable Energy (Vol.31, No.2) DOI 10.1002/ep

July 2012

251

and flowed across the sample containing boat. Bio-oil was
condensed in a glass beaker seated within an ice-cooled PVC
chamber. NCG was bubbled through two test tube traps con-
taining 1 N HCl in an attempt to capture aerosols containing
mineral nutrients.

Pyrolysis Using Bench-Scale Fluidized-Bed Reactor

The corn stover, sorghum, and switchgrass biomasses were

pyrolyzed using a bench-scale fluidized-bed fast pyrolysis
reactor. The reactor, located at the Eastern Regional research
Center in Wyndmoor, PA, comprised a state-of-the-art reactor

vessel system and associated auxiliary systems for biomass
feeding and injection, char collection, vapor condensation for
bio-oil recovery, and instrumentation for data acquisition and
control [6].

Biochar, Bio-Oil, and Gas-Trap Processing

For the fixed-bed reactor system, six subsamples of

each feedstock were pyrolyzed and biochar in combustion
boats was composited to represent a replication. Similarly,
bio-oil collected from the chilled beaker and acetone
washes of condenser and plumbing surfaces were compos-
ited to represent a replication for each feedstock source.
The acetone in the acetone–bio-oil mixture was evaporated
at room temperature before bio-oil analysis. The bio-oil
was ashed in porcelain crucibles at 800

8C for 4 h before

dissolution in 4 mL of concentrated HCl and brought to
volume with deionized water. Inductively coupled plasma
optical emission (ICP) spectroscopy was used to analyze
subsamples of each volume. A 200-mg sample of biochar
was ashed and similarly analyzed. Contents of the acid
trap for NCG’s were collected, diluted with deionized
water, and analyzed using ICP analysis. All biomass and
fluidized-bed reactor biochar was analyzed for nutrients
using a sulfuric acid digest [7], while all bio-oil was ana-
lyzed using the aforementioned ashing method and (ICP)
trace metal analysis.

For

the

fluidized-bed

system,

each

feedstock

was

pyrolyzed three times resulting in three replications for each
feedstock. Biochar and bio-oil from each replication was
collected and stored in separate containers. Bio-char was
analyzed for trace metals using both of the aforementioned
ashing (ICP) and sulfuric acid digestion methods, whereas
bio-oil was only analyzed for trace metals using the ashing
(ICP) method.

Statistical Analysis

For both reactor analyses, analysis of variance along with

Student’s t test was used for means comparison amongst
feedstocks. In addition, correlation analysis was used to
determine the degree of linear relationship between variables
(JMP, Version 8. SAS Institute, Cary, NC, 1989–2007).

Figure 1. Scatter plot matrix for correlation between DP
and mean percent recovery of total P (fixed-bed pyrolysis).
[Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]

Table 1. Mean percent recovery of total feedstock P, K, Ca, and Mg in pyrolysis co-products from fixed-bed slow pyrolysis
(ashing/solubilization method).*

,

**

Species

%P

Std Dev

%K

Std Dev

% Ca

Std Dev

%Mg

Std Dev

Biochar co-product

Corn stover

49.90b

20.50

30.10a

12.80

60.80a

26.70

61.50a

26.10

Sorghum

90.50a

20.30

4.80c

1.80

55.90a

12.60

38.30b

12.20

Rice

52.10b

3.40

18.40b

0.90

40.70b

2.80

45.10b

2.50

Bio-oil co-product

Corn stover

0.10b

0.10

0.06b

0.04

0.26b

0.11

1.70a

1.45

Sorghum

0.20a

0.20

0.07b

0.00

5.63a

5.69

2.77a

1.99

Rice

0.10b

0.10

0.09a

0.05

0.57b

0.37

2.41a

1.56

NCG co-product

Corn stover

0.16b

0.10

0.09a

0.03

2.09b

0.63

0.06b

0.04

Sorghum

1.00a

1.23

0.12a

0.11

5.41a

3.16

2.92a

1.41

Rice

0.11b

0.06

0.08a

0.02

4.34ab

4.17

0.08b

0.09

Co-products combined

Corn stover

50.09b

20.54

30.25a

12.77

63.18a

26.62

63.30a

25.42

Sorghum

93.63a

20.00

5.02c

1.75

68.89a

11.33

44.04b

13.12

Rice

52.22b

3.36

18.57b

0.96

45.63b

5.68

47.61b

3.17

*

Lower case a, b, and c indicates statistical differences (P

5 0.05).

**

NCG

5 non-condensable gas.

