Utilization of the Mangrove Forest for Sustainable Renewable Energy Production

The tropics are generally evergreen and have large tree populations that make up the bulk of plant biomass. The Niger Delta is rich in biodiversity and has the largest mangrove system in Africa and the Atlantic. Above ground biomass (ABG) is a good indicator of stand productivity in mangroves, and can be calculated with allometric method using tree structural characteristics of dbh and tree height. Red mangroves are the most dominant species, and the species mostly used for making firewood and charcoal. The carbon stock estimates was higher in locations with more red mangrove trees (66.1 ± 15.1 Mgha -1 ) than locations with fewer red mangrove trees (36.0 ± 12.8 Mgha -1 ), which indicates that they are excellent carbon sequesters. Mangrove forest therefore supplies low cost renewable energy and also reduces global warming through carbon sequestration. Already, utilization of firewood and charcoal for cooking is a booming business in many communities in the Niger Delta. But the issue is that deriving sustainable energy from mangrove forest requires modern technology. Energy production from mangrove raw material will reduce the burden of energy generation from petroleum. This will thus, save the environment from pollution from oil and gas exploration which has led to ozone layer depletion. Nonetheless, mangrove-derived biomass energy will thus save the environment from sulphur and radioactive contamination.

these countries most often lack the needed technology to convert biomass to liquid, gaseous or electrical energy. Globally, biomass is the fourth source of energy, but in Nigeria it is the second source of energy after petroleum [8,9]. Nigeria is the highest producer of crude oil and has the largest mangrove vegetation in Africa [2]. Therefore, the energy potentials of biomass in Africa and other developing countries of the world are great if it can be efficiently utilized. For instance, 96% of rural dwellers in Tanzania [10] and Utilizing firewood and charcoal is the highest bio-energy potential in Nigeria (805, 580 GJ) followed by animal (47, 718 GJ) and agricultural (325, 822 GJ) residues [11]. Majority of the firewood and charcoal utilized in Nigeria is derived from the stems and branches of the red mangrove trees (Rhizophora spp). Epileptic electrical power supply has made large number of people in cities to utilize firewood as the second option for cooking and heating. Increased use of firewood has its environmental implications, which include land degradation and deforestation, which can cause other unexpected effects such as increased soil erosion and flooding. Unregulated removal of trees can also result to barren lands, which eventually becomes wastelands. Continuous felling of trees can also result in the loss of other ecosystem services provided by mangroves globally (Figure 1) and in the Niger Delta in particular [12,13]. (Source: [13].

Solid Biomass Energy from Mangroves
There are over 150 species of mangroves globally, but the most dominant species in most parts of the world are the red mangroves (Rhizophora spp). In the Niger Delta there are three major species: the red (Rhyzophora racemosa), white (Avicennia germinans) and black (Laguncularia racemosa) mangroves and their percentage occurrence are 62.5%, 25% and 12.5% respectively [14,15]. All these three species are used to produce fire wood, but the most commonly used is the red mangrove because it has the highest potential (Table   1). This is followed by Sonneratia species. Rhizophora species is the best source of biomass energy because the stems are resilient and can catch fire and burn faster than stems of other species. Source: [15].

Figure 2:
Conversion of red mangrove stem (Rhizophora spp) to charcoal log, which is further broken into smaller pieces for local cooking and roasting [17].
The stems of red mangroves come in different sizes, which are later chopped into pieces and sold for domestic and industrial use ( Figure 2). For example, [16]  respectively [17,18]. The mature red mangrove stem is bulky and dense and can grow up to 20-30cm in diameter, which is an ideal range for fire wood and charcoal production ( Table 2). Red mangroves (Rhizophora) are the heaviest in density followed by Acacia and birch. The later species are common in the temperate regions, while mangrove species are more prominent in tropical regions where high temperature and precipitation influence their growth and survival. Source: Modified from [18].
The red mangrove is divided into sub-species, which include R. racemosa, R. mangle, R. harrisoni, and R. apiculata. Out of these four species R. apiculata is the best for fire wood and charcoal production [19]. In addition, the red mangrove stems stand out from other mangrove species because they have high calorific value (~5 calorie per gram), which is a significant factor in combustion (Table 3).  Source: [19].

Fire Wood is a Cleaner form Of Energy Production
Mangrove forests are renewable resource, which means they have a turnover period of 15 years in attaining maturity after they are planted as compared to crude oil from which petroleum is manufactured, which takes thousands and millions of years to form within the earth crust. Even though petroleum has the highest calorific value, and widely utilized round the world (Table   3), the problem it creates for the world's environment, may last for generations, and would eventually undo its usefulness in the long run due to emerging climate change problems caused by pollution from release of exhaust. Biomass generally produces less carbon dioxide (CO2) and sulphur dioxide (SO2) pollutants as compared to petroleum. The burning of firewood emits less carbon into the atmosphere as compared to petroleum products, which has flooded the earth's atmosphere with green house gases (methane, water vapor, carbon dioxide, nitrous oxide, ozone etc). This has resulted to biodiversity losses and species extinctions in many parts of the world. In the Niger Delta many species had gone extinct; a classical example is the local extinction of some species within the lepidopteran (butterfly) family that were in large population over 30 years ago as a result of oil and gas exploration.
Nevertheless, the role of mangrove forest as carbons sequester [20,21] had counteracted the adverse impact of firewood burning. This is because mangrove forest helps to filter the atmosphere of carbon dioxide, which is a major cause of global warming.

Carbon Stock Estimation of Red Mangrove Stems
To derive the above ground biomass firstly, the diameter at breast height (dbh) and the height of the trees are measured with a measuring tape using allometric method (Table 4). This study was carried out in two locations (i.e. Okrika and Buguma) in the Niger Delta, Nigeria by [23]. Secondly, to derive the estimate of carbon stock half of the above ground biomass (AGB) was calculated ( Table   5). The above ground biomass was calculated using the diameter at breast height by applying the formula of [24]. This equation is the Model 1 (diameter-height-wood density) mangrove biomass regression model (Eq. 1).

Recommendation
To advance firewood technology, a biomass energy processing plant can be installed across communities in Africa to accelerate the conversion of firewood and charcoal into liquid, gaseous and electrical energy. Since biomass energy require cutting down of trees a regulatory agency need to be established to monitor patterns of deforestation, and encourage controlled harvesting and aggressive afforestation programs to counter the negative consequences of tree removal such as climate change, soil degradation and habitat and biodiversity losses. Gradual increase in use of renewable energy can lead to the drastic reduction in the use of fossil fuel, which in the long run will be a win-win for the environment. The mangroves of the Niger Delta are excellent sequesters of pollutants which had reduced the quantity of pollutants in the skies of Africa.