Estimation of potential methane production through the mass balance equations from agricultural biomass in Korea

Abstract

Biomass is a renewable energy resource derived from all organic materials produced by both human and natural activities. Total biomass is amounted to be 58,010 Gg yr⁻¹ from agricultural sector during 2013 in Korea: livestock manure, crop residues, and agro-industrial wastes. Potential methane production from agricultural biomass was calculated based on IPCC guidelines using manipulated equations. The main parameters were emission factor, total waste amount, and physico-chemical properties of each waste to estimate methane production. Calculated total potential methane production from the different categories for livestock, crop residues, and agro-industrial wastes was 502 Gg yr⁻¹ in Korea. Poultry waste generated the highest methane potential with 227 Gg yr⁻¹ followed by 80 Gg yr⁻¹ from cattle waste. For crop residues and agro-industrial wastes, estimated methane production was 1 and 126 Gg yr⁻¹, respectively. Results of this study show that livestock manure gave the highest methane emission in the agricultural sector. With this, more effective management of livestock wastes is necessary to develop and maximize technology on harnessing methane as alternative energy.

Introduction

Global warming is becoming a critical issue around the world. Numerous researchers and organizations have been involved in reducing the greenhouse gases from various sources (Monni et al. 2004; Laifeld and Fuhrer 2005; Wang et al. 2011). Since many countries recognized the effects of greenhouse gases (GHG), including methane (CH₄), carbon dioxide (CO₂), and nitrous oxide (N₂O), the estimation of GHG emission was conducted for a comprehensive understanding of these effects in each country in terms of global warming and the significant mitigation potential (Janssen et al. 1999; Olivier et al. 1999; Yang et al. 2003; Wang et al. 2011).

Biomass is composed of carbon-rich materials including all plants, animals, nutrients, excrements, and bio-waste from households and industries (Deublin and Steinhauser 2011). Unused or discarded biomass residues from agricultural areas are potential energy resource, but at same time can be a source of GHG emissions, causing a significant environmental problem. Potential energy production from crop and animal residue is globally estimated to be about 34 EJ (exajoule = 10¹⁸ joules) out of a total 70 EJ (Bauen et al. 2004). In Korea, it is estimated that over 5 × 10⁴ Gg of organic wastes are produced every year in agricultural sector out of over 8 × 10⁴ Gg (MIFAFF 2010). The interest in biomass in resource-poor countries such as Korea is therefore increasing.

Anaerobic digestion (AD) is a biological process that converts the solid or liquid biomass into gas in the absence of oxygen (Khanal 2008). Many studies on anaerobic digestion in Korea have focused on pig manure and food wastes as substrates (Youn and Shin 2005; Shin et al. 2008; Yoon et al. 2011). Methane, the main product from this process, is a clean green fuel as well as a greenhouse gas.

Anaerobic digestion for methane production using organic waste in the agricultural sector is becoming economically feasible for intermediate to large-scale animal production operations. Methane produced by the digester can be used as an energy source for electricity and heat generation. This can reduce the treatment cost and methane emission into the atmosphere. However, some of the organic wastes are recycled as compost after an aerobic treatment, which end in landfills or dumped in the ocean. These practices are either consuming a lot of energy, i.e., aerobic digestion or a waste of resources.

Several studies on the agricultural biomass estimation were also conducted in Korea based on the crop-cultivated area (Hong et al. 1989), using an elemental analysis and heating value (Hong 2004). Using dry weight of crops and harvest index is being followed by Food and Agriculture Organization and the U.S. Department of Agriculture (NREL 2005). Estimating the methane production potential from the amounted biomass is a necessary process to make both greenhouse gases (GHGs) and bio-energy conversion inventoried in South Korea. However, an estimated potential methane production from biomass in Korea has not been studied.

The main objective of this study was to estimate the amount of potential methane production based on the total amount of production and characteristics of agricultural biomass in Korea.

