Recent trends in biochar production methods and its application as a soil health conditioner: a review

Abstract

Interest in biochar production from organic waste has been growing in recent years due to its broad applicability, availability, and smoother production. Biochar production techniques are being continuously modernized to improve the production rate and quality. Though numerous methods have been reported in the recent past, a systematic classification of the same is yet to be explored. Based on the advancement of the techniques being employed for biochar production and modification of conventional methods, we have categorized all major techniques of biochar production into two primary classes. In the traditional approach, ancient methods and conventional pyrolysis techniques (Slow and Fast pyrolysis) are included, whereas, in modern approaches, several advanced technologies such as Gasification, Torrefaction, Hydrothermal carbonization, Electro-modification, along with modified traditional methods (Flash pyrolysis, Vacuum pyrolysis, and Microwave pyrolysis) are comprised. Further, the systematic review was intended to evaluate various types of feedstocks (agricultural biomass, forest/woody biomass, aquatic biomass, urban waste, and paper waste) with their potential to produce biochar. It was observed that the feedstock containing high cellulose was found to be helpful in improving the overall properties of biochar, including enhanced adsorptive action and retention of nutrients.

Biochar and Its Broad Impacts in Soil Quality and Fertility, Nutrient Leaching and Crop Productivity: A Review

“…Archaeological records show that the production and use of biochar started thousands of years ago in the Amazon basin of South America [50]. In ancient times, biochar was produced in two ways: (i) people used to pile the wood in the soil pits, cover these pits with layers of soil, and then burn it slowly with absence of or limited air, or (ii) people used to burn the wood in open air and then immediately cover the half-burned wood with layers of soil [50,51]. Over time, these ancient methods of biochar production were replaced with clay burners, firebrick pits, and brick kilns that consist of a pit enclosed by clay, bricks, or metal [50].…”

“…Biochar has attracted significant attention in the current organic farming system over the last 20 to 25 years, although its origin is older and associated with the Amazon region, where is commonly known as "terra preta" soil [50]. These dark-colored, highly fertile soils have supported the food and feed requirements of the Amazonians for centuries.…”

“…The process generally occurs at atmospheric pressure with energy provided by an external source. In slow pyrolysis, the major product is biochar (35-45%) (Figure 1) followed by bio-oil (25-35%) and syngas (20-30%) [50,57]. This review describes the slow pyrolysis using an auger reactor, although rotary kilns and other reactors are also used in practice, because of its simplicity in design and operation and its wide use by small and mid-size industries [53,58].…”

“…In fast pyrolysis, small particles of feedstock >2 mm are used, which rapidly decompose to generate biochar, vapors, and organic volatiles (Figure 3) [61]. In this process, biochar yield is less (20%), syngas similar (20%), and bio-oil production higher (60%) compared to that produced with slow pyrolysis (Figure 1) [50]. Higher pyrolysis temperature increases both the surface area and C content of biochar due to the release of organic volatiles from feedstock particles.…”

“…Table 1. Average ranges for the process conditions in biochar production technologies most used at present, including slow and fast pyrolysis, gasification, and torrefaction, and yield of the main products for each technology [1,50,74].…”