(575f) Functionalization Using Minerals and Reactivity of Gasification Biochars

Introduction

A
challenge of this century is to develop industrial processes using renewable
energies to replace fossil fuels and thus to reduce our impacts on the
environment. High energetic gas called ?syngas? produced from the
thermochemical conversion of biomass and waste is considered as
environmental-friendly energy. This syngas composed of CO and H₂ at
convenient ratio, could be used for production of fuels or chemicals (ex.
diesel, methanol, ethanol), for steam generation via combustion boilers, or for
electricity production in a gas turbine or fuel cell. However the industrial
development of this gasification process is hold back by the production of
unwanted by-products: tars and char/ash (aromatic hydrocarbons and solid
residues respectively) which decrease the global efficiency yield and require
very costly treatments [1, 2]. At gasification temperature (ca. 700 to 900°C)
all co-products are in their gaseous forms while downstream the reactor some
co-products called tars start to condense which leads to fouling and blocking
process equipments as turbines and engines [3]. The gas phase is also
contaminated by co-products and pollutants which may contribute up to 20% of
the volatile material [4]. Therefore some recent studies investigated potential
applications to recycle these by-products and thus to reduce their economic
impacts. The challenge was to rely on the fact that these biochars from natural
feedstock already contain organic, minerals and more specifically metals which
make them very attractive for catalytic or sorbent applications. In this
context, the main options are to catalytically reform tar over biochars, or to
use biochars for hot gas cleaning. Biochars from gasification have recently
been recognized to be cheap candidates for catalytic applications related to
tar reforming and hot gas cleaning [4-10].These microporous materials of high specific
surface area (570 m²/g) contain active sites, such as endogenous Alkali
(Li, Na, K..), Alkaline Earth Minerals (Mg, Ca?), namely AAEM, and oxygen atoms.
Functionalization of the biochars surface is an opportunity for further development
as an inexpensive catalyst from renewable resources. The functionalization may
impact the carbonaceous matrix structure, the content of O-groups as well as
the increase in mineral content [9-10]. This study is focused on the impact of AAEM
content towards the reactivity of the functionalized biochars in methane
cracking.

Materials
and Methods

Chips
of poplar wood have been impregnated with Ca, K and a mixture of both of them
to increase their concentrations. The impregnation was carried out at room
temperature with concentrated aqueous solutions using KNO₃ and Ca(NO₃)₂.
Biochars from gasification were obtained using raw poplar wood and impregnated
wood in a steam atmosphere (90 vol% H₂O / 10 vol% N₂).

The
mineral composition in biochars was measured using X-ray fluorescence. Biochars
were also characterized using Scanning Electron Microscopy (Philips XL30 FEG)
to observe the texture and the distribution of minerals at the surface. The
porosity was measured with BET equipment (ASAP 2010, Micromeritics).

Methane
cracking experiments have been performed at 700°C in flow through micro-reactor
(ChemBet Pulsar-model 05090) under a mixture of 10 vol% CH₄ / 90% N₂
and a flow rate of 80 mL/min. prior to methane cracking, biochar was heated to
700°C at 20°C/min in N₂ atmosphere.

Results
and discussion

The
impregnation led to the increase of K and Ca concentrations from 0.6 wt% in the
raw biochar to 5 wt.% in the K-char and Ca-char respectively. In the sample
where Ca and K are impregnated simultaneously, the concentration of K and Ca
reached 2.2 wt.% in the biochar (Ca+K-char). The results show that the biochar
surface is coated with K and Ca resulting to the decrease of its porosity and
specific surface area. Indeed the specific areas are of 574, 504, 413 and 322 m²/g
for raw char, Ca-char, K-char and Ca+K-char respectively. It has been also seen
that the affinity of K towards the biochar surface is stronger than that of Ca
as the coating is more efficient (Figure 1). Indeed, the bright spots which are
due to minerals are strongly linked to the surface for K-char (b). The presence
of K in the impregnated Ca+K-char significantly improve the bounding of Ca at
the surface as shown in Figure 1 (c). These results are in agreement with BET
measurements as the specific surface area has been decreased by 12, 38 and 44%
for the Ca-char, K-char and Ca+K-char respectively.

Figure
1: SEM pictures of Ca-char (a), K-char (b) and Ca+K-char (c)

The
biochars produced have been used as a catalyst for methane cracking. The
efficiency of the conversion was evaluated following the production of
hydrogen. Figure 2 shows the cumulative H₂ production during the CH₄
cracking over the 4 biochars. Impregnated biochars demonstrated a catalytic
activity as the methane cracking was improved. The reactivity of the K-char
towards the conversion of methane into hydrogen is 4 times higher compared to
raw biochar. The Ca+K-char was more efficient than the Ca-char and the raw-char,
but less efficient than the Ca+K-char. The total content of K in the Ca+K-char
was lower than in the K-char as mentioned previously. In the literature, the
reactivity of AAEM in biochars was studied and 3 groups were established [4].
Group I corresponds to a biochar where the K+Na molar concentration is higher
than that of Ca. The reverse situation corresponds to the group II while group
III highlights high concentrations of Si. Literature shows that group I is more
efficient than the other ones towards catalytic efficiency [4]. The results obtained
are in agreement with the literature as the K-char belongs to group I while the
other ones should be considered in group II [4].

Figure
2: Cumulative hydrogen production over raw and impregnated chars during methane
cracking.

Conclusions

The
goal of the work was to use biochars from gasification for functionalization
using minerals and to compare their reactivity for methane cracking at 700°C. The
impregnation led to the increase of K and Ca concentrations from 0.6 wt% in the
raw biochar to 5 wt.% in the K-char and Ca-char respectively. In the sample
where Ca and K are impregnated simultaneously, the concentration of K and Ca
reached 2.2 wt.% in the Ca+K-char. The results show that the biochar surface is
coated with K and Ca resulting to the decrease of its porosity and the specific
surface area decreased by 12, 38 and 44% for the Ca-char, K-char and Ca+K-char
respectively. Although the specific surface area of the K-char was
significantly reduced the reactivity was the highest towards methane cracking.
The rank of efficiency is as follow K-char>Ca+K-char>Ca-char>raw char.
These results highlight the role and the type of metal ions on the catalytic
activity and are in agreement with literature where the high content of K
compared to Ca improves the catalytic reactivity.

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