Materials and methods

Data collection and sampling

In order to make a comprehensive inventory for estimating the potential methane production from livestock wastes, crop residues, and agro-industrial wastes, a field visit was conducted to characterize the waste management systems used and to verify the information collected through other sources by sampling the agricultural biomass. In addition, interviews with local experts from pertinent ministries (e.g., ministries of agriculture, environment, and energy), local Non-Governmental Organizations and engineering consulting companies working on agriculture and rural development, current users of AD technologies, and other stakeholders including the Ministry of Food, Agriculture, Forest, and Fisheries (MIFAFF), the Ministry of Environment, the Rural Development Administration (RDA), and the National Veterinary Research and Quarantine Service in Korea were done. Secondary data, including national statistical data, were used for estimating the methane yield from Korean statistics.

Calculation of agricultural biomass inventories

The produced amount of animal manure can be varied depending on the species and body weight of livestock, feed sources, and management systems. The agricultural and forestry biomass were calculated as total and volatile dry weights of livestock manure per year based on statistics (NIAS 2009). Fresh manure production from cattle, dairy, swine, and poultry were calculated using the average fresh manure production per Mg to livestock and average weights of corresponding animals in Korea. The average fresh manure productions for cattle, dairy, swine, and layer and broiler chickens were 8.0, 24.6, 1.6, 0.15, and 0.13 kg head⁻¹ day⁻¹, respectively (MIFAFF 2009). The average weights of these animals adopted were 350, 473, 111, 1.6, and 1.3 kg, respectively (MIFAFF 2009). The annual total dry weight and volatile dry weight for each type of fresh manure were calculated by multiplying the total annual mass of fresh manure by the average percent solid contents (3.8 % for cattle, 3.9 % for dairy, 4.1 % for swine, and 20.3 % for poultry) and by the average volatile solids fraction (2.7 % for cattle, 2.8 % for dairy, 2.6 % for swine, and 19.8 % for poultry) according to the National Academy of Agricultural Science (MIFAFF 2009). The total volatile dry weight for all agricultural manure was used for the calculation of potential methane production for this category of biomass residues.

As most of cattle are housed within the feedlots in Korea, a 97 % collectible factor was assumed, and the final calculations are shown in Eqs. 1, 2, 3, and 4 (NRC 1983).

$${\text{Total amount of cattle waste }} = \, ({\text{population of cattle)}}\, \times \, 3 6 5\, \times \, 1 3. 7\, \times \,0. 9 7,$$

(1)

where 13.7 = unit factors (kg head⁻¹ day⁻¹) and 0.97 = collectable factor.

There is no investigated population of dairy cows and calves in Korea; combined population of dairy was used for the calculation. An 85 % collection availability factor was used for the province in confined animal operation (JAYCOR 1990).

$${\text{Total waste amount from dairy }} = {\text{ total population}}\, \times \, 4 5. 6\, \times \, 3 6 5\, \times \,0. 8 5,$$

(2)

where 45.6 = unit factors (kg head⁻¹ day⁻¹) and 0.85 = collectable factor.

The calculation of total waste amount generated from swine was based on “Biomass Inventory and Bio-energy Assessment” in Washington, USA (Frear et al. 2005). Also, because most of the swine are raised in farm, the collectable factor was assumed to be 100 %.

$${\text{Total waste amount from swine }} = \, ({\text{total population}}\, \times \, 8. 6\,{\text{kg head}}^{ - 1} \, {\text{day}}^{ - 1} \, \times \, 3 6 5),$$

(3)

where 8.6 = unit factors (kg head⁻¹ day⁻¹).

To calculate the total waste amount from poultry, its total population was multiplied by the combined average manure production factor for egg layer and broiler in Korea. Also, an 80 % collectable factor was considered for the final calculation.

$${\text{Total waste amount from poultry }} = \, ({\text{total population}}\, \times \,0. 1 4\, \times \, 3 6 5\, \times \,0. 8 ),$$

(4)

where 0.14 = unit factors (kg head⁻¹ day⁻¹) and 0.80 = collectable factor.

The annual total dry weight of crop residues was derived from the report of Lee and Kim (2008).

