Biochar amendment for improved and more sustainable peat stabilisation

Abstract

Carbon-intensive binders such as cement are traditionally employed to stabilise peat. Few studies have investigated alternative materials such as biochar to improve peat stability while simultaneously sequestering carbon dioxide (CO2). This study explored biochar produced through pyrolysis of clean wood and leaves to stabilize peat from Tiller-Flotten, Norway. Unconfined compressive strength (UCS), water content and pH measurements on biochar, Portland composite cement (CEM II) and peat compositions and a sustainability assessment were conducted. It was found that biochar amendment increased strength and stiffness of peat and cement-stabilised peat. Biochar showed the potential to reduce the cement amount when stabilising peat while retaining geotechnical properties. Peat stabilised with 200 kg/m3 of biochar and 100 kg/m3 of cement exhibited comparable strength (63.3±4.2 kPa, n = 3) as samples with 200 kg/m3 of cement (63.2±1.3 kPa, n = 3), but with a negative carbon footprint. Adding biochar quantities greater than 27% of the cement quantities resulted in a climate-neutral stabilisation. At a carbon price of approximately 85 €/tonne, the biochar costs equalled the cement costs. The cement-only samples outperformed the ones with additional biochar in terms of shear strength/€, while future carbon prices increased the competitiveness of biochar amendments.

Stabilisation of Soft Clay, Quick Clay and Peat by Industrial By-Products and Biochars

“…This implies that stabilisation with biochar improves stiffness to a lesser degree than strength. For the peat specimens, the ratio E 50 /τ max varied between ~40 and ~100, independent of IBP or biochar, in line with previous studies on biochar in peat [27]; however, there was also an exceptionally low ratio for the CEM-PSA-specimen, where E 50 /τ max was ~13. The Tiller-Flotten clay (see Figure 4c,d) showed a considerably greater strength development, where 𝜏 ranged from just below 200 kPa up to approximately 550 kPa.…”

“…The biochars had varying w, ρ s and SFA. The high SFA of the biochars BC1, BC2 and BC3 was expected [26,27].…”

“…Table 1 shows the complete laboratory strength testing programme. Based on the results from previous research [27,29,46], different binder quantities (α) and combinations were used for the clays and peat and for the IBPs and biochars. For the clays stabilised with IBPs, mixtures of 50 wt.% CEM and 50 wt.% IBP were used, with a total α of 60 kg/m 3 .…”

“…Biochar is manufactured by the pyrolysis of biomass, i.e., combustion in an inert atmosphere (i.e., the absence of oxygen), resulting in a non-biodegradable carbonaceous material with typical carbon contents exceeding 75%. It is characterised by a high porosity and large surface area and, hence, a high water-absorption capacity [26,27], and has several potential benefits, including soil fertility improvement and contaminant immobilisation [28]. Recent studies have also shown that biochar in mixtures with cement can have a positive stabilisation effect in soils [27,[29][30][31][32][33][34][35][36][37][38][39], meaning that it can be used as an alternative binder along with its carbon sequestration potential.…”

“…It is characterised by a high porosity and large surface area and, hence, a high water-absorption capacity [26,27], and has several potential benefits, including soil fertility improvement and contaminant immobilisation [28]. Recent studies have also shown that biochar in mixtures with cement can have a positive stabilisation effect in soils [27,[29][30][31][32][33][34][35][36][37][38][39], meaning that it can be used as an alternative binder along with its carbon sequestration potential. Since 1.0 kg of biochar contains a minimum of approximately 0.75 kg of carbon (C), it can sequester over ~2.7 kg of CO 2 eq as no decomposition or erosion takes place in low permeable thick clay deposits.…”