252

July 2012

Environmental Progress & Sustainable Energy (Vol.31, No.2) DOI 10.1002/ep

RESULTS AND DISCUSSION

Fixed-Bed Reactor Study

The recovery of total feedstock P, K, Ca, and Mg varied

amongst species (Table 1). The recovery of total P in sor-
ghum was statistically higher than corn stover or switchgrass.
The recovery of total K in corn stover was statistically higher
than sorghum or rice. The low recoveries of K in sorghum
are similar to those observed by Schnell et al. [2]. The high
ashing temperature has been suggested to result in K volatili-
zation [8]. Husmoen [9] utilized identical methodology
(i.e., ashing/solubilization) for the analyses of sorghum
biochar and reported

>85% recovery of both P and K. The

recovery of total Ca also varied amongst species. Ca recovery
for corn stover and sorghum was statistically similar and
higher than rice. Total recovery of Mg also varied amongst
species. For Mg, corn stover was statistically higher than
sorghum and rice, which were statistically similar.

To demonstrate that total mineral nutrient recovery was

related to the amount of nutrient mass lost during conversion
from dry feedstock to biochar a parameter, (

Dnutrient) was

calculated. For phosphorus, (

DP) was defined as the mass

loss of phosphorus (wt %) during conversion to biochar from
dry feedstock. For the fixed-bed system, correlation analysis
was done between mean percent recovery of total P (total
P of all three coproducts) and

DP.

The Pearson’s correlation coefficient in Figure 1 suggests

a linear relationship between

DP and mean percent recovery

of total P. High Pearson’s correlation coefficients were also
found for the remaining macronutrients which suggested
potential linear relationships. Similar results were found for
the fluidized-bed system.

For this study, percent mean recovery varied amongst

species for a given nutrient. However, as mentioned above,
low mean percent recovery of K in sorghum has been
observed in similar studies. In addition, mass balances
revealed low recovery of mass. For corn stover, sorghum, and
rice stover coproduct mass balances were 29%, 36%, and 37%,
respectively. The varying results indicate that optimal nutrient
recovery is not exclusive to a single species. Hence, a given
nutrient’s recovery in biochar following pyrolysis is specific to
feedstock and potentially to reactor operating conditions.

Fluidized-Bed Reactor Study

The recoveries of P, K, and Ca in each biochar and bio-oil

were statistically similar for corn stover and sorghum (Table
2). For biochar, corn stover had similar mean percent recov-
eries of P whereas corn and sorghum had similar recoveries
for K. Both corn and sorghum had higher recoveries of Ca
and Mg than switchgrass. Overall, the nutrient recoveries
were significantly lower than those reported by Mullen et al.

Table 3. Mean percent recovery of feedstock P, K, Ca, and Mg in pyrolysis co-products from fluidized-bed fast pyrolysis
(ashing/solubilization method).*

Species

%P

Std Dev

%K

Std Dev

% Ca

Std Dev

%Mg

Std Dev

Biochar co-product

Corn stover

63.35a

25.52

56.95a

0.67

61.89a

15.13

53.60a

0.23

Sorghum

53.26a

11.52

52.38a

4.92

59.05a

9.69

32.40b

6.33

Switchgrass

34.32b

8.37

4.02b

0.45

45.14b

7.27

21.67c

1.95

Bio-oil co-product

Corn stover

5.23a

1.96

2.05a

0.65

5.30a

1.31

7.74a

2.04

Sorghum

2.03a

2.00

0.86a

0.63

2.27a

1.91

3.28a

1.69

Switchgrass

5.49a

4.35

2.12a

1.50

6.09a

4.71

7.66a

4.46

Biochar and bio-oil combined

Corns stover

68.58a

3.56

59.03a

4.01

67.19a

6.82

61.34a

1.81

Sorghum

55.30a

9.90

53.24a

4.65

61.32a

8.74

35.68b

6.62

Switchgrass

39.81b

6.86

6.14b

1.47

51.23b

4.26

29.33c

3.18

*

Lower case a, b, and c indicates statistical differences (P

5 0.05).

Table 2. Mean percent recovery of feedstock P, K, Ca, and Mg in pyrolysis co-products from fluidized-bed fast pyrolysis
(sulfuric acid digestion method).*

Species

%P

Std Dev

%K

Std Dev

% Ca

Std Dev

%Mg

Std Dev

Biochar co-product

Corn stover

65.44a

16.77

53.08a

3.90

63.17a

10.62

57.80a

2.97

Sorghum

56.52a

9.74

53.95a

8.33

57.88a

8.18

35.97b

5.29

Switchgrass

30.05b

6.68

8.88b

1.20

38.49b

5.84

14.32c

1.44

Bio-oil co-product

Corn stover

5.23a

1.96

2.05a

0.65

5.30a

1.30

7.74a

2.04

Sorghum

2.03a

2.00

0.86a

0.63

2.27a

1.91

3.28a

1.69

Switchgrass

5.49a

4.35

2.12a

1.50

6.09a

4.71

7.66a

4.46

Biochar and bio-oil combined

Corns stover

70.78a

14.81

55.13a

3.25

68.47a

17.67

65.54a

7.10

Sorghum

58.55a

8.45

54.81a

8.04

60.15a

4.72

39.26b

3.55

Switchgrass

35.54b

9.57

10.99b

1.72

44.58b

5.66

21.98c

0.97

*

Lower case a, b, and c indicates statistical differences (P

5 0.05).