Estimation of potential methane production related with manure

The 2006 IPCC Guidelines (GLs) for National Greenhouse Gas Inventories Tier 2 method (IPCC 2006) was used for estimating the methane production from each commodity group in the livestock production sector. Using the Tier 2 method, methane potential production for each livestock commodity group (M) and existing manure management system (S) and climate (k) combination were estimated (see below) using Eq. 5;

$${\text{CH}}_{{ 4 { }({\text{M}})}} \, = \,\left( {{\text{VS}}\, \times \,{\text{H}}_{{({\text{M}})}} \, \times \, 3 6 5 {\text{ days}}/{\text{year}}} \right)\, \times \,\left[ {{\text{B}}_{{{\text{o}}({\text{M}})}} \, \times \,0. 6 7 {\text{ kg CH}}_{ 4} /{\text{m}}^{ 3} {\text{CH}}_{ 4} \, \times \,{\text{MCF}}_{{{\text{S}},{\text{ k}}}} } \right],$$

(5)

where CH_{4 (M)} = estimated methane potential production from manure for livestock category M, kg CH₄ per year, VS_(M) = average daily volatile solids excretion rate for livestock category M, kg volatile solids per animal-day, H_(M) = average number of animals in livestock category M, B_o(M) = maximum methane production capacity for manure produced by livestock category M, m³ CH₄ per kg volatile solids excreted, and MCF_(S,k) = methane conversion factor for manure management system S for climate k, decimal.

As shown in Eq. 5, estimated methane production requires an average daily volatile solids excretion rate for the livestock category. Therefore, the measured values of volatile solids for dairy cows, cattle, swine, and poultry were used for this calculation.

For the livestock wastes, contents of total solid (TS) and volatile solid (VS) were similar for cattle, dairy, and swine, and their ratios of VS/TS averaged at 70.8, 71.8, and 63.4 %, respectively. However, TS and VS values for poultry were much higher than the other livestock species. Moisture content of poultry manure was averaged at 74 % for broilers and layers. Total suspended solid and VS were measured on a dry basis, and BOD₅ and COD were also measured for estimating the methane potential (Table 1).

Table 1 Sources of livestock wastes in Korea

Realistic estimates of methane production also require identification of the appropriate MCF, which is a function of the current manure management system and climate. MCFs for various types of manure management systems for average annual ambient temperatures were referred to the 2006 IPCC GLs for National Greenhouse Gas Inventories. Specification of the potential methane production (Bo) for the type of manure was considered according to the 2006 IPCC GLs for National Greenhouse Gas Inventories.

Estimation of potential methane production from crop residues and agro-industry wastes

To calculate the potential methane production from crop residues and agro-industrial wastes, two different calculation methods were used based on data availability.

For crop residues, the method of calculation from IPCC GLs was adapted because of the lack of available information for element analysis of crop residues. For emission factor of anaerobic digestion, a dry weight basis emission factor of 1 g CH₄ kg⁻¹ waste treated was used (Table 2).

$${\text{CH}}_{4} \,{\text{emission}} = \sum\limits_{i} {(M_{i} \cdot EF_{i} )} \cdot 10^{ - 3} - R,$$

(6)

where CH₄ emission = total CH₄ emissions in inventory year, Gg CH₄, M _i  = mass of organic waste treated by biological treatment type i, Gg, EF = emission factor for treatment i, g CH₄ kg⁻¹ waste treated, i = composting or anaerobic digestion, R = total amount of CH₄ recovered in inventory year, Gg CH₄.

Since no distinctive statistical information was available for agro-industrial wastes, the average value of elemental analysis was used for calculation. When information on elemental analysis was available, theoretical methane production, as shown in Eq. 7, was applied.

$${\text{C}}_{a} {\text{H}}_{b} {\text{O}}_{c} {\text{N}}_{d} \, + \,\left(\frac{4a - b - 2c + 3d}{4}\right)\,{\text{H}}_{2} {\text{O}}\, \to \,\left(\frac{4a + b - 2c - 3d}{8}\right)\,{\text{CH}}_{4} \, + \,\left(\frac{4a - b + 2c + 3d}{8}\right)\,{\text{CO}}_{2} \, + \,d{\text{NH}}_{3}$$

(7)

Table 2 Default emission factor for methane emission from biological treatment of waste

Results

Estimation of livestock waste

Populations of different livestock (cattle, dairy, and swine) and poultry species were calculated and examined. The amount of manure mainly depends on the rate of waste production per animal and the number of animals. The main parameters for calculating the total waste amount from livestock are total population of livestock, unit factor, and collectable factors. For determining collectable factors, the management practice of each livestock was considered. Similarly, dairy, swine, and poultry are raised in confined animal feeding operations in Korea. Therefore, the collectable factors were assumed to be 85, 100, and 80 %, respectively, based on the previous study (JAYCOR 1990).