Environmental Progress & Sustainable Energy (Vol.31, No.2) DOI 10.1002/ep

July 2012

253

[1], who reported corn biochar P and K recoveries of 102
and 89%, respectively.

Recoveries of P, Ca, and Mg in biochar (Table 3) were

similar for each feedstock when compared with results in Ta-
ble 2. However, recovery of K in biochar remained low
amongst all tested species. When methods are compared in
Tables 2 and 3, the combined coproduct recoveries for P, K,
Ca, and Mg did not differ vastly for the two analysis meth-
ods, again further suggesting that the use of either method
was not the reason for low recovery of nutrients.

The recovery of key plant nutrients within the biochar

and lack of these nutrients in the bio-oil of all three suggests
that nutrients are being lost through the gas phase or due to
‘‘slagging’’ in the reactor piping. The fluidized-bed fast pyrol-
ysis setup did not allow for elemental analyses of the the
NCG products. The very low recovery of P, K, Ca, and Mg in
the switchgrass suggests that recovery of overall nutrients
and a given nutrient is species dependant. When both reac-
tors are compared, fixed-bed slow pyrolysis had the highest
recovery of sorghum P in pyrolysis coproducts; whereas the
corn stover coproducts from the fluidized-bed fast pyrolysis
system had the highest recovery of P. High recoveries of P in
sorghum could have been a result of higher carbohydrate
concentration in the relatively green sorghum, whereas the
other feedstocks had reached senescence. Calcium and Mg
recovery in corn stover and sorghum was similar for both
reactors, while recovery of K for both reactors was low.
Combined biochar and bio-oil mass balances for this system
were 47% for sorghum and 57% for both corn stover and
switchgrass. In a study of the combustion of softwood saw-
dust by Boman, it was summarized that K losses could be a
result of K vaporization into KCl, (KCl)

2

, K

2

SO

4

, and KOH

during the pyrolysis or ashing processes [10]. Other studies
have found that considerable amounts of K was found in fil-
tered Beech wood pyrolysis oil, which likely came from the
vapor phase [11]. Losses due to K vaporization escaping with
NCG’s could also be a possibly related to the N

2

flow rates

by both reactors. Another reason for K losses could result
from the lack catalyst anchoring sites, such as carboxyl and
hydroxyl groups, which are important for preventing K loss
during pyrolysis and gasification [12]. There are a wide range
of additional factors that could also influence the recovery of
mineral nutrients. Factors such as reactor O

2

content [13] and

the cooling effect of water vapors on solid pyrolysis copro-
ducts [14] can negatively affect nutrient recovery. Other fac-
tors such as low Si/K ratio, especially for bioenergy rice, can
cause slagging and result in nutrient losses within the copro-
ducts [15]. High moisture content of pyrolysis biomass could
also adversely affect the recovery of mineral nutrients [16].
Other factors not evaluated in this study that might address
why recoveries of these key plant nutrients were low include
(1) physiological maturity of feedstock, (2) overall nutrient
profile of feedstock, (3) growing conditions of the feedstock,
and (4) drying, grinding, and overall handling of feedstock
before pyrolysis. In addition, evaluation of complete mass
balances needs further investigations. These factors likely
contribute to significant geographically specific biochar nutri-
ent conservation.

CONCLUSIONS

The recovery of P, K, Ca, and Mg varied among feed-

stocks for both systems. Volatilization losses from either the
pyrolysis process or the high temperature nutrient analysis
ashing process could reduce recovery of nutrients, including
P and K. Low recovery of mineral nutrients could also be a
result of low mass recovery for both systems. From this
study, it can be concluded that the recovery of feedstock
nutrients: (1) varies amongst species and (2) reactor design,
and (3) is highly correlated to feedstock nutrient mass loss in
the biochar coproduct. However, if the primary goal is the

recycling of P and K through biochar derived from sorghum
and corn stover, either system is suitable. Fixed-bed slow py-
rolysis is potentially more effective for the recovery of P in
sorghum biochar than fluidized-bed fast pyrolysis.

ACKNOWLEDGMENTS

Fellowship support of Jatara Wise was provided from the

United States Department of Agriculture National Needs Fel-
lowship in Bioenergy. In addition, partial support was pro-
vided the Department of Energy North Central SunGrant Pro-
gram, the Alfred P. Sloan Foundation Minority PhD program,
and the Hispanic Leaders in Agriculture and the Environment
(HLAE) program at Texas A&M University.

Abbreviations

ICP

Inductively coupled plasma optical emission

NCG

Noncondensable gases

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