The number of cattle, swine, and poultry has increased by 60.5, 10.6, and 38.0 %, respectively, from 2005 to 2013, while dairy has gradually decreased during this period countrywide (Table 3).

Table 3 Population of the different livestock species in Korea (MIFAFF, 2006–2014)

Based on Eqs. 1, 2, 3, and 4, the calculated total waste amount related to livestock manure was 57,456 Gg yr⁻¹ in 2013. Among the livestock species, swine had the highest amount of waste with 31,114 Gg yr⁻¹, which accounts for 54.2 % of the total wastes (Fig. 1).

Fig. 1

Percentage of waste generation from different livestock species (Grand total amount of waste was 57,456 Gg yr⁻¹)

The waste produced from cattle was 24.6 % (14,153 Gg yr⁻¹) of the total waste followed by dairy with 10.4 % (6001 Gg yr⁻¹) (Table 4). The least amount of waste came from poultry because of its lower unit factor compared to other livestock species.

Table 4 Annual waste amount estimated from different livestock species in Korea

Estimation of crop residues and agro-industrial wastes

Crop residues are also one of the main organic biomasses produced from agricultural practices. Biomass from 13 kinds of crop residues was investigated (Table 5). The total amount of crop residues was estimated at 7820 Gg yr⁻¹ in 2013. For rice and barley, the estimated total biomass was 6741 and 38 Gg yr⁻¹, respectively. In the pulse category, the biomass related to soybeans and peanuts was determined for each province in Korea and summed together to calculate the total amount. Estimated total biomass for soybeans was 218 and 22 Gg yr⁻¹ for peanuts.

Table 5 Estimated total organic wastes of crop residues and agro-industrial wastes in Korea

In the vegetable category, four vegetables, namely, watermelon, carrot, garlic, and green onion were investigated. Estimated total biomass were 9.4 Gg yr⁻¹ for watermelon, 1.2 Gg yr⁻¹ for carrot, 72.2 Gg yr⁻¹ for garlic, and 29.5 Gg yr⁻¹ for green onion. Among the four vegetables, biomass of garlic was the highest. Estimated total biomass for sweet potatoes was 280.1, and it was 131 Gg yr⁻¹ for potatoes. For the oil-seed crops, sesame and perilla seeds were investigated. The total biomass was estimated at 71.9, 204 and 751 Gg yr⁻¹, respectively (Table 5).

Agro-industrial wastes include the generated wastes from food manufacturing and processing, slaughterhouses, and tobacco manufacturing and processing. The total amount of agro-industrial wastes was estimated at 845 Gg yr⁻¹ in 2007.

The average carbon contents of the crops’ different portions, namely the leaf, grain or fruit, and root were presented in Table 6. In the leaf part, rapeseed showed the highest carbon content followed by perilla seed and sesame. For grain or fruit and root parts, rapeseed and peanut showed the highest carbon contents with 57.6 and 59.6 %, respectively. Since the biomass of rice was the highest among other crops, the estimated total carbon content of rice was 87 % of the total carbon content among the 13 crops (Table 6).

Table 6 Average carbon content of each crop (Lee and Kim 2008)

Agro-industrial wastes have no category distinction in Korea. For the different sources of agro-industrial wastes (Table 7), the value of pH ranged from 3.9 to 7.3. Moisture content also varied and was observed to be highest in food manufacturing with 95.3 % followed by 82.3 % in slaughterhouse waste. In case of elemental analysis, the average carbon content was 47.0 % for all examined agro-industrial wastes, and the highest carbon content was observed in food manufacturing residues.

Table 7 Different sources of agro-industrial wastes in Korea

Estimation of potential methane production from manure

In order to estimate the methane (CH₄) production during storage and treatment of manure, two main factors including the amount of manure produced and the portion of manure that decomposes anaerobically, were considered. Using Eq. 5, estimated total methane production potential was 375.5 Gg yr⁻¹ in Korea (Table 8). The highest methane production is estimated to be 227.3 Gg yr⁻¹ for poultry waste, and the lowest was 12.1 Gg yr⁻¹ for swine waste.

Table 8 Estimated methane production from manure-related waste

Potential methane production from crop residues and agro-industrial wastes

To calculate the methane production from crop residues and agro-industrial wastes, two different calculation methods were used based on data availability.

For the crop residues, the calculation method from IPCC GLs was adapted because of the lack of available information for element analysis of crop residues. For emission factor of anaerobic digestion, a wet weight basis emission factor of 1 g CH₄ kg⁻¹ waste treated was used. In addition, two assumptions were made to calculate the methane production from crop residues. If this is used for alternative energy production, it can be assumed as following: first, 30 % of the total production of each crop residues is collectable from the field, and second, the utilization rate of crop residues for anaerobic digestion ranges from 10 to 30 % of collectable crop residues.

Based on those two assumptions, methane potential production from crop residues was estimated using Eq. 6 (Table 9).

Table 9 Estimated methane production from crop residues

When information on elemental analysis was available, theoretical methane production was applied (Eq. 7). Based on the calculation, the potential methane production from agro-industrial wastes was estimated at 125.7 Gg yr⁻¹ (Table 10).

Table 10 Estimated methane production potential from agro-industrial wastes

Summary of potential methane production from agricultural biomass

Potential methane production for bio-energy from agricultural biomass in Korea was estimated based on animal manure, crop residues, and agro-industrial wastes (Table 11).

Table 11 Estimated total potential methane production from agricultural wastes in Korea

Total methane potential production from agricultural biomass was estimated to be 502 Gg yr⁻¹. Among the three categories, estimated methane production from animal manure was the highest at 74 % of the total methane production. It is considered that methane production from crop residues was the smallest because of high recycled efficiency for animal feeding stock. With fossil energy sources eventually dwindling and becoming increasingly more expensive, bio-energy systems are likely to have future attraction. The improvements in digestion system obtained from better preprocessing and recovery techniques would also help improve their viability in general.

Discussion

This research was conducted to suggest an alternative calculation for constructing the typical Korean-specific GHG inventory as well as the bio-energy conversion potential in agricultural sector. Significant amounts of agriculture-based biomass are produced from animal wastes as cattle, dairy, swine and poultry; crop residues as mainly rice and barely straws; and agro-industrial wastes, in agricultural sector across Korea. Large sources of biomass are often located at a distance from potential energy production sites. Since most of the cattle are housed within feedlots in Korea, a 97 % collectable factor was assumed (NRC 1983). However, a significant amount of livestock manure, as much as 25–35 % of the total residues, may remain un-recoverable in the field (Woods and Layzell 2003). The collection, transport, and processing of biomass also pose a significant challenge to their use in energy production (Abu-Ashour et al., 2010). With regard to the utilization of agricultural biomass, the costs of these feedstocks are directly proportional to the costs of collection and transportation to energy production sites (Overend 2002). Agricultural biomass in Korea totally generated could be 68,011 Gg year⁻¹. It has been widely reported that the maturity stage of the plant is of major importance for specific methane yield since it determines biomass composition (Amon et al. 2007; Kaparaju et al. 2002; Gunaseelan 2004). The biggest contributor was animal manure which was estimated to be 375.5 Gg year⁻¹ for a potential methane production. On the other hand, crop residues and agro-industrial wastes were calculated to be 1.2 and 125.7 Gg year⁻¹, respectively. Agricultural biomass each year in Korea can currently offer the greatest potential for alternative bio-energy production with various points. Our assessments indicated that this potential methane yield is supposed to be about 2 % of annual amount of total energy consumption (23,650 Gg yr⁻¹) in Korea (RDA 2009). However, the potential methane yield was the greatest in animal manure, which is equivalent with 5 % of 6.8 Tg yr⁻¹ in the EU (Moss et al. 2000). Results of this study indicated that animal manure was the main source of methane production in agricultural sector. However, it is acknowledged that assessment of potential methane production has some limitations due to non-application of various bio-energy production technologies with different feeding stocks. In spite of having some limitations, this assessment provides useful data that can be used in prioritizing agricultural biomass for further analysis, research exploitation as well as identifying the biomass sources which may have higher significance. Good management practices for livestock wastes are necessary to develop and maximize the technology on harnessing methane using agricultural wastes as alternative energy.